Polymerizations of Acrylic and Methacrylic Esters

Free-radical bulk polymerizations of acrylate esters exhibit rapid rate accelerations at low conversions. This often results in formation of some very high molecular weight polymer and some cross-linked material. The cross-linking is a result of chain transferring by abstractions of labile tertiary hydrogens from already formed “dead” polymeric chains [204]. Eventually, termination by combination of the branched radicals leads to cross-linked structures. Addition of chain transferring agents, like mercaptans (that reduces the length of the primary chains), helps prevent gel formation. There are no labile tertiary hydrogens in methacrylic esters. The growing methacrylate radicals are still capable of abstracting hydrogens from the a-methyl groups. Such abstractions, however, require more energy and are not an important problem in polymerizations of methacrylic esters [205]. Nevertheless, occasional formation of cross-linked poly(alkyl methacrylate)s does occur. This is due to chain transferring to the alcohol moiety [206, 210]. The termination reaction in free-radical polymerizations of the esters of acrylic and methacrylic acids takes place by recombination and by disproportionation [206, 207]. Methyl methacrylate polymerizations, however, terminate at 25C predominantly by disproportionation [205]. Oxygen inhibits free-radical polymerization of a-methyl methacrylate [208]. The reaction with oxygen results in formation of low molecular weight polymeric peroxides that subsequently decom pose to formaldehyde and methyl pyruvate [210]:

Oxygen is less effective in inhibiting polymerizations of acrylic esters. It reacts 400 times faster with the methacrylic radicals than with the acrylic ones. Nevertheless, even small quantities of oxygen affect polymerization rates of acrylic esters [216]. This includes photopolymerizations of gaseous ethyl acrylate that are affected by oxygen and by moisture [217]. Acrylic and methacrylic esters polymerize by free-radical mechanism to atactic polymers. The sizes of the alcohol portions of the esters determine the Tg values of the resultant polymers. They also determine the solubility of the resultant polymers in hydrocarbon solvents and in oils.

Solvents influence the rate of free-radical homopolymerization acrylic acid and its copolymerization with other monomers. Hydrogen bonding solvents slow down the reaction rates [219]. Due to electron withdrawing nature of the ester groups, acrylic and methacrylic ester polymerize by anionic but not by cationic mechanisms. Lithium alkyls are very effective initiators of a-methyl methacrylate polymerization yielding stereospecific polymers [213]. Isotactic poly(methyl methacrylate) forms in hydrocarbon solvents [214]. Block copolymers of isotactic and syndiotactic poly(methyl methacry late) form in solvents of medium polarity. Syndiotactic polymers form in polar solvents, like ethylene glycol dimethyl ether, or pyridine. This solvent influence is related to Lewis basicity [215] in the following order:

tetrahydrofuran>tetrahydropyran>dioxane>diethyl ether

Furthermore, polymerizations in solvating media, like ethylene glycol dimethyl ether, tetrahydro furan, or pyridine, using biphenylsodium or biphenyllithium yield virtually monodisperse syndiotactic poly(methyl methacrylate) [216]. The nature of the counterion in anionic polymerizations of methyl methacrylate in liquid ammonia with alkali metal amide or alkali earth metal amide catalysts is an important variable [217]. Lithium and calcium amides yield high molecular weight polymers, though the reactions tend to be slow. Sodium amide, on the other hand, yields rapid polymerizations but low molecular weight polymers. Polymers formed with sodium amide, however, have a narrower molecular weight distribution than those obtained with lithium and calcium amides. Calcium amide also yields high molecular weight polymers from ethyl acrylate and methyl methacrylate monomers in aromatic and aliphatic solvents at temperatures from 8 to 110C. When, however, tetrahydrofuran or acetonitrile is used as solvents much lower molecular weight products form [218]. Products from anionic polymerizations of methyl methacrylate catalyzed by Grignard reagents (RMgX) vary with the nature of the R and X groups, the reaction temperature, and the nature of the solvent [219–221]. Secondary alkyl Grignard reagents give the highest yields and the fastest rates of the reactions. Isotacticity of the products increases with the temperature. When anion-radicals from alkali metal ketyls of benzophenone initiate polymerizations of methyl methacrylate, amorphous polymers form at temperatures from 78 to +65C[222]. Sodium dispersions in hexane yield syndiotactic poly(methyl methacrylate) [223]. A 60–65% conversion is obtained over a 24-h period at a reaction temperature of 20–25C. Lithium dispersions [224], butyllithium [203], and Grignard reagents [225, 226] yield crystalline isotactic poly(t-butyl acrylate). The reactions take place in bulk and in hydrocarbon solvents. Isotactic poly(isopropyl acrylate) forms with Grignard reagents [226, 227]. Coordination polymerizations of methyl methacrylate with diethyliron–bipyridyl complex in nonpolar solvents like benzene or toluene yield stereo block polymers. In polar solvents, however, like dimethylformamide or acetonitrile, the products are rich in isotactic placement [229]. There are many reports in the literature on polymerizations of acrylic and methacrylic esters with Ziegler–Natta catalysts [230–233]. The molecular weights of the products, the microstructures, and rates of the polymerizations depend upon the metal alkyl and the transition metal salt used. The ratios of the catalyst components to each other are also important [234, 235]. In 1992 Yasuda et al. [236, 237] reported that organ lanthanide complexes of the type Cp*₂Sm-R (where Cp* is pentamethyl cyclopentadienyl, and R is either an alkyl, alkylaluminum or a hydride) initiate highly syndiotactic, living polymerizations of methacrylates. It was also reported that lanthanide complexes such as Cp*2Yb (THF)_1–3, Cp*₂Sm (THF)₂, and (indenyl)₂Yb (THF)₂ can also initiate polymerizations of methyl methacrylate [238]. Although very low initiator efficiencies were observed, these were living polymerizations. The polymers that formed had the dispersity of 1.1 and were high in syndiotactic sequences. Novak and Boffa, in studying lanthanide complexes, observed an unusual facile organometallic electron transfer process takes place that generates in situ bimetallic lanthanide (III) initiators for polymerizations of methacrylates [239]. They concluded that methyl methacrylate polymerizations initiated by the Cp*₂Sm complexes occur through reductive dimerizations of methyl methacrylate molecules to form “bisinitiators” that consists of two samarium (III) enolates joined through their double bond terminally [239]. Their conclusion is based on the tendency of Cp*₂Sm complexes to reductively couple unsaturated molecules:

Montei and coworkers [240] reported that Nickel complexes [(X, O) NiR (PPh3)] (X = N or P), designed for the polymerization of ethylene, are effective for home- and copolymerization of butyl acrylate, methyl methacrylate, and styrene. Their role as radical initiators was demonstrated from the calculation of the copolymerization reactivity ratios. It was shown that the efficiency of the radical initiation is improved by the addition of PPh₃ to the nickel complexes as well as by increasing the temperature. The dual role of nickel complex as radical initiators and catalysts was exploited to succeed in the copolymerization of ethylene with butyl acrylate and methyl methacrylate. Multiblock copolymers containing sequences of both ethylene and polar monomers were thus prepared.

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الآخبار الصحية

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متى يصبح محيط الخصر "مؤشر خطر"؟

دراسة تكشف دور "العوامل الوراثية" في تحديد عمر الإنسان

رائحة كريهة في الجسم.. "علامة خفية" أخطر مما تتصور

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مايكروسوفت تكشف عن الجيل الثاني من شرائحها للذكاء الاصطناعي

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المزيد

مواضيع ذات صلة

Thermoplastic and Thermoset Acrylic Resins

Acrylic Elastomers

Polymers of Acrylic and Methacrylic Esters

ABS Resins

High-Impact Polystyrene

Polymers from Substituted Styrenes

Polystyrene Prepared by Ionic Chain-Growth Polymerization

Preparation of Polystyrene by Free-Radical Mechanism

GR-N Rubber

GR-S Rubber

Cyclopolymerization of Conjugated Dienes

Special Polymers from Dienes