Thermosetting Polymers Part 2 potx

2 Chemistry of Crosslinked Polymer Synthesis 2.1 GENERAL CONSIDERATIONS A comprehensive classification of both linear and crosslinked polymers may be based on the mechanism of the polymerization process. From the point of view of the polymer growth mechanism, two entirely different processes, step and chain polymerization, are distinguishable. Step-growth polymerization proceeds via a step-by-step succession of elementary reactions between reactive sites, which are usually functional groups such as alcohol, acid, isocyanate, etc. Each independent step causes the disappearance of two coreacting sites and creates a new linking unit between a pair of molecules. To obtain polymers, the reactants must be at least difunctional; monofunctional reactants interrupt the polymer growth. In chain-growth polymerization, propagation is caused by the direct reaction of a species bearing a suitably generated active center with a mono- mer molecule. The active center (a free radical, an anion, a cation, etc.) is generated chainwise by each act of growth; the monomer itself constitutes the feed (reactive solvent) and is progressively converted into the polymer. For both mechanisms of polymer growth, if one of the reactants has a functionality higher than 2, branched molecules and an infinite structure can be formed. To summarize both mechanisms it may be stated that: 1. A step-growth polymerization (with or without elimination of low-molar-mass products) involves a series of monomer + monomer, monomer + oligomer, monomer or oligomer + macromolecule, and macromolecule + macromolecule reactions. The molar mass of the product grows gradually and the molar mass distribution becomes continuously wider. Functionalities of monomers and the molar ratio between coreactive sites are the main parameters for controlling the polymer structure. 2. A chainwise polymerization proceeds exclusively by monomer + macromolecule reactions. When the propagation step is fast com- pared to the initiation step, long chains are already formed at the beginning of the reaction. The main parameters controlling the polymer structure are the functionalities of the monomers and the ratios between the initiation and propagation rates and between initiator and monomer concentrations. Thermosetting polymers may be formed in two ways: 1. By polymerizing (step or chain mechanisms) monomers where at least one of them has a functionality higher than 2. 2. By chemically creating crosslinks between previously formed linear or branched macromolecules (crosslinking of primary chains, as vulcanization does for natural rubber). In fully reacted polymer networks, practically all constituent units are covalently bonded into an infinite three-dimensional structure. It means that during polymerization or crosslinking the system evolves from a collection of molecules of finite size to an infinite network, proceeding through the gel point at which the infinite network structure appears for the first time. This transformation is called gelation. As polymer networks are very often prepared in bulk, vitrification, which is the transformation from a liquid or rubbery state to a vitreous state, can also take place. These transformations are discussed later (Chapters 3, 4, and 6), but one question that concerns chemistry is the possible effect of these transformations on the mechanisms and kinetics of the reactions. Chemistry of Crosslinked Polymer Synthesis 7 2.2 STEP-GROWTH POLYMERIZATIONS: POLYCONDENSATIONS AND POLYADDITIONS 2.2.1 General Aspects Based on the classical definitions given in organic chemistry, the step-growth polymerization process can involve either condensation steps or addition steps. The former proceeds with elimination of by-products while the latter takes place without elimination of by-products. This is illustrated by Eqs (2.1) and (2.2), for the particular case of difunctional molecules: n A − R − BA − RC { + b y -products} n-1 − R −B ð2:1Þ n AR 1 A + n BR 2 BA R 1 CR 2 C R 1 CR 2 B { + b y -products} 2n-2 ð2:2Þ Each reaction step causes the disappearance of two reactive sites, A and B, converted into a linking unit C, and leaves two reactive sites at the ends of any growing molecule irrespective of its size. The difference between polycondensation and polyaddition is only the formation of by-products during each reaction step. The extent of reaction (or conversion) at any stage can be expressed by the fraction of total reactive sites that have been consumed. Reactive sites usually display the same reactivity regardless of the size of the molecule to which they are linked. The polymerization process has the characteristics of a statistical combination of fragments. In this way, a distribution of pro- ducts from the monomer to a generic n-mer is obtained (Table 2.1), with average molar masses increasing continuously with conversion. If the initial polymerization system contains a single monomer as in Eq. (2.1), the constitutional repeating unit (CRU) of the polymer will con- tain only one monomer-based unit and the structure of the CRU will be derived from the monomer (polyaddition case), possibly through the elim- ination of a small molecule (polycondensation case). If the initial polymerization system contains two different monomers (Eq. 2.2), the CRU will contain two monomer-based units. When at least one of the monomers bears more than two reactive functional groups, the formation of a polymer network is possible. When the concentrations of A and B may be varied independently (Eq. 2.2), the stoichiometric ratio of functionalities is defined by r ¼ A 0 =B 0 , where A 0 and B 0 are the initial concentrations of functional groups A and B. As will be shown in Chapter 3, this ratio is very important in designing and controlling a step-growth polymerization. Statistical parameters at any 8 Chapter 2 conversion may be correlated with the initial composition and the number of functional groups per molecule. Many reactions familiar to organic chemists may be utilized to carry out step polymerizations. Some examples are given in Table 2.2 for poly- condensation and in Table 2.3 for polyaddition reactions. These reactions can proceed reversibly or irreversibly. Those involving carbonyls are the most commonly employed for the synthesis of a large number of commercial linear polymers. Chemistries used for polymer network synthesis will be presented in a different way, based on the type of polymer formed (Tables 2.2 and 2.3). Several different conditions may be chosen for the polymeriza- tion: in solution, in a dispersed phase, or in bulk. For thermosetting poly- mers the last is generally preferred. Chemistry of Crosslinked Polymer Synthesis 9 TABLE 2.1 The step-growth polymerization process or or + + + + + + tetramer pen t amer etc. x -mer + y -mer (x + y ) -mer dime r trimer 10 Chapter 2 TABLE 2.2 Typical polycondensations A and B reactive sites By-product Linkage C formed Type of polymer Carboxylic acid + alcohol H 2 O Ester of carboxylic acid + alcohol ROH –CO 2 – Polyester Anhydride of dicarboxylic acid + alcohol H 2 O } Carboxylic acid + amine H 2 O –CONH– Polyamide Anhydride of dicarboxylic acid + amine H 2 O –CO–N–CO– | Polyimide Ketone + amine H 2 O \ / C¼N– Polyazomethine Phenol + phosgene (COCl 2 ) HCl –OCO 2 – Polycarbonate Phenol (+ NaOH) + aryl halide NaCl Ar–O–Ar Polyether Phenol + formaldehyde H 2 O Ar–CH 2 – ‘‘Phenolic resin’’ Urea (or melamine) + formaldehyde H 2 O –NH CH 2 – ‘‘Amino resin’’ Isocyanate (2A,noB)CO 2 –N¼C¼N– Polycarbodiimide Chlorosilane + H 2 O HCl – \ / SiO– Polysiloxane Chemistry of Crosslinked Polymer Synthesis 11 TABLE 2.3 Typical polyadditions A and B reactive sites Linkage C formed Type of polymer Isocyanate + alcohol Polyurethane Isocyanate + amine Polyurea Epoxy (or oxirane) + amine Polyepoxy Epoxy (or oxirane) + isocyanate Poly(2-oxazolidone) Cyanate–ester (3A,noB) Polycyanurate or triazine Thiol + double bond Polysulfide for example, amine + fumarate double bond Michael-type additions Diels–Alder reaction – O – CO – NH –NH – CO – NH– − CH − CH 2 − N OH O N O O C N N CH C N O O –CH 2 – CH 2 S– 2.2.2 Organic Acid Reactions a. Mechanism: An Example of Polycondensation Involving Carbonyl Groups Reactions of this type are employed for the synthesis of a large number of commercial (linear) polymers such as polyesters and polyamides. A small molecule, water, is split out during these condensation reactions: R 1 - C - OH + R 2 - OH O O R 1 - C - OR 2 + H 2 O ð2:3Þ O R 1 - C - OH + R 2 - NH 2 O R 1 - C - NHR 2 + H 2 O ð2:4Þ As a result of reactions (2.3) and (2.4), an equilibrium between reac- tants and products would be reached in a closed system. The removal of water is necessary in order to allow the reaction to proceed to high conver- sions. Instead of carboxylic acids, other carbonyl compounds can be used: acid halides, esters, amides, etc. The commonly accepted general mechanism for these reactions consists of the initial nucleophilic addition of an active hydrogen compound to the electron-poor carbonyl carbon atom of the R 1 COOH molecule, with the formation of a metastable intermediate that can undergo a subsequent elimination reaction: R 1 - C - X + AH R 1 - C - X R 1 - C - A + HX O _ O O A-H + ð2:5Þ Such a mechanism can explain why aromatic R 1 acids have a higher reactivity than aliphatic ones. AH is a Lewis base, which carries an unshared pair of electrons on the A atom. Hence an increase in its nucleophilic char- acter should facilitate the addition to the electron-poor carbonyl carbon atom and should make easier the elimination of X as a negative ion. The use of a catalyst is frequently necessary in order to obtain high- molar-mass products. Strong protonic acids or carboxylic organic acids are frequently employed as catalysts: 12 Chapter 2 R 1 - C - X R 1 - C - X R 1 - C - X R 1 - C+ R 1 - C - A +AH -H + -HX AH + +H + A + O OH OH OH O ð2:6Þ Different types of metal compounds can also be used as catalysts; for example, zinc acetate, titanium alkoxide, phosphorus derivatives, etc. Cyclic anhydrides are diacids with one molecule of water eliminated from the condensation of the two acid groups. They can be useful for the synthesis of polyesters. The reaction proceeds in two steps because the free acid formed in the first step (Eq. 2.7) is much less reactive than the original anhydride: R 1 C C O + R 2 OH HO - C - R 1 - C - OR 2 O O O O ð2:7Þ HOC - R 1 - C - OR 2 + R 2 OH O O R 2 O - C - R 1 - C - OR 2 + H 2 O O O ð2:8Þ The reaction of cyclic anhydrides with amines can be different from that with alcohols, because in the case of amines, the amido acid formed during the first step (Eq. 2.9) can close a cycle to give an imide group (Eq. 2.10): R 1 C C O + R 2 NH 2 O O R 1 C - NHR 2 C - OH O O ð2:9Þ R 1 C - NHR 2 C - OH O O R 1 C C N - R 2 + H 2 O O O ð2:10Þ The reaction can be very fast, even at low temperature without a catalyst. Chemistry of Crosslinked Polymer Synthesis 13 b. Some Examples of Polymer Networks Based on Esterification Direct polyesterification can be used to prepare polymer networks for coat- ing applications. In this case it is necessary to increase the reactivity of the system by using anhydrides instead of a diacid (glycero-phthalic or glyptal ‘‘resins,’’ Eq. 2.11) or activated alcohols (powder coatings, Eq. 2.12). Phthalic anhydride + glycerol OH OH OH CO CO O + CH 2 - CH - CH 2 ð2:11Þ Diacid + activated alcohol HO - C - R - C - OH + (CH 2 ) 4 CO N (CH 2 CH 2 OH) 2 CO N (CH 2 CH 2 OH) 2 O O d ð2:12Þ c. Synthesis of Unsaturated Polyesters, UP Oligomers Maleic anhydride [R 1 equals –CH¼CH– in Eq. (2.7); cis isomer] is reacted with aliphatic diols to form low molar mass unsaturated polye- sters, UP. For molar masses higher than 1000 g/mol, products are diluted with a liquid vinyl monomer, most often styrene. This reactive mixture, generally called ‘‘unsaturated polyester, UP resin,’’ can be transformed into crosslinked polymers through a free-radical chain polymerization (see Sec. 2.3). Equations (2.13) and (2.14) (Table 2.4) describe the synthesis of UP oligomers. This is usually carried out in bulk at elevated temperatures. During a first step, the temperature is kept in the range of 60–1308C and is increased up to 160–2208C in a second step. During this second step most of the maleate groups (cis isomer) are isomerized into fumarate groups (trans isomer), Eq. (2.15) (Table 2.4). The degree of isomerization is deter- mined by the esterification conditions (temperature, acid content, catalyst, nature of the diol). It must be carefully controlled because the content of fumarate units determines many properties of UP networks. Since esterification is a reversible process, water must be efficiently removed, especially in the last stages of the reaction. These stages are usually carried out under a vacuum with the difficulty to avoid losses of other volatile reactants such as diols. 14 Chapter 2 Chemistry of Crosslinked Polymer Synthesis 15 TABLE 2.4 Main reactions occurring during UP synthesis Monoester formation: O O + HO - R 2 - OH OO HO - C - CH = CH - C - OR 2 OH O ð2:13Þ Polycondensation (polyesterification): n HO-C-CH = CH-C-OR 2 OH O O HO C-CH = CH-C-OR 2 O H + (n-1) H 2 O n OO ð2:14Þ Maleate–fumarate isomerization: HO OR 2 O H O O H H HO O H H OR 2 OH O ð2:15Þ Ordelt saturation of a monoester by a diol: HO-C-CH = CH-C-OR 2 OH + H O R 2 OH O O HO-C-CH 2 -CH-C-OR 2 OH R 2 - OH O OO ð2:16Þ Ordelt saturation of a monoester by a monoester: HO - C - CH = CH - C - OR 2 OH + HO - C - CH = CH - C - OR 2 OH O O OO HOC - C H = CH - C - O O O HO - C - CH 2 - CH - C - O R 2 OH R 2 O OO ð2:17Þ [...]... CH2-CH-CH2-O C O-CH2-CH-CH2 O CH3 O OH O-CH2-CH-CH2 O CH3 n (b) Diglycidyl ether of bisphenol F, DGEBF CH2 CH2- CH- CH2 -O CH2 O-CH2-CH-CH2 O O n (c) Epoxy novolac, EN O O O O -CH -CH2 CH2 -CH -O O -CH -CH2 CH2 CH2 n (d) Triglycidyl ether of tris(hydroxyphenyl)methane HC O-CH2-CH-CH2 O 3 (e) N,N-diglycidylaniline, DGA N CH2-CH-CH2 O 2 (f) Triglycidylparaaminophenol, TGpAP O O N CH2 -CH -CH2 2 CH2 O-CH2-CH-CH2... C - A O 2: 31Þ Chemistry of Crosslinked Polymer Synthesis TABLE 2. 5 19 Main reactions of isocyanate groups Reaction with water: R1NCO + H2O R1NH2 + CO2 R1NHCOOH (22 .2) carbamic acid Amine addition: R1NCO + R2NH2 R1NHCONHR2 urea (2. 23) R1NHCOOR2 urethane (2. 24) allophanate (2. 25) biuret (2. 26) oxazolidone (2. 27) Alcohol addition: R1NCO + R2OH Urethane addition: R1NCO + R1NHCOOR2 R1NHCONCOOR2 R1 Urea... TABLE 2. 10 networks 33 Main reactions during the synthesis of UF or MF oligomers and  Urea ¼ H2 N–C–NH2 || O melamine ¼ H2N N C NH2 C N N C NH2  Addition (T < 1008C) U - NH2 + CH2O U = H+ or B C or _ U - HN - CH2OH (2. 48) C with O N  Condensation - U - NH - CH2OH + -U -NH2 2 - U - NH - CH2OH -U - NH -CH2O -CH2 -NH -U- - U - NH - CH2 - NH -U - + H2O (2. 49) - U - NH - CH2 O - CH2 - NH -U - + H2O (2. 50)... Triglycidylparaaminophenol, TGpAP O O N CH2 -CH -CH2 2 CH2 O-CH2-CH-CH2 O OH N -CH2 -CH -CH2 2 26 Chapter 2 TABLE 2. 7 Continued (g) N,N,N0 ,N0 -tetraglycidyl-4,40 -methylene dianiline (TGMDA) O O N CH2 -CH -CH2 N -CH2 -CH -CH2 CH2 2 2 (h) 3,4-epoxycyclohexyl methyl 3 0 ,4 0 -epoxy cyclohexane carboxylate O C CH2 O O E -CH -CH2 O O + E -CH -CH2 -N -A OH H k2 E - CH - CH2 - N - A OH 2 2: 38Þ Usually when the concentration of... groups: 28 Chapter 2 TABLE 2. 8 Main diamine hardeners used in epoxy systems (a) Aliphatic  H2N NH CH2 NH2 CH2 2 2 liquid i Diethylene triamine, DETA, i ¼ 1; triethylene tetramine, TETA, i ¼ 2; tetraethylene pentamine, TEPA, i ¼ 3  N-aminoethylpiperazine H2N N (b) Cycloaliphatic CH2 2 NH2 NH2 CH3 liquid  Isophorone diamine, IPD CH3 CH3 CH2 NH2  4,40 -Diamino dicyclohexyl methane, PACM H2N CH2 Tm $... sulfone, DDS H2N SO2 Tm $ 1758C NH2 (d) Latent hardener Tm $ 20 78C  Cyanoguanidine or dicyanodiamide, Dicy H2N C N CN NH2 OH CH2 -CH - CH2 A-N O CH2 -CH - CH2 OH N-E A -NH2 + N-E CH2 -CH - CH2 CH2 -CH - CH2 O OH H A-N CH2 -CH - CH2 O CH2 -CH - CH2 N-E 2: 40Þ d Other Polyaddition Reactions of Epoxy Groups Some of these reactions are listed in Table 2. 9, but they are not as clear as they are described in... R1NHCONHR2 R1NHCONCONHR 2 R1 Epoxy addition: CH2 - CH - R2 R1 - N R1 NCO + R2 - CH - CH2 O C O O Homopolyaddition (with catalyst like triphenylphosphine): O C 1 2 R NCO N-R1 1 R -N uretdione (2. 28) isocyanurate (2. 29) carbodiimide (2. 30) C O Homopolyaddition (base as catalyst): O C 1 3 R NCO 1 N - R1 R -N C C O N O 1 R Homopolycondensation: 2 R1 NCO R1 - N = C = N - R1 + CO2 20 Chapter 2 TABLE 2. 6 Main... cyclization occurs and bismaleimides are obtained (Eq 2. 20) (with chloroform, acetone, or toluene as solvent and acetic anhydride for cyclization) O O O + H2 N - R2 - NH2 2 O O N - R2 - N O + 2H2O O 2: 20Þ The intramolecular reaction (Eq 2. 20) is easier with aromatic diamines than with aliphatic ones Reactions of BMI monomers will be discussed later (Sec 2. 2.7) 2. 2.3 Isocyanate Reactions a Mechanism: An Example... + H2O O  metal ions chelation Cu O O P - O - C - CH = C - CH3 CH3 (2. 62) 38 Chapter 2 TABLE 2. 13 Some synthesis of aceto–acetoxy functional polymers or oligomers, P * (1) Radical chain-growth copolymerization with aceto-acetoxy ethyl methacrylate AAEM CH3 CH3 (2. 63) CH2 = C -COOR + CH2 = C -C -O CH2 -O -C -CH2 -C -CH3 2 O O O (2) Use of diketene O C O CH2 C P -OH + P -O -C - CH2 -C -CH3 O CH2 (2. 64)... aprotic solvents can be used to prepare these monomers/oligomers: Ar1 − ONa + Cl − CH2 Ar2 Ar1 − ONa + Cl Ar2 Ar1 − O − CH2 − Ar2 + NaCl Ar1 − O − Ar2 + NaCl n X − Ar − SH X − Ar−S − H + (n−1) HX 2: 70Þ 2: 71Þ 2: 72 n Ar1 + 2 X − C − Ar2 O AlCl3 Ar2 − C − Ar1 − C − Ar2+ 2HX O 2: 73Þ O 2. 3 CHAIN POLYMERIZATIONS 2. 3.1 Description of the Different Stages a Three Stages An addition chain polymerization . addition: R 1 NCO + R 2 NH 2 R 1 NHCONHR 2 urea (2. 23) Alcohol addition: R 1 NCO + R 2 OH R 1 NHCOOR 2 urethane (2. 24) Urethane addition: R 1 NCO + R 1 NHCOOR 2 R 1 NHCONCOOR 2 R 1 allophanate (2. 25) Urea. DGEBA O C O - CH 2 -CH-CH 2 O CH 2 -CH-CH 2 -O O-CH 2 -CH-CH 2 C CH 3 CH 3 CH 3 CH 3 O n OH (b) Diglycidyl ether of bisphenol F, DGEBF O O-CH 2 -CH-CH 2 O CH 2 - CH- C H 2 -O O-CH 2 -CH- C H 2 CH 2 O n CH 2 OH (c). obtained (Eq. 2. 20) (with chloroform, acetone, or toluene as solvent and acetic anhydride for cyclization). O O O N O OO O 2 + H 2 N - R 2 - NH 2 - R 2 - N + 2 H 2 O 2: 20Þ The intramolecular