Handbook of Polymer Synthesis Second Edition Episode 4 pdf

4 Polymers of Acrylic Acid, Methacrylic Acid, Maleic Acid and their Derivatives Oskar Nuyken Technische Universita ¨ tMu ¨ nchen, Garching, Germany I. ACRYLATES AND METHACRYLATES (This section was prepared by O. Nuyken, G. Lattermann, H. Samarian, U. Schmelmer, C. Strissel, L. Friebe.) This section is supposed to be a review of the background and possibilities of acrylate and methacrylate polymerization with a main focus on recent developm ents. Additional information and examples are given in the first edition of this book [1]. A. Introduction 1. Formula and History The esters of acrylic and methacrylic acid, whose polymerization reactions are described in this chapter, are unsymmetrically substituted ethylenes of the general formula ð1Þ with R ¼ H for acrylates and R ¼ CH 3 for methacrylates. The substituents R 0 may be of a great variety: from n-alkyl chains to more complicated functional groups. In the following chapters these compounds are generally named acrylic esters, although in literature, esters of other a-substituted acrylic acids (e.g., R ¼ –CN, –Cl, –C 2 H 3 ) are sometimes included in this term. The first report of a polymeric acrylic ester was published in 1877 by Fittig and Paul [2] and in 1880 by Fittig and Engelhorn [3] and by Kahlbaum [4], who observed the polymerization reaction of both methyl acrylates and metha crylates. But it remained to O. Ro ¨ hm [5] in 1901 to recognize the technical potential of the acrylic polymers. He continued his work and obtained a U.S. patent on the sulfur vulcanization of acrylates in 1914 [6]. In 1924, Barker and Skinner [7] published details of the polymerization of Copyright 2005 by Marcel Dekker. All Rights Reserved. methyl and ethyl methacrylates. In 1927 [8], based on the extensive work of Ro ¨ hm, the first industrial production of polymeric acrylic esters was started by the Ro ¨ hm & Haas Company in Darmstadt, Germany (since 1971, Ro ¨ hm GmbH, Darmstadt). After 1934, the Ro ¨ hm & Haas Co. in Darmstadt was able to produce an organic glass (Plexiglas) by a cast polymerization process of methyl methacrylate [9]. Soon after, Imperial Chemical Industries (ICI, England), Ro ¨ hm & Hass Co. (United States), and Du Pont de Nemours followed in the production of such acrylic glasses. Nowadays poly(m ethyl methacrylate) (PMMA) as homo- or copolymer exceeds by far the combined amount of all other polyacrylic esters produced [10]. 2. Monomers The most common procedure for the technical synthesis of the monomer methyl methacrylate (MMA) is the reaction of acetone cyanhydrine with water and methanol in the presence of concentrated sulfuric acid [11]: ð2Þ ð3Þ Many other processes and reactions of the monomer synthesis are described exten- sively in literature [12–14]. For different acrylic esters, especially on a laboratory scale, the alcoholysis of the corres ponding acid chlorides as well as direct esterification reactions of methacrylic acid, but also transesterification reactions of MMA, are often preferred [13–15]. The physical properties of various monomers are well summarized in literature [16,17]. 3. Reactions Acrylic esters have two functional groups, where reactions occur: the ester group and the double bond. Reactions on the ester group are carried out under conditions that prevent polymerization of the double bond (i.e., the use of polymerization inhibitors and low reaction temperatures are necessary). Typical reactions of the ester function are: saponification, transesterification, aminolysis, and Grignard reaction [10,17]. Reactions of the double bond beside polymerization reactions are Diels-Alder reaction; Michael addition; and addition of halogens, dihalocarbenes, hydrogen halogenides, alcohols, ammonia and amines, nitroalkanes, or sulfur compounds such as hydrogen sulfide or mercaptanes [10,17]. Most acrylates are polymerized by both radical and anionic initiations, with the former being the more commonly used. In all cases the heat of polymerization must be carefully controlled to avoid runaway reactions. The values of the heat of polymerization for selected methacrylates are listed in literature [18]. In general, the rate of polymeriza- tion and the average molar mass must be controlled by the initiator and monomer concentration and the reaction temperature. In all cases the use of high-purity monomers is important for proper polymerization conditions. Therefore, the removal of inhibitors is necessary. Phenolic inhibitors such as hydroquinone, 4-methoxyphenol, or aromatic amines are usu ally removed by alkaline or acidic extraction [11,19]. Otherwise, the Copyright 2005 by Marcel Dekker. All Rights Reserved. monomers are distilled from inhibitors of low volatility such as dyes (methylene blue, phenothiazine), aromatic nitro or copper compounds. To prevent inhibition by dissolved oxygen, acrylic monomers must be carefully degassed before polymerization [19]. After the polymerization step, the isolation of the product is often necessary. Depending on the polymerization technique, this may be achieved by different procedures (e.g., precipitation, spray drying, breakdown of a colloidal system, etc.). Purification of soluble polymers can be achieved by repeated cycles of precipitation, or in the case of water solubility, by dialysis. The removal of solvent may often be very difficult because of strong polymer–solvent interactions. Therefore the polymer is slightly heated above T g under high vacuum, spray-dried, or freeze-dried. Freeze-drying with benzene, dioxane, or water results in a very dry, highly porous material. B. Processing 1. Bulk Polymerization In contrast to acrylic monomer s the bulk polymerization of methacrylic esters is very important in manufacturing sheets, rods, tubes, and moldin g material by cast molding techniques [9–11]. Three important properties are characteristic of the bulk polymerization of acrylates. First, a strong volume contraction, being relatively high compared with other monomers, occurs during the polymerization reaction (see Table 1). It may be overcome either by using ‘prepolymers’ (i.e., solution of polymers in their monomers, usually prepared by bulk polymerization until a desired viscosity level of the mixture [20]) or by forming rigid polymer networks even at low conversion through cross-linking agents. Second, the polymerization process is accompanied by a considerable reaction heat (see Table 1), which is higher for acrylates than for methacrylates. Therefore, after 20 to 50% conversion, causing an increased viscosity of the system, a drastic autoacceleration process may be possible, known as gel or Trommsdorff effect [11,21,22]. Thus it is necessary to regulate very carefully heat removal during the polymerization in bulk. Third, at high conversion, branching and cross-linking reactions, leading finally to insoluble networks, may occur [23–25 ]. This is due to chain transfer involving abstraction of hydrogen from the polymer chain, subsequent branching, and combining two branch radicals. Bulk polymerization is commonly started by radical initiators such as azo compounds and peroxides; however, some examples of thermal self-initiation of bulk Table 1 Shrinkage and reaction heat of various methacrylates. a Methacrylates Shrinkage/% ÁH/(kJ/mol) Methyl 21.2 54.5 Ethyl 17.8 59.1 Butyl 14.3 56.6 Isobutyl 12.9 Source: Refs. [26] and [19]. ‘ a The percent shrinkage can be calculated by using the following equation: % shrinkage ¼ 100 Â (D p ÀD m )/D p (D m ¼ monomer density at 25  C; D p ¼ polymer density at 25  C. Copyright 2005 by Marcel Dekker. All Rights Reserved. polymerization of MMA [27] and octylacrylate [28] are described. For MMA, which cannot form a Diels-Alder adduct, diradicals are believed to play a role in the thermal initiating mechanism [29–31]. Different descriptions of general procedures for the bulk polymerization of acrylates (sheets, molding material) are given in Refs. [12] and [19]. The bulk polymerization of g-alkoxy-b-hydroxypropylacrylates is described in Ref. [32]. Bulk atom transfer radical polymerization is reviewed in Ref. [33]. 2. Solution Polymerization Several general disadvantages of bulk polymerization (removal of the reaction heat, shrinkage, nonsolubility of the resulting polymer in the monomer, side reactions in highly viscous systems such as the Trommsdorff effect or chain transfer with polymer) are responsible for the fact that many polymerization pr ocesses are carried out in the presence of a solvent. A homog eneous polymerization occurs when both monomer and polymer are soluble in the solvent. When the polymer is insoluble in the solvent, the process is defined as solution precipitation polymerization. Other heterogeneous polymeriza- tion reactions in liquid–solid or liquid–liquid systems such as suspension or emulsion polymerizations are described later. Conventional solution polymerization is compared with solution precipitation polymerization for the synthesis of acrylic resins in Ref. [34]. In homogeneous systems including inert solvents, the reaction rate decreases with decreasing monomer concentration. In solution precipitation polymerization, kinetics may deviate from that in homogeneous solution. In nearly every polymerization system the influence of the solvent on the course of the reaction is important. Thus chain transfer reactions with active chain ends occur in radical polymerization. The solvent can also influence the stereoregularity of the product in anionic polymerizations. The boiling range of the solvents should correspond to that of the monomer s and to the decomposition temperature of the initiators. Thus common polymerization temperatures are often between 60 and 120  C (under reflux of the solvent). A general procedure for the radical homopolymerization of acrylates in solution is given in Ref. [35]. Not only acrylic esters that have intermediate solubility in water due to additional hydroxy or amino groups can be polymerized in water, but also conventional acrylic monomers with a relatively low water solubility (MMA: 15 g/L at room temperature) [36] can be polymerized in water. Acrylate monomers of intermediate solubility in water, such as hydroxyalkyl acrylates and methacrylates or aminoalkyl acrylates or methacrylates, undergo free- radical polymerization with a variety of initiator systems. Both monomer classes have been reviewed in the literature [37]. Highly soluble monomers such as 2-sulfoethyl methacryla tes or the corresponding alkali salts are easily polymerized to high molar mass by hydrogen peroxide in aqueous solut ion [38]. Anionic initiation has been accomplished in a variety of so lvents, both polar and nonpolar. Isolation and purification of the product is performed, for example, by addition of a nonsolvent, leading to polymer precipitation or by removal of the solvent by spray drying or by freeze drying in benzene, dioxane, or water. Polymer precipitation should be quantitative. However, PMMA with a degree of polymerization less than 50 is still soluble even in methanol; thus petroleum ether is necessary to precipitate the low-molar-mass PMMA [39]. Numerous solvents and nonsolvents for polymers are reviewed in Refs. [40] and [41]. Copyright 2005 by Marcel Dekker. All Rights Reserved. In industrial processes it is sometimes ad vantageous to have a strong solvent– polymer interaction. Thus solution polymerization is often performed for applications in which the solvent remains present (e.g., in protective coatings, adhesives, and viscosity modifiers). 3. Suspension Polymerization The term suspension polymerization, often a lso called aqueous suspension polymerization or pearl or bead polymerization, means a process where liquid monomer droplets are suspended in an aqueous phase under vigorous stirring. This process can be regarded as a bulk polymerization within the monomer droplets, where the polymerization heat can easily be dissipated by the surrounding water. To prevent the coalescence of the droplets, the presence of suspension stabilizers or suspending agents is necessary. Two classes of suspension stabilizers are known [42,43]: 1. Water-soluble polymeric compounds. These can be natural or modified natural products such as gelatine, starch, or carbohydrate derivatives such as methyl cellulose, hydroxyalkyl cellulose, or salts of carboxymethyl cellulose. Synthetic polymers such as poly(vinyl alcohol), partially hydrolyzed poly(vinyl acetate), sodium salts of poly(acrylic acids), methacrylic acids, and copolymers thereof are widely used in quantities between 0.1 and 1% related to the aqueous phase. 2. Powdery inorganic compounds. Earth alkaline carbonates, sulfates, phosphates, aluminum hydroxides, and various silicates (talc, bentonite, Pickering emulga- tors) are used in quantities between 0.001 and 1%. The initiator systems are the same as for radical bulk or solution polymerization processes (e.g., peroxides or azo compounds). A typical recipe is given in Ref. [44]. 3. Nonaqueous dispersion polym erization is defined as the polymerization of a monomer, soluble in an organic solvent, to produce an insoluble polymer whose precipitation is controlled by an added stabilizer or dispersant. The resulting stable colloidal dispersion ensures good dissipation of the polymerization heat. Stabilization of the polymeric particles is generally achieved by a lyophilic polymeric additive. PMMA is mostly homo- or copolymerized in aliphatic hydrocarbon dispersions, using different rubbers, polysiloxanes, long-chain polymethacrylates, or different block and graft copolymers as stabilizers. An interesting variant of the dispersion polymeriza- tion of acrylates is carried out in supercritical carbon dioxide [45,46]. Transition-metal- mediated living radical suspension polymerization is discussed in Ref. [47]. Common radical initiators are described in Refs. [48] and [49]. The entire field is reviewed extensively in Ref. [50]. 4. Emulsion Polymerization An emulsion polymerization system can comprise three phases: (1) an aqueous phase, containing the water-soluble initiator, the micelle-forming surfactant, and a small amount of the sparingly soluble monomer; (2) monomer droplets; and (3) latex particles, consisting of the polymer and some monomer. The locus of polymerization is predominantly inside the latex particles. Usual free-radical water- soluble initiators are used, such as potassium persulfate for higher reaction Copyright 2005 by Marcel Dekker. All Rights Reserved. temperatures and redox systems [e.g., Fe(III) salts, cumene hydroperoxide] for low- temperature polymerizations. Three types of surfactants are known: (1) electrostatic (anionic or cationic) low-molecular mass surfactants; (2) steric stabilizers such as poly(vinyl alcohol), or a combination of (1) and (2); and (3) electroste ric stabilizers such as polyelectrolytes. Furthermore, many other additives (protecting agents, cosolvents, chain transfer agents, buffer systems, etc.) are often necessary. The entire field is reviewed in Ref. [51], comprising the special kinetics of particle growth and form ation, particle size, and molecular mass distribution. Various emulsion polymerization procedures for the thermal and redox initiation of acrylic monomers are given in Refs. [52] and [53]. Methyl, ethyl, and n-butyl acrylates and methacrylates are found to form high-molecular-mass compounds quite easily through a plasma-induced emulsion polymerization system [54]. Emulsions are thermo- dynamically unstable, although they often may have an appreciable kinetic stability. The use of a co-emulsifier (e.g., long-chain alkanes , alkanol or ammonium salts, or block copolymers of ethylene and propylene oxide) can produce microemulsions. They are thermodynamically stable systems, exhibiting an average particle size of about 100 nm [55]. Thus transparent microemulsions of MMA can be obtained which have been photo- polymerized together with a photosensitizer [56]. The field of microemulsion is reviewed in Ref. [57]. A emulsifier-free emulsion polymerization of acryl ates is possible by the use of 2-hydroxyethyl methacrylate [58]. Acrylate block copolymers (P(MMA-b-MAA)) were used as surfactants in emulsion polymerization of acrylate monomers [59]. 5. Irradiation Polymerization Irradiation-induced bulk polymerization can be divided into two types: solid-state polymerization and polymerization in the liquid state, classified as follows: 1. UV light: the initiation process is thought to oc cur via a free-radical mechanism. 2. g-radiation: the induced polymerization process involves free radicals or ionic species, depending on monomer, temperature, dose rate etc. [60]. 3. Electron-beam, x-ray, or ion-beam radiation. Since most of the monomers do not produce initiating species with a sufficiently high yield upon UV exposure, it is necessary to introduce a photosensitive initiator. The photo initiator (PI) will start the polymerization upon illumination. Thus, the PI plays a key role in light-induced polymerization for it absorbs the incident light and generates reactive radicals or ions and it controls the reaction rate and the depth of cure profile within the sample. There are various photoinitiators used in UV-curing applications which can be classified into three categories, depending on the way the initiating species are generated: 1. Radical formation by photocleavage: aromatic carbonyl compounds that undergo homolytic C–C bond cleavage upon UV exposure with formation of two radical fragments like benzoin ether derivatives, hydroxyalkylphenones, a-amino ketones, morpholinoketones (MoK) and bisacylphosphine (BAPO) from Ciba Specialty [61]. Phosphine oxides undergo fast photolysis to generate non-colored products (Scheme 4). Their higher initiation efficiency is caused by a disaggregation that is fast, as the rate of initiation is directly related to the rate of the PI photolysis. Copyright 2005 by Marcel Dekker. All Rights Reserved. ν ν ð4Þ 2. Radical generation by hydrogen ab straction: some photoinitiators tend to abstract a hydrogen atom from a H-donor molecule via an exciplex, to generate a ketyl radical and the donor radical. The H-donor radical initiates the polymerization, the inactive ketyl radical disappears by a radical coupling process (5). This type of photoinitiators includes benzophenone and thiox- anthone. ν ð5Þ 3. Cationic photoinitiators: like protonic acids. Oxygen as an initiatior in photo-initiated free-radical polymerization and cross- linking of acrylates is reviewed in Ref. [62]. Methyl methacrylate does not appear to polymerize in the solid state upon simple UV radiation [63,64]. However, under pressure sufficiently high to solidify the monomer at a relatively high temperature or in a ‘solid solution’ in paraffin wax, polymerization was found to be possible. It is remarkable that the g-radiation-induced solid-state polymerization is influenced significantly when the polymerization proceeds in tunnel clathrates [1]. Copyright 2005 by Marcel Dekker. All Rights Reserved. Another possibi lity for irradiation-induced solid state polymerization is that in mono- or multilayers. Thus acrylates or methacrylates with different long-chain ester groups are polymerized by UV light, g -radiation, or electron-beam radiation [65–67]. The majority of the examples given in the literature for irradiation-induced bulk polymeriza- tion deal with monomers in the liquid state as pure compounds. Some examples are given for polymerization in the presence of inclusion compounds or related polymer matrices (see Refs. [60,68–72]). Another possibility ha s been described as photopolymerization of an oriented liquid crystalline acrylate [73]. Photo- or radiation-initiated bulk polymerization of acrylates is often used for the production of thick coatings or sheets. Demonstration experiments are given in Refs. [12] and [19]. For many purposes (e.g., photocoating, embedding media, etc.) casting resins often contain multifunctional cross-linking compounds [74,75]. A review of the chemistry of photoresists, reacted by UV, eximer laser (deep UV), x-ray, electron-beam, and ionbeam irradiation is given in Ref. [76]. In general, most industrial processes use a large variety of copolymerization reactions. Besides the above noted polymerization techniques photocuring is a special process that transforms a multifunctional monomer into a crosslinked macromolecule by a chain reaction initiated by reactive species generated by UV irradiation [77]. Three basic components are needed for photocuring: 1. The already mentioned photoinitiator; 2. A functionalized oligomer, which by polymerizing will constitute the backbone of the three dimensional polymer network formed; 3. A mono or multifunctional monomer, which acts as reactive diluent and will thus be incorporated into the network. UV-curable resins of acrylate and methacrylate monomers gained great commercial success because they offer high reactivity and the possibility of creating a large variety of crosslinked polymers with tailormade properties. On the other hand there are problems like early gelation of the irradiated sample and mobility restrictions of the reactive sites during the preceding reaction and also with increased monomer functionality. Novel acrylate monomers seem to circumvent these problems. Very promising results have been obtained by introducing a carbamate or oxazolidone group into the structural unit of a monoacrylate [77]. As shown by the RTIR profiles, the light-induced polymerization was found to occur faster than with typical monoacrylates or diacrylate monomers. The UV-cured polymers based on the novel acrylate monomers show some advantages: completely insolubility in organ ic solvents which makes these very reactive photoresists well suited for imaging applications; high crosslink density; good resistance to moisture, strong acids, weathering and thermal treatment [78]. Photopolymerization in micellar systems is useful for the synthesis of polymers displaying high molecular weights [57]. The model of photopolymerization used to describe a micellar polymerization does not differ from the one in bulk or solution photopolymerization [79]. 6. Plasma Polymerization A general introduction to the field of plasma polymerization is given in Ref. [31]. The plasma used in polymerization processes is the low-temperature plasma or low-pressure plasma, which is usually created by an electric glow discharge caused by, for example, Copyright 2005 by Marcel Dekker. All Rights Reserved. microwave power sources. There are two general methods in use to polymerize pure monomers. First, in plasma-state polymerization the plasma reacts directly within the vapor phase of a monomer, resulting in the vacuum deposition of polymers [31,80,81]. Here the course of the initiation reaction depends on the bombardment of the monomer by excited species such as radicals, ions, metastable particles, and on the absorption of UV radiation emitted by the different excited species. Concerning the UV-induced part of plasma polymerization, the propagation will be maintained by a free-radical mechanism. Acrylic monomers are not described as undergoing such processes. The second way, plasma-induced polymerization, is characterized by the formation of initiating species under the influence of a plasma and subsequent polymerization in the condensed phase. One possibility for the initiation process is that it can take place by exposing liquid monomers to a plasma of different gases (helium, argon, nitrogen, NO, CO 2 ,O 2 ,CF 4 ) [82] for several minutes. The presence of radical initiators, photo-initiators, and photosensitizers can influence the course of the polymerization reaction [83–86]. This technique is used to polymerize thin films for coating purposes. Another possibility in plasma-induced polymerization is to expose the vapor phase over a liquid monomer [31,87], volatile initiator, or monomer solution to the plasma for several seconds only. Chain propagation occurs in the liquid phase during a longer period of postpolymerization in the absence of plasma. The unique feature of this way of plasma- induced polymerization is that the formation of initiating species takes place in the gas phase, presumably creating diradicals with a very long lifetime [31]. In most cases the molar mass increases with reaction time (i.e., conversion). This is not the case in conventional free-radical polymerization, although the tacticity of the resulting acrylic polymers corresponds to that observed in free-radical polymerization. Some similarities of polymer characteristics (gel permeation chromatography, thermogravimetry, differential scanning calorimetry) can be observed between plasma-induced and thermal polymerization, the initiation process of the latter also being caused by diradicals. C. Mechanism 1. Free Radical Polymerization The kinetic scheme of this type of polymerization is equivalent to other classical vinyl polymerizations, including initiation, propagat ion, chain transfer, and termination (Scheme 6). Initiation: I À! k d 2R . R . þ M À! P 1 . Propagation: P 1 . þ n À 1M À! k p P n . Chain transfer: P n . þ M À! k c,M P n þ M . P n . þ L À! k c, L P n þ L . Termination: P n . þ P m . À! k t,r P n ÀP m Recombination P n . þ P m . À! k t, d P n þ P m Disproportionation ð6Þ Common solvents include toluene, ethyl acetate, acetone, and 2-propanol. The boiling range of the solvents should correspond to that of the monomers and to the Copyright 2005 by Marcel Dekker. All Rights Reserved. decomposition temperature of the initiators. Thus common polymerization temperatures are often between 60 and 120  C (under reflux of the solvent). Most common initiators are compounds decomposing to starting radicals by thermolysis. The main classes for both organic and aqueous media systems are reviewed according to the following main groups: 1. Azo and peroxy like azobisisobutyronitrile (AIBN) and dibenzoyl peroxide (BPO) initiators [88]. 2. Redox initiators such as peroxide tertiary amine systems or those based on metals or metal complexes [89]. 3. Ylide initiators such as b-picolinium-p-chlorophenacylide or others [90]. This initiating system is especially interesting with respect to alternating copolymers of MMA. 4. Thermal iniferters [91], a class of initiators that not only can start a polymeric chain but can also undergo a termination reaction by chain transfer (initiator, transfer agent, chain terminator). The resulting end group is thermally or photochemically labile, being able to undergo reversible homolysis to regenerate a propagating radical. These materials have been applicated in the synthesis of block and graft copolymers. General conditions for a successful application of radical initiators are [92]: 1. The initiator decomposition rate must be reasonably constant during the polymerization reaction. The ‘cage effect’ (recombination of initiator radicals before starting a polymer chain) should be small, which is generally more the case with azo compounds than with peroxides. 2. Side reactions of the free radicals (e.g., hydrogen abstraction with dialkyl peroxides and peresters) should be reduced. 3. In addition to initiators, accelerators and chain transfer agents are sometimes used. Thus, with accelerators (often redox activators (e.g., ZnCl 2 [93], cobalt salts, tertiary amines [94]), the reaction temperature can be drastically reduced; with chain transfer agents the average molar mass of the resulting polymer can be regulated. Concerning the growing radicals in polymerization reactions, they can be studied directly by ESR spectroscopy as in the case of triphenylmethyl methacrylate and MMA [95]. In the latter case it was concluded that there are two stable conformations of the propagating radicals. The steric effect of the a-methyl group of MMA is not only responsible for the comparatively low heat of the polymerization reaction, but also for a certain control of the propagation steps. Therefore, in radical solution polymerization the polymethacrylates exhibit in most cases a favored syndiotacticity. With respect to the termination mechanism in radical acrylate polymerization, some results are reviewed in Ref. [96]. In MMA polymerization the preferred termination mechanism is solvent dependent (e.g., disproportionation is being favored in benzene). For alkyl acrylates termination involves predominantly combination. As mentioned earlier, a general procedure for the radical homopolymerizaiton of acrylates in solution is given in Ref. [35]. With a-substituted acrylates other than methacrylates, isotacticity is somewhat enhanced [97]. Tacticity of acrylate or methacrylate polymers obtained by radical initiators is an important matter of research, as it influences the physical properties of the acrylate polymers: for example, the higher the syndiotacticity, the higher the glass transition Copyright 2005 by Marcel Dekker. All Rights Reserved. [...]... syndiotacticity increases from 50% to 80% [2 34, 244 , 245 ] A comparison of the triad distribution for anionic and GT polymerizations of MMA with the same counterions under the same conditions shows that the tacticities of both polymerization types are consistent [97, 246 ] Some selected examples of the influence of different polymerization parameters on tacticity are given in Ref [ 245 ] The living character and different... PDI Conversion/% 32.5 38.2 36 .4 10.6 24. 6 24. 8 24. 4 6.1 1.32 1. 54 1 .49 1.73 89.3 83.0 65.2 33 .4 about 5% and a low molecular mass, generally below 40 00, the careful control of the amount of oxygen allows to continue polymerization to higher conversions, rarely exceeding 20% Similar results were obtained using initiator/nitroxide adducts for the control of the initial amount of excess free nitroxide [268,269]... of t-C4H9Li and MeAl(ODBP)2 can also provide stereoregular statistical copolymers of methacrylates acrylates [1 74, 175] as well as stereoregular block copolymers and block copolymers [176,177] via living polymerization Replacement of the methyl group in MeAl(ODBP)2 by other alkyl groups (Scheme 10) resulted in an increase of syndiotacticity with the size of the alkyl rest as it is shown in Table 4 polymerization... Anionic polymerization of MMA in THF at various temperatures using DPHLia as initiator in the presence of DLiTG.b Temp./ C 40 À20 À20 À20 0 0 0 a [DPHLi]/ (mmol/L) [MMA]/ (mol/L) [DLiTG]/ [DPHLi] Yield/ % 10À3 Mn,calcc 10À3 Mn d PDI 1.015 0.08 0.952 0.335 1.90 1 .41 0 .47 0.228 0.2 24 0.267 0.303 0.312 0.330 0.300 10 0 4 10 0 5 10 100 95 100 100 80 90 100 22 .4 28 28.0 90.5 16.3 23 .4 63.8 25.0 42 .2 36 .4 100.5... reactions of OH-containing polymers, such as poly(vinyl alcohol) [ 142 – 144 ] or poly(hydroxyethyl methacrylate) [ 144 ], but also natural products such as cellulose [ 145 , 146 ] or gelatine [ 147 ] with, for example, Ce4þ are used to graft MMA side chains Otherwise, hydroxyl functions in starch have been reacted with methacrylic anhydride Subsequently, MMA was radically grafted from these sites [ 148 ] Other... cross-linking of acrylamide, 2-hydroxyethyl methacrylate, and ethylene dimethacrylate to a copolymer gel is given in Ref [133] Radical techniques are also used for the synthesis of graft polymers The grafting polymerization of MMA or its mixture with other comonomers from diene units containing rubbers, in bulk or suspension [1 34 140 ], and from a terpolymer of styrene, MMA, and t-butylperoxy acrylate [ 141 ] Furthermore,... the absence of organic solvents Due to the increased diffusivity of monomer dissolved in the supercritical CO2 and the plasticization of the polymer, the rate of polymerization of the second block can be increased, and thereby, a one pot synthesis of block copolymers becomes possible [282,283] Copyright 2005 by Marcel Dekker All Rights Reserved Table 8 Molecular mass and polydispersity of synthesized... of living anionic polymerization is the availabilty of telechelic polymers [207] and macromonomers, which are of specific interest for the preparation of comb-like (if monofunctional) and network (if difunctional) structures [208,209] In addition, due to its ‘living’ nature, anionic polymerization provides a versatile synthetic route for the synthesis of a wide range of well defined polymer structures... C2H5 i-C4H9 a Yield/% Mn c Mw c =Mn mm mr rr H CH3 t-C4H9 Br H H 100 97 100 100 100 30 7,510 6, 040 8,170 6,360 6 ,49 0 4, 990 1.13 1.12 1.10 1.08 1.09 1. 14 7.3 6.9 6.2 13.8 0.0 0.3 87.6 67.5 84. 3 82.7 8.1 17.5 5.1 25.6 9.5 4. 1 91.9 82.2 EMA 10 mmol, t-C4 H9 Li 0.2 mmol, toluene 10 mL, alkylaluminum phenoxide 1.0 mmol; bsee Scheme (10); c Determined by SEC; dDetermined by 13C-NMR this kind of polymerization... this kind of polymerization is often limited by the occurrence of side reactions, including (1) the attack of the initiator at the carbonyl double bond of the monomer or polymer, (2) chain transfer of a-situated protons, (3) 1 ,4- addition via the enolate oxygen instead of 1,2-addition through the carbanionic centers [see Scheme 11], and (4) coordination of the counterion of the active centers with carbonyl . terpolymer of styrene, MMA, and t-butylperoxy acrylate [ 141 ]. Furthermore, redox reactions of OH-containing polymers, such as poly (vinyl alcohol) [ 142 – 144 ] or poly(hydroxyethyl methacrylate) [ 144 ], but. the synthesis of graft polymers. The grafting polymerization of MMA or its mixture with other comonomers from diene units containing rubbers, in bulk or suspension [1 34 140 ], and from a terpolymer. existence of two types of active species [169]. The addition of (CH 3 ) 3 Al to the polymerization of EMA recently has been found to have the beneficial effect of allowing the synthesis of highly