Luận án tiến sĩ: Kinetics and mechanism of vinyl chloride polymerization: Effects of additives on polymerization rate, molecular weight and defect concentration in the polymer

38 Table 2.2 Molecular weights and molecular weight distributions for PVCs prepared by bulk polymerization at 55 oC in the presence of organic additives……….... 73 Table 2.7 Summary of ki

Introduction

Poly(vinyl chloride) (PVC), is one of the most widely produced polymeric materials in the world; the annual production of the PVC is second only to that of polyethylene 1 In industry, PVC is generally synthesized by free radical polymerization of vinyl chloride (VC) Radiation-induced 2 , anionic 3 , and modified Ziegler-Natta catalyst 4 methods have also been reported in the literature Vinyl chloride differs from monomers like styrene, methyl methacrylate or vinyl acetate principally by the insolubility of the polymer in its monomer Its key feature is that poly(vinyl chloride) is insoluble in its monomer, but slightly swollen by it Polymerization of vinyl chloride also differs from the heterogeneous polymerization of monomers, such as acrylonitrile Vinyl chloride partly swells its polymer but acrylonitrile does not It also differs from the conventional emulsion polymerization of unsaturated monomers in which the polymer particles are swollen by their monomer

Vinyl chloride is classified as a non-conjugated weak electron-withdrawing vinyl monomer with Q- and e-values of 0.056 and 0.16, respectively, determined from radical copolymerization 5 This indicates that the VC monomer has low reactivity, but its radical is highly reactive The propagating radical is so reactive that it tends to chain transfer to all substances in the polymerization system, such as monomer, initiators, solvents, and the resulting polymer As a consequence, radical polymerization of VC produces anomalous units in the chain, which decrease its thermal stability Generally, the termination reaction is dominated by chain transfer to monomer, if there are no other

- 2 - chain-transfer agents in the system The overall polymerization of vinyl chloride can be described as follows:

Here I is the initiator, R • the primary radical, M the monomer, M • the monomeric radical, Mi • the growing radical with i monomer units, P dead polymer, P • the polymer radical generated by a chain transfer reaction, I • the initiator radical, S the solvent, S • the solvent radical, k’s corresponding rate constants.

A brief History of PVC

The monomer, vinyl chloride, was first discovered in 1835 by Henri Victor Regnault 6-9 , a young Frenchman, born in Aix-la-Chapelle in 1810 Regnault came to

- 3 - work for a short time in the winter of 1834-35 in Justus von Liebig’s laboratory at Giessen, Germany He described the preparation of vinyl chloride as follows:

When one mixes an alcoholic solution of caustic potash with oil of the Dutch chemists (i.e dichloroethane), one can observe after some time the appearance of a precipitate which continues to increase When one takes the vessel containing the reaction mixture into one’s warm hands, the liquid begins to boil and a large amount of gas with an ether-like smell is evolved This burns with a yellow flame with a green mantle 10

He examined the white precipitate and identified it as potassium chloride The gas however proved less simple He found, it can be condensed between –15 o and –18 o C, and it is soluble in alcohol and ether in all proportions and to a much lesser degree in water The formula of the new compound was described as C H 2 3 Cl He named it

‘chloraldehydene’ In the same year 11 , he also prepared the bromine and iodine analogues of ‘chloraldehydene’: C2H3Br and C2H I It was Kolbe3 12 who first named Regnault’s compound as ‘vinyl chloride’ in 1854, although the origin of the term ‘vinyl’ was not discussed It was not until 1870 that the structure of vinyl chloride was finally established by F.V Kekule 13

It was not clear whether Regnault observed poly(vinyl chloride) in his early investigations The first report of vinyl halide polymer was made in 1860 by A W Hofmann 14 He noted the change of vinyl bromide monomer to a white mass without compositional change Actually, Hofmann saw the polymerization of vinyl bromide but had little idea about the nature of the change Since the polymer concept had not been

- 4 - developed yet, he described the change as ‘metamorphosis’ He was followed by Eugen Baumann 15 who described the preparation of poly(vinyl chloride) in 1872; almost 50 years before the macromolecular concept was developed He detailed the sunlight induced change of vinyl chloride monomer to solid products, which he thought to be the isomers of the monomer The properties described by him are those we ascribe, today, to poly(vinyl chloride), or PVC

The further development of poly(vinyl chloride) is a tale of two continents as well as different reasons and objectives Actually, it took 50 years for the issue of a German patent for the manufacture of vinyl chloride by reaction of acetylene and hydrogen chloride In 1912, it was Frits Klatte 16 who was assigned by his superiors at Chemische Fabrik Griesheim-Elektron to find uses for excess acetylene It was no longer used for lighting because new efficient electric generators were developed which ended the acetylene lamp business He reacted some acetylene with hydrochloric acid (HCl) Now this reaction will produce vinyl chloride, but at that time no one knew what to do with it, so he put it on the shelf, where it polymerized over time He and his company Griesheim- Elektron patented the material in Germany in 1913 It was the first PVC patent in the world They never figured out a use for their poly(vinyl chloride) product, and in 1925 their patent expired

In 1926, an American chemist, Waldo Semon 17 , working at B.F Goodrich, invented plasticized PVC, or vinyl, ‘by accident’, as he later claimed Actually, his original assignment was to make an adhesive from a simple synthetic organic polymer

His first attempts, using reclaimed crude rubber and a German prototype of synthetic rubber, were unsuccessful When he used up all his supply of rubber, he began experimenting with synthetic organic polymers—including poly(vinyl chloride), a substance at that time considered no more valuable than refuse Because the polymer was stiff at room temperature, Semon heated it in a solvent with a high boiling point, e.s., tritolyl phosphate, and got a jelly that was elastic and flexible after cooling, but was not adhesive It demonstrated the plasticizing of PVC Semon instinctively realized that he was halfway to a major breakthrough He kept experimenting with poly(vinyl chloride) until he succeeded in plasticizing the polymer PVC was always more durable than crude rubber Semon's first breakthrough made it elastic as well as resilient; and his second breakthrough made it moldable into whatever shape was required Semon's first applications included a golf ball and shoe heels, as well as a number of useful coatings for tool handles, wire, and other items By the 1930s, BFGoodrich had begun to produce and market the first of the hundreds of commercial applications that would be found for plasticized PVC

In the ’50s, five companies in the Unites States were producing PVC, and the number of producers increased to 20 by the middle ’60s The number remained constant in the ’70s and ’80s Currently, the largest PVC maker in the U.S is Shintech, followed by Oxy Vinyls a joint venture between Occidental Chemical and PolyOne and Formosa Plastics and Georgia Gulf, which are tied for third 18

PVC is now the second most used plastic in the world, next to polyethylene, the source of billions of dollars of revenue every year The PVC industry employs more than 100,000 people in the United States alone Figure 1.1 shows a graphic evolution of world PVC production from 1950 to 2003 PVC is a very versatile plastic This is exemplified by the wide variety of end use applications that include toys, food packaging, furniture, transportation, electronics, medical blood bags and prosthetic devices, wire and cable insulation, water and sewer pipes, window frames, etc Figure 1.2 illustrates the World PVC applications in 2003

Evolution of World PVC Production

PVC Production (Million Metric Ton)

Figure 1.1 Evolution of world PVC production (*source: CMAI)

Figure 1.2 2003 World PVC applications (source: CMAI)

Methods of Vinyl Chloride Polymerization

PVC is commercially manufactured by four major processes: suspension, emulsion, bulk, and solution polymerization Suspension polymerization is the most widely used procedure 1 , followed by emulsion and bulk polymerization Solution polymerization is reserved for a few specialty copolymers, or where the application makes it appropriate, as in solution coatings §1.3.1 Polymerization in Bulk

The bulk polymerization of VC is the third most important manufacturing process for PVC The advantage of bulk polymerization, in contrast to the common suspension or

- 8 - emulsion polymerization is that the products are free of protective colloids, suspending agents, surfactants, buffers, water, additives, or solvents There is, however, one great problem for technical application This is to remove the heat generated during polymerization and to control the rate of reaction The industrial-scale bulk polymerization is based on the Pechiney-Saint-Gobain process 19 , a two-stage process 20, 21

In the first step VC is prepolymerized to approximately 10% conversion Then the reacting mass is dropped into a second autoclave, and more monomer and initiator are added The polymer beads grow larger and the mixture takes on the appearance of a dry powder The reactor is specially designed to stir powdery material and is equipped with a condenser To avoid agglomeration of the beads, it is very important to control the rate of agitation PVC produced in this way is the purest product available on the market Experimental procedures for bulk polymerization on a laboratory scale are relatively simple Normally, the monomer is heated in the presence of a small amount of a monomer-soluble initiator, such as azobis(isobutyronitrile) or dicapryloyl peroxide, under a suitable condensing or pressure system until the desired conversion of monomer into polymer has been achieved The VC can be recovered by distillation in an effective hood

Some monomer-soluble initiators mentioned in many patents are di(2- ethylhexanoyl) peroxide, 3,5,5-trimethylhexanoyl peroxide, di(t-butyl) peroxyoxalate, di(carballyloxyisopropyl peroxydicarbonate) di-2-butoxyethyl peroxydicarbonate, di-4- chlorobutyl peroxydicarbonate, azobis(isobutyronitrile) and azobis-(cyclohexyl- carbonitrile)

As mentioned earlier, suspension polymerization is the most widely used method today, producing over 80% of the world’s PVC The usual manufacturing procedure is a batch operation, although continuous processes have been described A typical formula for such batch polymerization might be as follows:

Here lauroyl peroxide is the initiator, which is monomer-soluble Poly(vinyl alcohol) acts as a suspending agent, which is necessary for stabilizing the monomer droplets to avoid coagulation and to control the dimension of the particles Sometimes small amounts of emulsifiers such as sulfonated oils are also used to increase the porosity of the polymer particles The water/monomer ratio can be raised up to 4:1 and the polymerization temperatures range from 40-60 o C When a pressure drop is observed, the polymerization is almost finished The slurry from the autoclave is then discharged into an evacuated tank and unreacted monomer is pumped out, condensed, and after processing fed back into the polymerization The water is removed by centrifugation

- 10 - accompanied by washing with distilled water to remove all soluble electrolytes Finally the PVC product is dried under reduced pressure at about 50 o C §1.3.3 Polymerization in Emulsion

Emulsion polymerization is mainly used for manufacturing vinyl chloride copolymers Sometimes the polymerization product, in latex form, is used as it comes from the reactor Sometimes it is converted to a dry powder The homopolymer is never used in latex applications since the temperature required for fusion of the homopolymer particles is rather high

In contrast to bulk and suspension polymerizations, where monomer-soluble initiators are used, in emulsion polymerization a water-soluble initiator is used Some initiators used are: potassium persulfate, ammonium persulfate, sodium percarbonate, peracetic acid, hydrogen peroxide and cumene-hydroperoxide (water-soluble), etc The ratio between water and VC is approximately 2:1 By analogy with suspension polymerization, the rate of agitation is very important for the preparation of useful emulsion resins Generally, the speed of agitation is more moderate than in suspension polymerization Sometimes it is desirable to keep the pH value constant during the reaction Therefore, a buffer is added to the reaction system The advantage of emulsion polymerization for latex production is obvious However, there are other advantages of the process compared to suspension polymerization: a) the initiation and propagation steps can be controlled more independently than in suspension polymerization which has essentially the features of a bulk process; b) polymer particles have relatively little

- 11 - internal void volume as compared with the suspension polymerization product, and c) monomer can be added during the polymerization to maintain polymer composition constant This is very important in copolymerization §1.3.4 Polymerization in Solution

Since PVC is not soluble in its own monomer, it is necessary to find a solvent for the polymer for solution polymerization Such a solvent could be tetrahydrofuran, acetone, cyclohexanone, alkyl acetates, chlorinated alkyls, diethyl oxalate, etc The azo and organic peroxo compounds used in bulk and suspension polymerization are suitable for initiation Actually solution polymerization is very complex due to chain transfer to the solvent and the solubility of the polymer Because of the cost of solvents and their recovery, this process is rarely used in industry.

Structure and Properties of PVC

Poly(vinyl chloride) has a mainly ‘head-to-tail’ main chain structure, in which a chlorine atom is situated on alternate carbons of the polymer chain:

-CHCl-CH -CHCl-CH -CHCl-CH -CHCl-CH -CHCl-CH -CHCl~~~

The structure of the polymer leads to a very rigid and relatively tough plastic in the unplasticized state In the presence of a plasticizer, however, the dipole bonding between polymer chains is much reduced, leading to increased freedom of chain movement and thus to a flexible material By suitable choice of plasticizer level and type a whole range of products from flexible elastomers to rigid compounds can be produced

Materials can be made that are virtually unbreakable, that are weatherable with good property retention for over 30 years, that have stiff melts and little die swell for outstanding dimensional control in profile extrusion, or low viscosity melts for thin walled injection molding

Many materials can be used as plasticizers for PVC; the important ones include diisooctyl phthalate, tritotyl phosphate, dibutyl sebacate and epoxidized soya bean oil Generally, 40-60 parts of plasticizer per 100 parts of PVC polymer are used for most common applications PVC containing this level of plasticizer is a flexible rubber-like material From Figure 1.2 we find 45% of PVC is used in a plasticized form and 55% in the rigid form The presence of chlorine in the molecule makes PVC particularly versatile since it makes it compatible with a wide range of other materials The chlorine content also helps to make PVC flame retardant PVC polymer is light, neutral, durable, robust, non-toxic and non-flammable

Some chain branching is present in PVC; perhaps 6-10 branches per molecule The branching makes the PVC imperfect This is one example of a “structural defect”, which will be discussed in detail in the following section.

Structural Defects in PVC

In spite of its enormous technical and economic importance, PVC still poses many problems: its rather low stability towards heat and light results in discoloration, HCl loss and serious corrosion phenomena, which require stabilization of the polymer for

- 13 - practically all technical applications Many people agree that normal PVC with a head-to- tail structure should be quite stable to heat It is generally assumed, therefore, that structural abnormalities in the polymer chains are responsible for the relative instability of PVC Possible defect structures in PVC are branching, chloroallyl end groups and head-to-head structures In addition to these abnormalities the steric order of the monomer units, i.e the tacticity, may have some influence on the degradation process §1.5.1 Short-chain Branching

There are mainly four short-chain PVC branches; these have been studied in detail in the literature 22-24 They are chloromethyl branches (MB), 2-chloroethyl branches (EB- I), 1,2-dichloroethyl branches (EB-II), and 2,4-dichlorobutyl branches (BB) These have the following formulas

-CHCl-CH 2 -CH-CHCl-CH 2 - -CH 2 -CCl-CH 2 -CHCl-

CH 2 Cl CH 2 -CHCl-CH 2 CH 2 Cl

-CH 2 -CCl-CH 2 -CHCl- -CH 2 -CH-CH 2 -CHCl-

The chloromethyl branches (MB) and 1,2-dichloroethyl branches are mostly generated by an intramolecular process first proposed by Rigo et al 22 and proved by

Starnes et al 23, 24 This mechanism is outlined in Scheme 1.1:

~CH 2 -CHCl-CH-CH 2 Cl

~CH 2 -CH-CHCl-CH 2 Cl

~CH 2 -CH=CH-CH 2 Cl

CHCl-CH 2 Cl CH 2 Cl

VC VC VC VC VC k 1 k 2 k -2 k 6 k 4 k 5 k 3 k p k p k p k p

~CH 2 -CH-CH-CH 2 -CH2-CH

Scheme 1.1 Mechanistic sequel of head-to-head addition during the free radical polymerization of vinyl chloride (VC), where P • is the propagating head-to-tail macroradical, and k’s are rate constants 24

An occasional head-to-head monomer addition can be followed by either an ordinary addition or a radical rearrangement The latter implies 1,2-Cl migration forming a secondary radical, which presumably is more stable than the primary radical Propagation will then result in a pendent chloromethyl group, shown as follows:

Cl Cl -CHCl-CH 2 -CH-CH-CH 2

-CHCl-CH 2 -CH + CH=CH 2 -CHCl-CH 2 -CH-CH-CH 2

-CH 2 -CH-CH-CH 2 CH 2 -CHCl

-CHCl-CH 2 -CH-CH -CH 2 -CH-CH-CH 2 CHCl

The forming of 1,2-dichloroethyl branches was proposed by Starnes et al This involves four steps, shown below:

-CHCl-CH 2 -CH + CH=CH 2 -CHCl-CH 2 -CH-CH-CH 2

-CHCl-CH 2 -CH-CH-CH 2

-CHCl-CH 2 -CH-CH migration

-CHCl-CH 2 -CH CH 2 =CHCl

-CHCl-CH 2 -CH-CH 2 -CHCl

The first two steps are head-to-head addition of monomer to the propagating macroradical and subsequent rearrangement of the ensuing radical, via a 1,2-chlorine shift The next step is more conjectural It involves a 2,3-Cl shift, followed by normal

- 16 - chain propagation, which generates the 1,2-dichloroethyl branch structure Its formation can be regarded as having proceeded via a modified “billiard-ball” route in which the first chlorine that migrates is quickly replaced by another one It has been pointed out that a tertiary carbon at the 1,2-dichloroethyl branch point is bound to a hydrogen, while the tertiary carbon bonds with a chlorine in the 2-chloroethyl branch

The 2-chloroethyl branches are suggested to be formed by “backbiting” of normal propagating macroradical end groups, through a 1,3-hydrogen shift, indicated as following:

-CHCl-CH 2 -CHCl-CH 2 -CHCl 1,3-Hydrogen migration

-CHCl-CH 2 -CCl CH 2 =CHCl

CH 2 -CH 2 Cl CH 2 -CH 2 Cl

Finally, 2,4-dichlorobutyl branches are formed by “backbiting” of the normal propagating macroradical end groups through a 1,5-hydrogen shift, involving a 6- member ring transition state It is shown as follows:

The tertiary carbon bonds with a chlorine in the 2, 4-dichlorobutyl branch §1.5.2 Long-chain Branching

The long-chain branches have the following structures:

The tertiary carbon at the LCB-II type point is bound to a hydrogen atom, while in the LCB-I type branching the tertiary carbon bonds with a chlorine atom This labile chlorine is removed easily at elevated temperatures by dehydrochlorination The propagating macroradical chain transferring to polymer initiates chain growth from the polymer to form long chain branches For instance, the macroradical could attack the

- 18 - methyne group in the polymer to form a 1,3-dichloroalkane chain end and a tertiary carbon radical bound with chlorine This tertiary carbon radical initiates a type-I long- chain branch If the macroradical attacks a methylene group of the polymer, then a type II long-chain branch can be formed, shown as follows:

~CH-CH 2 -CH + ~CH 2 -CH-CH 2 ~ ~CH-CH 2 -CH 2 + ~CH 2 -C-CH 2 ~

Cl Cl Cl Cl Cl Cl

CH 2 -CHCl-CH 2 -CHCl-CH 2 ~

~CH-CH 2 -CH + ~CH-CH 2 -CH~ ~CH-CH 2 -CH 2 + ~CH-CH-CH~

Cl Cl Cl Cl Cl Cl

~CH-CH-CH~ ~CH 2 -C-CH 2 ~

CH 2 -CHCl-CH 2 -CHCl-CH 2 ~

If the methylene radical loses one chlorine to monomer, an internal double bond can be formed:

The chlorine on a carbon next to the double bond is an allylic chlorine, which functions as a labile chlorine in PVC dehydrochlorination at elevated temperaturess §1.5.3 Unsaturation

Unsaturated groups in PVC include terminal and internal double bonds Internal double bond formation was described in the previous section Terminal double bonds can be formed through the following mechanism:

~CH 2 -CHCl -CH- CH 2 Cl

~CH 2 -CH-CHCl- CH 2 Cl

~CH 2 -CH=CH-CH 2 Cl ~CH 2 -CHCl-CH=CH 2

We can see that there are two types of terminal double bonds One is vinyl end group and the other can be called pseudo-terminal double bond

The PVC chain consists mainly of a head-to-tail arrangement of monomer units However, a head-to-head PVC structure could be formed through the following mechanism: k p '' k p ' k p

~CH 2 -CH-CH 2 -CH-CH 2 -CH

~CH 2 -CH-CH-CH 2 -CH 2 -CH

Cl Cl head-to-tail head-to-head

The head-to-head structure could lower the thermal stability of PVC.

Stereoregularity of PVC

When vinyl chloride monomer adds to a growing PVC chain, the terminal group takes one of two possible orientations relative to the penultimate group If the backbone is in an all trans conformation, isotactic structures have adjacent chlorine atoms oriented at the same side of the carbon-carbon plane Syndiotactic structures have the chlorine atoms alternating in their placement relative to the carbon-carbon-carbon plane Atactic PVC is a mixture of the two stereoisomers A schematic representation of isotactic, syndiotactic and atactic PVC structure is shown as in Figure 1.5 It was claimed that

- 21 - syndiotactic PVC can crystallize easily, and it is very important in understanding the properties of PVC

Cl Cl Cl Cl Cl

Figure 1.3 Schematic representations of (a) isotactic, (b) syndiotactic, and

PVC is one of the oldest plastics; 28 million tons per year are manufactured worldwide today PVC owes its popularity to its versatility and low cost The polymer itself is chemically inert and nonflammable, burning only in the presence of a source of ignition It is compatible with many additives, including plasticizers, heat stabilizers, lubricants, fillers, and a wide range of other polymers All of these features make PVC one of the world’s major bulk polymeric materials that have a huge impact on our everyday lives

However, PVC still has a number of drawbacks One serious problem for PVC is its rather low thermal stability The dehydrochlorination of PVC starts at about 100 o C, and is the reason for its discoloration during extrusion, due to the formation of polyene sequences As PVC is one of the relatively ‘old’ plastics, research in this field is not as attractive as that on new, pioneering polymers Nonetheless, owing to its enormous commercial importance, the research and study of this polymer continues to attract the attention of thousands of polymer scientists and engineers Many important basic problems that relate to PVC and vinyl chloride polymerization still remain to be fully understood or solved Research on new methods of VC polymerization, the mechanism and modeling of VC free-radical polymerization, the relationship of the microstructure and stability of PVC, and the PVC degradation and stabilization will continue to be of interest

A number of years ago, our group demonstrated that solvent interaction could be used to increase the stereoregularity in radical polymerization of poly(methacrylic acid) from 58% to 95% syndiotactic triads 25, 26 Hydrogen bonding between the pen- penultimate unit and the carboxyl group in the radical was found to be the important interaction Lack of hydrogen bonding interaction resulted in relatively low syndiotactic content as did too strong a hydrogen bonding interaction with solvent or monomer A solvent such as 2-propanol with its moderate hydrogen bonding capability gave the highest syndiotacticity It was also found that too bulky a hydrogen bonding solvent gave lower syndiotacticity

This concept may be extended to the radical polymerization of vinyl chloride Since poly(vinyl chloride) forms compatible mixtures with poly(methyl methacrylate) and poly(carbonate) 27, 28 , the hydrogen on the chloromethylene(~CHCl~) group of poly(vinyl chloride) is acidic enough to hydrogen bond with weak proton accepting groups A terminal radical hydrogen should be more acidic than the internal hydrogens These weak proton accepting compounds can be weakly basic compounds, high dipole carbonyl compounds, molecules having two connected noncarbon atoms, and some heteroaromatics

Our hypothesis was that small quantities of those compounds, so-called

‘additives’, in the vinyl chloride polymerization system could preferentially form hydrogen bonds with the propagating chloromethylene hydrogen This interaction might create steric hindrance that could direct the propagating radical to add vinyl chloride

- 24 - monomer in a way that forms syndiotactic rich polymer PVC Additionally, such interactions could reduce ‘backbiting’ and chain transfer to monomer or polymer A more linear PVC structure could be formed that has higher molecular weight, higher thermal stability and/or higher crystallinity, depending on the additive

The goal of this dissertation was to investigate the polymerization of vinyl chloride in the presence of small amounts of organic additives The organic additives selected were weakly basic compounds, high dipole carbonyl compounds, high dipole ether compounds, and some heteroaromatics Various methods were applied to evaluate the influence of the additives on the polymerization and the resulting PVC polymer structures The following is a summary of this dissertation:

Chapter 1 was a general introduction to vinyl chloride polymerization and the history of poly(vinyl chloride) The microstructure of the PVC and its bulk polymerization, suspension polymerization, emulsion polymerization, and solution polymerization were briefly discussed

Chapter 2 described in detail the free radical polymerization of vinyl chloride in the presence of various organic additives Gel Permeation Chromatography (GPC) was used to determine the molecular weights and the molecular weight distributions of the resulting poly(vinyl chloride) A kinetic model was developed to describe the polymerization rate under the influence of additives and the molecular weights of the resulting polymers

In Chapter 3, Differential Scanning Calorimetry (DSC) and Thermo-gravimetric Analysis (TGA) were used to characterize the crystallization and stability of the resulting polymers The possible additive effect on the crystallinity of the resulting PVCs was evaluated

In Chapter 4, the dehydrochlorination of PVCs prepared in the presence of various additives was measured and the possible additive effect on the dehydrochlorination rate of the resulting polymers was discussed A combination of 1D and 2D Nuclear Magnetic Resonance (NMR) spectroscopy was used to identify and quantify the structural defects of the resulting PVCs The additive effect on the formation of some structural defects was investigated A correlation was found between the dehydrochlorination rate and the labile structure concentration in the resulting polymers

And finally, in Chapter 5, general conclusions were drawn based on the results of the GPC, DSC, TGA, NMR and dehydrochlorination of the resulting polymers

Free Radical Polymerization of Vinyl Chloride in the presence

Introduction

PVC with good stability has long been the objective of intense studies since the low stability of commercial PVC makes it difficult to process The defect structures in the PVC backbone are considered to be the main factors in lowering its thermal stability PVC degrades by releasing hydrogen chloride, with yellowing and subsequent blackening of the product The present methods for enhancing the thermal stability of PVC involve modifying the polymer by copolymerization with another monomer, by adding some organic or inorganic compounds or blending with other polymers after polymerization Those methods have nothing to do with the defect structures of PVC itself The big challenge in the PVC community is to prepare PVC with fewer defects, thus having an enhanced thermal stability

In the early 1960s, Burleigh first prepared so-called ‘crystalline’ poly(vinyl chloride) with a free-radical initiator using aliphatic aldehydes, especially n- butyraldehyde as a polymerization medium 1-2 The crystallinity of the PVC obtained reached 42%, significantly exceeding the value for the commercial PVC (4-10%) Highly crystalline PVC was also obtained by the polymerization of vinyl chloride in the presence of acetaldehyde 3 , dialkyl phosphites 4 , triethylamine 5 , carbon tetrachloride 6 , and other compounds 7 However the yields of the crystalline PVC were usually low and the degree of polymerization was only from 20 to 80 It was proposed that the high crystallinity was due to the ability of the additives to form donor-accepter or π-bond complexes with the propagating radicals, which promoted stereospecific addition of the vinyl chloride to the growing chains 2, 4, 8 This mechanism was questioned and other authors pointed out that

- 30 - the highly crystalline PVC had a low degree of polymerization and was obtained when vinyl chloride was polymerized with solutes or additives 9 ; low-molecular-weight PVC characteristically has an increased crystallinity and tacticity

In 1981, Turska et al 10 reported on vinyl chloride polymerization in the presence of small quantities of 2,5-dimercapto-1,3,4-thiodiazol (bismuthiol), epoxidized soybean oil, and Ergowax GS-1 (a mixture of mono and diesters of glycerol and stearic and palmitic acids) The thermal stability of the PVC was 20-30% higher than normal PVC with no evident molecular weight change The syndiotacticity of the resulting PVC was also higher than similar commercial PVC, but dropped to its usual value, i.e 54.5% of syndiotactic dyads, as conversion increased No further studies of mechanism were performed since then Recently, studies on the polymerization of vinyl chloride by the pseudo-living or ‘living’ radical polymerization method have been published 11-14 Grishin et al 11, 12 studied the radical polymerization of vinyl chloride in the presence of catalytic amounts of stable nitroxyl radicals or their sources, such as C-phenyl-N-tert-butylnitrone (PBN), 2-methyl-2-nitrosopropane(MNP), or 1-tert-butyl-3-phenyl-1-oxytriazene(BPT) Studies of the polymerization kinetics of vinyl chloride and molecular weight characteristics of the resulting PVC show that the reaction occurs by a pseudoliving chain mechanism, thus opening the opportunity to prepare PVC with controlled chain structure and molecular weight Percec and coworkers 13, 14 prepared poly(vinyl chloride) by single electron transfer-degenerative chain transfer ‘living’ radical polymerization of vinyl chloride initiated with iodoform and catalyzed by nascent Cu 0 /tri(2-aminoethyl)amine or

Na2S2O4 in water at 21 o C The resulting PVC had a controlled molecular weight up to

120,000 (relative to polystyrene standards), a molecular weight distribution around 1.7- 2.3, free of structural defects, and higher syndiotacticity (62%) than commercial PVC

Poly(vinyl chloride) can form compatible blends with poly(methyl methacrylate) (PMMA) and polycarbonate (PC) It was proposed that the miscibility was due to a special hydrogen bonding interaction between the carbonyl groups (>C=O) of PMMA or

PC and hydrogen from chloromethylene (~CHCl~) groups of PVC 15, 16 The hydrogen on the chloromethylene groups of PVC may be acidic enough to hydrogen bond with compounds with weak proton accepting groups Such weak proton accepting compounds can be weak base compounds, high dipole carbonyl compounds, molecules having two connected noncarbon atoms and some heteroaromatics In this dissertation, such compounds were chosen as additives in the vinyl chloride polymerization system to investigate their influence on the resulting PVC The interaction may reduce ‘backbiting’ and chain transfer to monomer or polymer, to form PVC with higher molecular weight, higher thermal stability and higher crystallinity.

Experimental

Vinyl chloride (VC) monomer, polymerization grade, provided by BFGoodrich, was used without further purification 2,2’-Azobisisobutyronitrile (AIBN), purchased from Aldrich, was recrystallized three times from methanol and dried in vacuum at 45 o C Tetrahydrofuran (HPLC grade, Fischer) was refluxed with 2,6-di-tert-butyl-4- methylphenol (BHT) and distilled under nitrogen Trimethylphosphine oxide (TMPO,

- 32 - mp: 140-141 o C) and triethylphosphine oxide (TEPO, mp: 48-50 o C) were purchased from Alfa-Aesar and used as received Triphenylphosphine oxide (98%, Aldrich) (TPPO) and tributylphosphine oxide (95%, Aldrich) (TBPO) were recrystallized from ethanol 2- Methyl benzothiazole (99%, Aldrich) (MBTZ) was distilled under vacuum (65 oC/1mmHg); 2-methylpyrazine (99+%, Aldrich), (MPZ) was distilled under reduced pressure (95 o C/18mmHg) Ethylene carbonate (98%, Aldrich) (EC) was distilled under vacuum (95 o C/1mmHg, mp: 37-39 o C) 2,6-Dichloropyridine (98%, Aldrich), (DCPY) was recrystallized twice from ethanol and dried under vacuum (mp: 86-88 o C)

Pyridine (99%, Fisher) (PY) and 2,6-dimethylpyridine (2,6-lutidine) (99%, Aldrich) (DMPY) were refluxed with sodium hydroxide (NaOH) pellets and distilled at one atmosphere pressure 2,4,6-Trimethylpyridine (2,4,6-collidine)(99%, Aldrich) (TMPY) was refluxed with NaOH pellets and distilled under reduced pressure (61 o C/13mmHg) 2,2’-Bipyridyl (BPY) was recrystallized from hexane/ethanol and dried under vacuum (mp: 70-71 o C) 1,3-Dimethyl-2-imidazolidinone (98%, Aldrich) (DMI) was distilled under vacuum (100 o C/1mmHg) ε-Caprolactone (98%, Aldrich), (CPLA) was distilled under vacuum (100 o C/13mmHg), and γ-butyrolactone (99+%, Aldrich) (GBL) was purified by passing through a neutral alumina column 2-Coumaranone (97%, Aldrich) (CMN) was recrystallized from hexane/methanol (mp: 47-50 o C); dimethyl terephthalate (99+%, Aldrich) (DMT) was recrystallized from methanol (mp:140-142 o C); trimethyl-1,3,5-benzene-tricarboxylate (98%, Aldrich) (TMB) was recrystallized from methanol (mp1: 132-134 o C; mp2: 142-145 o C); 1,4-dimethoxy-benzene (hydroquinone dimethyl ether) (99%, Aldrich) (DMB) was recrystallized from hexane/methanol (mp:

56-59 o C) Methanol was purchased from Fisher and used as solvent or precipitant without further purification §2.2.2 Polymerization Apparatus

The polymerization was carried out in an ACE-8648 Pressure Tube with Ace- Thred and Plunger Valve The tube is a heavy-wall tube (max pressure: 10 atmospheres), with bushing and plunger valve that allows purging of the tube The closed bottom plunger has a hole inside that when positioned in relation to O-Ring seal, will open the tube to the atmosphere; i.e., pull to close, push to open as shown in Scheme 2.1:

Scheme 2.1 Schematic illustration of Ace-8648 Pressure Tube

The polymerization tube was connected with a high vacuum line, which had multiple polymerization tubes attached, shown in Scheme 2.2:

Scheme 2.2 Schematic of the polymerization set-up with a vacuum line and pump system; a: Nitrogen flow; b1 to b7: two-way stopcock; c: three- way stopcock; A: vacuum pump; B: trap; C: vinyl chloride tank; D: recycle pressure tube; E1 & E2: polymerization pressure tubes (Ace-

8648); F: digital vacuum gauge §2.2.3 Bulk Polymerization of Vinyl Chloride

For all bulk polymerization runs, the following procedure was used

1 Put required amount of initiator AIBN and additive into the polymerization pressure tubes E1 and E2 and connect them to the vacuum line as shown in Scheme 2.2 Cool E1 and E2 to -205 o C in a liquid nitrogen bath and cool the trap B in liquid nitrogen

2 Open b3, b4, b5, b6, close b1, b2, b7, switch 3-way-c to trap B and evacuate to high vacuum (~10μ) for about 15 minutes

3 Then close b5 and open b1 to charge vinyl chloride into the pressure tubes E1 and E2 Close b1 when vinyl chloride reaches the marked level

4 When vinyl chloride is completely frozen by liquid nitrogen, open b5 to evacuate the charged polymerization tubes E1 and E2, and keep the whole system at high vacuum for 10 minutes Then switch 3-way-c to nitrogen flow to let nitrogen purge the polymerization tubes E1 and E2 When the pressure reaches one atmosphere, switch 3- way-c back to the vacuum pump system

5 Repeat the purging and pumping three times, then close b4 and b3 and remove liquid nitrogen bath to warm the vinyl chloride After the vinyl chloride is melted, freeze it again in the liquid nitrogen bath and repeat procedure 4 After the freezing-pumping- melting cycle has been performed three times, close b3 and b4 and seal the polymerization tubes E1 and E2 by pulling up the plunger, and then disconnect them from the vacuum line

6 Open b7, and stop the vacuum pump Keep the pressure tubes E1 and E2 sealed Warm and weigh the tubes to calculate the exact amount of vinyl chloride added Put the sealed pressure tubes into a 55 ±0.5 o C oil bath for 1-2 hours Mark the starting time, when poly(vinyl chloride) (white powder) is seen to precipitate from the vinyl

- 36 - chloride monomer (colorless liquid) Stop the polymerization at low conversion by cooling in a liquid nitrogen bath

7 Re-connect the pressure tubes into the vacuum line and recycle the un-reacted vinyl chloride monomer into pressure tube D Start the vacuum pump to remove residual vinyl chloride from the polymer particles Weigh the pressure tubes again to measure the amount of poly(vinyl chloride) produced and the conversion

8 Dissolve the poly(vinyl chloride) in fresh tetrahydrofuran (THF) and precipitate the product into a large amount of methanol (10 times larger than THF) Repeat the dissolving and precipitating procedure twice to completely remove the additive and unreacted AIBN, and dry the final PVC in a vacuum oven at 60 o C for 2 days §2.2.4 Suspension Polymerization of Vinyl Chloride

Suspension polymerization of vinyl chloride was performed in a 3L pressure reactor at OxyVinyls’ pilot plant The polymerization temperature was 53±0.5 o C and the initiator used was diisobutyl peroxydicarbonate (SBP), synthesized by reaction of sec- butyl chloroformate with H2O2 plus NaOH The polymerization was stopped at a specified pressure drop or time limit by adding diethylhydroxylamine (DEHA) A recipe for the suspension polymerization of vinyl chloride in the presence of dimethyl terephthalate (DMT) is given in Table 2.1

Table 2.1 A selected recipe for suspension polymerization of vinyl chloride at 53 o C

Sodium hydroxide 0.009 0.94g with shortstop a): Polyvinyl alcohol (degree of hydrolysis: 71.5-73.5 mole%), commercially available from Synthomer b): A cellulose ether, commercially available from Dow c): A partial hydrolyzed polyvinyl alcohol, commercially available from Air Products &

Chemicals, Inc d): Acronym SBP, commercially available from Akzo Nobel Polymer Chemicals §2.2.5 GPC Characterization of PVC Samples

GPC measurements were performed using a Waters GPC system equipped with a

Model 510 Pump, Model 410 Refractive Index Detector, Model 996 Photodiode Array

Detector and a Column Heater Module with a Styragel HR 5E column and a Styragel HR

4E column (5μm, 100Å, 7.8x300mm) in series The GPC system was controlled by a stand-alone workstation through a Waters Bus LAC/E Card HPLC grade tetrahydrofuran

- 38 - was used as the mobile phase and the flow rate was set to 1.0 mL/min The Styragel columns were kept at 35 o C in the column heater module Poly(vinyl chloride) narrow molecular weight distribution standards (purchased from Polymer Standards Service- USA) with molecular weights of 327,000, 213,000, 146,000, 119,000, 94,000, 64,000, 48,000, 36,000, and 14,200 were used to make a GPC calibration curve All the dried PVC samples, including the standards, were dissolved in cyclohexanone (CHX) solvent overnight and heated at 90-120 o C for 60 minutes prior to injection The hot solution was injected to avoid aggregation The concentration of injected PVC solution was 1~2 mg/mL and a volume of 100~200 μL solution was injected into the GPC system The data acquisition and processing was handled by Millennium 32 Software provided by Waters The number-average molecular weight (Mn), weight-average molecular weight (Mw), z-average molecular weight (Mz) and the polydispersity index (PDI) of the samples were calculated from the chromatograms §2.2.6 Dynamic Thermal Stability Test of PVC Samples

Some suspension polymerization PVC samples were chosen to be studied using the Dynamic Thermal Stability (DTS) test in a Brabender Plasticorder equipped with a mixing chamber, operating at 190 o C with a rotor speed of 60 rpm About 63 g of virgin PVC resin were mixed with specified plasticizers and lubricants and the melted mixture was extruded strip by strip at 2-minute intervals; the strips were cooled and pasted onto a record pad The test was stopped when the extruded PVC strip had turned black The DTS time was defined as the time required, taken at 2-min intervals, for the melted mass in Brabender chamber to turn a relatively dark color

- 39 - §2.2.7 Static Thermal Stability Test of PVC Samples

12 PVC samples were dissolved in dichlorobenzene in 12 5-mm NMR tubes The concentration was 5wt% for each sample and the tubes were held at 90±0.5 o C for 14 hours; then all tubes were removed and the color changes for the polymer solution were recorded and compared by visual inspection.

Results and Discussion

§2.3.1 Molecular Weight and Molecular Weight Distribution of PVCs §2.3.1.1 Polymer aggregation in solution

Gel Permeation Chromatography (GPC), also called Size Exclusion Chromatography (SEC), is a very rapid analytical technique for measuring the molecular weight (MW) and the molecular weight distribution (MWD) of polymers GPC separation is based on the molecular size of the dissolved polymer; thus the polymer must be dispersed at a molecular level in the solution However, the molecular weight and size information derived from PVC solutions is often distorted because the polymer trends to form aggregates even in dilute solution

Macromolecular aggregation of PVC was first described by Doty and coworkers 17 back in 1947 The association of PVC molecules into densely packed aggregates in dilute dioxane was demonstrated by osmotic-pressure, light-scattering, and ultracentrifuge measurements 17 Since then, various procedures have been used to disintegrate these aggregates into single PVC molecules in solution to prevent anomalous

- 40 - molecular weight measurements 18-24 Lyngaae-Jφrgensen 18, 19 found that heating the PVC solution in THF at 120 o C for 3 hours could dissociate the aggregates Rudin and Benschop-Hendrychova 20 reported that the ultrasonic treatment of the PVC solution in THF at room temperature for 15 minutes destroyed the aggregates; the simultaneous degradation of PVC was prevented by adding a small amount of a nonionic surfactant to the THF solution Abdel-Alim and Hamielec 21 reported that heating the solution at 90 o C for 10 min was adequate for PVC prepared at polymerization temperatures between 30 and 70 o C However, for PVC prepared at -50 o C, a temperature of 200 o C was needed to dissociate the polymer into single PVC molecules 22 Pang and Rudin 23 reported the preparation of aggregate-free PVC solution in 1,2,4-trichlorobenzene (TCB) by heating at

120 o C for 12 hrs, followed by GPC measurements at 110 o C, with TCB as the mobile phase Recently, Manabe and coworkers 24 reported a new method PVC was dissolved in TCB, held at 130-140 o C for 6 hours, precipitated in methanol, and dried Aggregate-free PVC solution could be prepared by dissolving the pre-treated PVC in THF at room temperature They also pointed out that the existence of aggregates sometimes could not be observed by a refractive index detector, but a light-scattering detector showed them clearly

Generally, the molecular aggregation in PVC solution is affected by many factors, such as the temperature, the molecular weight and the tacticity of PVC (polymerized at different temperatures), the concentration of the polymer, the solvent used, the method of dissolution and so on The most important factor is the molecular weight of the PVC In this dissertation, the number average molecular weights of the PVC samples were mostly

- 41 - in the range of 40,000-60,000 (P n d0~960), and cyclohexanone (CHX) was used to dissolve PVC samples A 0.1-0.2% CHX solution of PVC was prepared and heated at

110 o C for various periods of time prior to injection A volume of 100~200 μL solution was injected into the GPC system Since PVC does not have UV sensitive groups, only the RI defector gave positive signal peaks The RI chromatogram for PVC/control is shown in Figure 2.1 (Chromatograms for other samples are shown in Figures 2.3, 2.4 and 2.5 They are discussed later.)

Figure 2.1 GPC traces of PVC/control prepared by bulk polymerization at 55 o C, initiated by AIBN (⎯) before heating treatment; (⎯) after heating at 110 o C for 60 minutes

Figure 2.1 shows two peaks for the untreated PVC/control, a small, higher molecular weight one at ~13.5 min, and a large, lower molecular weight one at ~15.5 min After the solution was heated at 110 o C for 1 hour, the peak at 13.5 min disappeared; the lower molecular weight peak was unchanged and showed unimodal distribution The high molecular weight shoulder peak can be ascribed to molecular aggregation of PVC in

- 42 - cyclohexanone (CHX) solution By comparison with the calibration curve, it was found that the molecular weight of the ‘aggregate peak’ in Figure 2.1 was about 6-8 times that of the corresponding disaggregated PVC Since aggregated polymers occupy a smaller volume than the unaggregated polymers do, a molecular weight increase of 6-8 times probably means 15-20 molecules in the aggregate After disaggregation treatment, the molecular weights of the PVC polymerized at 55 o C in the absence of additives were calculated as MnC,700, Mw,100 (Mw/Mn=1.83) For most of the PVC samples studied, the aggregates could be effectively disaggregated by heating at 110 o C for 60 minutes; this pretreatment method was applied to all of the PVC samples polymerized in bulk or in suspension, in the presence or absence of organic additives, to determine the weight average and number average molecular weights §2.3.1.2 GPC data for bulk polymerized PVCs

Representative GPC data for PVCs prepared by bulk polymerization initiated by AIBN in the presence of various additives are listed in Table 2.2 The number average molecular weight of PVC prepared in the absence of additive is about 43,600 and the molecular weight distribution is about 1.83 For PVC prepared in the presence of various additives, the molecular weight varies PVCs prepared in the presence of 1 mole% of 1,3- dimethyl-2-imidazolidinone (DMI), 2-methyl benzothiazole (MBTZ), 2-methylpyrazine (MPZ), pyridine (PYR), and 2,4,6-trimethylpyridine (TMPY) have lower molecular weights (20~40% loss compared to normal polymer); these additives are relatively strongly basic nitrogen compounds Figure 2.2 shows that the dichlorobenzene solutions of PVCs prepared in the presence of DMI, MBTZ, MPZ, PYR, and TMPY darkened after

- 43 - heating in dichlorobenzene at 90 o C for 14 hours However, the other samples remained colorless after the same treatment It is very interesting that, for PVC prepared in the presence of a less basic compound, 2,6-dichloropyridine (DCPY), the molecular weight did not decrease 14 hours at 90 o C static thermal stability test also shows that the resulting polymer is reasonably stable Further studies on the effect of DCPY will be discussed in section 2.3.7 The molecular weights slightly decreased or stayed the same for PVCs prepared in the presence of 2-coumaranone (CMN), 1,4-dimethoxybenzene (DMB), and phosphine oxide compounds However, the number average molecular weights increased about 8~15% for PVCs prepared in the presence of 1.0% of dimethyl terephthalate (DMT), ethylene carbonate (EC) or trimethyl-1,3,5-benzene-tricarboxylate (TMB) A detailed discussion of the influence of the individual additive on the molecular weights of resulting PVCs will be given in sections 2.3.7, 2.3.8, and 2.3.9

Table 2.2 Molecular weights and molecular weight distributions for PVCs prepared by bulk polymerization at 55 o C in the presence of organic additives at given AIBN concentration

Sample designation a) Mw Mn Mz Mw/Mn Mz/Mw

TPPO01b 85,500 38,400 145,100 2.23 1.70 a): Samples prepared in the presence of additives are named by additive abbreviation plus a 2 or 3 digit number followed by a lower-case letter (the number represents the mole ratio of additive to monomer(decimal point omitted), and the lower-case letter identifies different runs); GBL: γ-butyrolactone; CMN: 2-coumaranone; DCPY: 2,6- bichloropyridine; DMB: 1,4-dimethoxybenzene; DMI: 1,3-dimethyl-2-imidazolidinone;

DMPY: 2,6-dimethyl-pyridine; DMT: dimethyl terephthalate; EC: ethylene carbonate;

MBTZ: Methyl benzothiazole; MPZ: 2-methyl-pyrazine; PYR: Pyridine; TEPO: triethylphosphine oxide; TMB: trimethyl-1,3,5-benzenetricarboxylate; TMPO: trimethylphosphine oxide; TMPY: 2,4,6-trimethyl-pyridine; TPPO: triphenylphosphine oxide b): Control polymerization was initiated by AIBN in the absence of additives; samples were named as AIBN plus a random 3-digit number to distinguish different runs

PYR01a TMPY05c TMPY01a DMI05d DMI01a MBTZ01b TPPO01b DCPY01a TBPO03d EC01c EC04d AIBN319 Figure 2.2 Thermal stability test for some PVC samples Test conditions: 5% solution in dichlorobenzene, 90 o C, 14 hours

- 47 - §2.3.1.3 PVC/DMT01b and PVC86k aggregation in solution

Figure 2.3 GPC traces of PVC/DMT01b prepared by bulk polymerization at 55 o C, initiated by AIBN in the presence of 1.0 mole% of DMT:

(⎯) before heating treatment; (⎯) after heating at 110 o C for 60 minutes

The GPC chromatograph of PVC/DMT01b, obtained by polymerization at 55 o C in the presence of 1.0 mole% of dimethyl terephthalate (DMT), is shown in Figure 2.3 It can be seen that PVC/DMT01b also has a bimodal distribution for untreated sample as did the PVC/control There is a small peak at 13.2 min, and a large peak at 15.3 min After the solution was heated for one hour at 110 o C, the peak at 13.2 min disappeared and the peak at 15.3 min remained but with a very small shoulder on the high molecular weight side After heating for 3 more hours at 110 o C, the shape for the 15.3 min peak did not change, which means that high molecular weight shoulder is not aggregated polymer peak The reason for having that shoulder is not clear The molecular weights were calculated as MnP,200, Mw,500, with a molecular weight distribution of 1.78 The

- 48 - weight average molecular weight of PVC prepared in the presence of 1.0 mole% of DMT is about 12% higher than that of PVC/control, prepared without additive (MnC,700, Mw,100, Mw/Mn=1.83) The GPC curves for disaggregated PVC/DMT01b and PVC/control are shown in Figure 2.4 It can be seen that the main eluent peak for PVC/DMT01b came out 0.2 min earlier than that for the PVC/control

Figure 2.4 GPC traces of PVC/DMT01b and PVC/control

(both dissolved samples were heated in cyclohexanone at 110 o C for 60 minutes)

In our polymerization runs, we found that after degassing, vinyl chloride monomer in Ace-8648 pressure tube spontaneously polymerized at room temperature under visible light The ‘spontaneous’ polymerization was extremely slow After 7 days a very thin white film formed inside the tube wall and the colorless monomer liquid became opaque After venting the unreacted monomer, the white polymer film was collected and the conversion was less than 0.1% This type of polymer was purified by

- 49 - the same procedure as other PVCs and named PVC86k (86k is its number-average molecular weight)

Figure 2.5 GPC traces of PVC86k prepared by bulk polymerization at room temperature, initiated by visible light:

(⎯) before heating treatment; (⎯) after heating at 110 o C for 60 minutes

Conclusions

The polymerization rate for bulk polymerization of vinyl chloride was found to be proportional to the 0.62 power to the initiator concentration A proposed mechanism suggested that, under the heterogeneous condition, mutual propagating radical/propagating radical termination was relatively rare and the dominant termination reaction was the reaction of propagating radicals with small radicals, formed by chain transferring to monomer or additive A kinetic model was developed based on the proposed mechanism The rate constant for the cross-termination reaction was determined to be about 10 9 -10 10 L•mole -1 •s -1

Various organic materials were added to vinyl chloride for bulk as well as suspension polymerization The initial polymerization rates and the molecular weights of the resulting polymers increased in the presence of weakly ‘basic’ compounds such as dimethyl terephthalate (DMT), ethylene carbonate (EC), γ-butyrolactone (GBL), tributylphosphine oxide (TBPO) and trimethyl-1,3,5-benzene tricarboxylate (TMB) A modified kinetic model was developed for the bulk polymerization of vinyl chloride in the presence of these weakly basic additives, assuming a hydrogen-bond complex formed between an additive and the terminal hydrogen of the propagating radical The H-bonded propagating radicals could grow, on the average, longer chains, and the molecular weights of the resulting polymers increased The kinetic model was tested on four sets of experimental data and the calculated results were in good agreement with the experimental findings

Differential Scanning Calorimetry (DSC), Nuclear Magnetic Resonance (NMR) analysis and dehydrochlorination of the PVC samples, prepared in presence or absence of organic additives, will be discussed in Chapter 3 and Chapter 4

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Chapter 3 Differential Scanning Calorimetry and Thermogravimetric Analysis of Poly(vinyl chloride)s

Introduction

It is well-known that commercial poly(vinyl chloride) (PVC) is a low crystallinity polymer PVC prepared at 50-60 o C contains approximately 10% crystallinity But this relatively low level of crystallinity has a surprisingly significant effect on its processing and properties Many techniques have been used to investigate the crystalline nature of the polymer These include electron and x-ray diffraction 1-14 , vibrational spectroscopy such as infrared and Raman6, 9-10, 15-18, thermal analysis such as differential scanning calorimetry (DSC) 19-25 , density determination 10, 26-28 and nuclear magnetic resonance spectroscopy 29 Among these, wide angle X-ray scattering and infrared spectroscopy were the most extensively used techniques DSC was the most widely used thermal analysis method It should be pointed out that each method examines somewhat different aspects of the crystalline nature of the polymer Because the methods look at different aspects of the crystallinity, each method tends to call different levels of order crystalline, and draws the demarcation line between crystalline and non-crystalline regions differently, according to its own limit of detection It is therefore very important to remember what each technique is actually measuring Also, the level of order in any given sample is a very strong function of thermal history So, it is not surprising that reported PVC crystallinity varies widely from polymer to polymer, as well as between measurement methods

Natta and Corrandini 1 first studied the unit cell structure of PVC using X-ray diffraction in 1956 They reported that PVC crystalline unit cell is orthorhombic and its lattice parameters are a=1.04nm, b=0.53nm, c=0.51nm, respectively Each cell contains

- 126 - two chains arranged in a planar syndiotactic conformation In 1973, Wilkes, Folt and Krimm 4 prepared low molecular weight PVC by polymerization in butryraldehyde and, after purification, produced a single crystal mat and studied its X-ray diffraction patterns They found a more compact orthorhombic crystal structure with lattice constants of a=1.024nm, b=0.524nm, and c=0.508nm The nature of larger scale order is still far from clear Both nodular and lamellar crystalline structures have been proposed in the literature 7, 8 Blundell 8 favored the former, but accumulating evidence appears to point to lamellar-type structures Wenig 7 carried out a detailed study of commercial PVC samples, using small-angle x-ray scattering (SAXS), wide-angle x-ray diffraction (WAXD), and small-angle light scattering (SALS) Wenig concluded that most crystallites were lamellar, though some rodlike entities were also present Biais et al 9 suggested that PVC crystallites are ribbonlike and narrow in both b- and c-directions, which would be consistent with Wenig’s rodlike crystallites

Both X-ray diffraction and thermal analysis provide evidence for the atypical crystalline nature of PVC The X-ray diffraction trace for so-called amorphous PVC is bimodal 6, 30 , rather than having a broad unimodal diffraction peak as is found for most amorphous polymers The crystalline diffraction peaks for non-oriented PVC are unusually broad, suggesting the presence of small, imperfect crystallites A DSC trace of PVC powder shows a very broad endotherm, covering the range from just above the glass transition temperature to above 200 o C The rapid crystallization of PVC has been demonstrated by thermal analysis 19-25 and is one of the reasons why it is difficult to get a 100% amorphous sample, which is sometimes required for measurement of crystallinity

In this chapter, Differential Scanning Calorimetry (DSC) was used to study the crystallization behavior of PVC samples (preparation discussed in the previous chapter) The annealing effect on crystallinity was extensively studied for PVC samples prepared in the presence of various additives, as well as samples prepared without additives A subtraction method was used to calculate the crystallinity of PVC samples Crystallization kinetics of PVC is discussed based on the crystallinity obtained as a function of annealing time The additive effect was evaluated by comparing the crystallinity of different PVCs after the same thermal cycling The degradation behavior of a few representative samples was studied using dynamic thermogravimetric analysis (TGA) The overall degradation activation energy was determined by several different methods and the result is in agreement with the literature results.

Experimental

PVCs prepared by free radical bulk and suspension polymerization methods as described in Chapter 2 were studied The polymers were precipitated twice from a 5% THF solution into a large volume of methanol, and dried in a vacuum oven at 60 o C for

48 hours The purified PVC samples were white, cotton-like fibers

- 128 - §3.2.2 DSC Measurement of PVC Samples

Differential Scanning Calorimetry (DSC) measurements were carried out using a

TA 2910 modulated differential scanning calorimeter calibrated using Indium (melting point: 156.60 o C, heat of fusion: 28.71 J/g) The data was collected using Thermal Advantage version 1.1A instrument control software and analyzed by Universal Analysis

2000 version 3.1E software A heating rate of 20 o C/min and a nitrogen flow rate of 50ml/min were used for the calorimetric measurements The PVC sample used weighed about 10~15mg For the annealing studies, PVC samples were heated under a nitrogen atmosphere in the DSC cell to 230 o C, rapidly cooled to the annealing temperature, held there for a specified time, and then cooled rapidly to -60 o C The samples were then scanned at 20 o C/min to about 250 o C to observe the melting behavior For the quenching studies, the samples were heated to 230 o C, kept there for one minute, then rapidly cooled to -60 o C, using a dry-ice filled cold cylinder Then the samples were scanned to 250 o C Quenched PVC DSC curves were used as a reference to measure the increase of crystallinity of annealed samples §3.2.3 TGA Measurement of PVC Samples

The thermogravimetric experiments were carried out using a TA 2950 thermogravimetric analysis instrument with platinum crucibles The weights of the samples were about 4-8 mg Small PVC samples were used to reduce the temperature difference between the sample and the crucible, and minimize any heat transfer barriers between the organic polymer and metal crucibles The tests were performed in a dynamic

- 129 - mode, going from room temperature to 600 o C Experiments were carried out under nitrogen, with a flow rate of 100 ml/min in order to remove the evolved corrosive gases rapidly Heating rates of 2, 5, 10, 15 and 20 o C/min were used for selected samples.

References

radical polymerization of vinyl chloride……… 15

Correlation between Structural Defects and the

References

Conclusions

Poly(vinyl chloride) (PVC), is one of the most widely produced polymeric materials in the world; the annual production of the PVC is second only to that of polyethylene 1 In industry, PVC is generally synthesized by free radical polymerization of vinyl chloride (VC) Radiation-induced 2 , anionic 3 , and modified Ziegler-Natta catalyst 4 methods have also been reported in the literature Vinyl chloride differs from monomers like styrene, methyl methacrylate or vinyl acetate principally by the insolubility of the polymer in its monomer Its key feature is that poly(vinyl chloride) is insoluble in its monomer, but slightly swollen by it Polymerization of vinyl chloride also differs from the heterogeneous polymerization of monomers, such as acrylonitrile Vinyl chloride partly swells its polymer but acrylonitrile does not It also differs from the conventional emulsion polymerization of unsaturated monomers in which the polymer particles are swollen by their monomer

Vinyl chloride is classified as a non-conjugated weak electron-withdrawing vinyl monomer with Q- and e-values of 0.056 and 0.16, respectively, determined from radical copolymerization 5 This indicates that the VC monomer has low reactivity, but its radical is highly reactive The propagating radical is so reactive that it tends to chain transfer to all substances in the polymerization system, such as monomer, initiators, solvents, and the resulting polymer As a consequence, radical polymerization of VC produces anomalous units in the chain, which decrease its thermal stability Generally, the termination reaction is dominated by chain transfer to monomer, if there are no other

- 2 - chain-transfer agents in the system The overall polymerization of vinyl chloride can be described as follows:

Here I is the initiator, R • the primary radical, M the monomer, M • the monomeric radical, Mi • the growing radical with i monomer units, P dead polymer, P • the polymer radical generated by a chain transfer reaction, I • the initiator radical, S the solvent, S • the solvent radical, k’s corresponding rate constants §1.2 A brief History of PVC