Chain-growth polymerization copolymerization

Free-radical polymerization is the most widely used process for polymer synthesis. It is much less sensitive to the effects of adventitious impurities than ionic chain-growth reactions. Free-radical polymerizations are usually much faster than those in step-growth syntheses, which use different monomers in any case. Chapter 7 covers emulsion polymerization, which is a special technique of free-radical chain-growth polymerizations. Copolymerizations are considered separately in Chapter 8. This chapter focuses on the polymerization reactions in which only one monomer is involved. 

We can create crosslinks during chain growth polymerization by copolymerizing dienes with vinyl monomers. When the two vinyl functions of the diene are incorporated into separate chains, a crosslink is formed. This process is shown in Fig. 2.18. When we use a low concentration of dienes, we produce a long chain branched polymer, while high concentrations of dienes create a highly crosslinked polymer network... 

Comparison of the Two Reactions Step-Growth Polymerization in More Detail Making PET in the Melt Interfacial Poly condensation Chain-Growth Polymerization in More Detail Free Radical Chain Polymerization Going One Step Better Emulsion Polymerization Copolymerization Ionic Chain Polymerization It Lives ... 

In the present chapter, the basic principles of chain polymerizations in which the reactive centers are free radicals will be considered in detail, focusing on the polymerization reactions in which only one monomer is involved. Copolymerizations involving more than one monomer are considered separately in Chapter 7. Chain-growth polymerizations in which the active centers are ionic are reviewed in Chapter 8. 

To address polymer network formation from nonlinear chain-growth polymerization (or copolymerization), kinetic methods are more appropriate [23, 75-83], Some of the most successful kinetic models to address this type of system are based on the method of moments [23, 75-77, 79, 80, 82, 84], Some divergence problems at the vicinity of the gelation point are common with the method of moments, although there are practical ways to avoid this situation [80], A more refined kinetic method to address the issue of modeling the dynamics of gelation in... 

It can be formed by suspension polymerization. One procedure is to carry out the reaction in an aqueous solution of lithium bromide at -25 °C with magnesium carbonate as the suspending agent. No initiator is added and the reaction takes about 20 hours. Because the reaction is inhibited by hydroquinone and accelerated by ultraviolet light, it is believed to take place by a free-radical mechanism. Whether it is chain-growth polymerization, however, is not certain. A1 1 copolymer is always formed regardless of the composition of the monomer feed, and the copolymerization takes place only at low temperatures. At elevated temperatures, however, cyclic oxazetidines form instead ... 

Three events are involved with chain-growth polymerization catalytic initiation, propagation, and termination [3], Monomers with double bonds (—C=C—R1R2—) or sometimes triple bonds, and Rj and R2 additive groups, initiate propagation. The sites can be anionic or cationic active, free-radical. Free-radical catalysts allow the chain to grow when the double (or triple) bonds break. Types of free-radical polymerization are solution free-radical polymerization, emulsion free-radical polymerization, bulk free-radical polymerization, and free-radical copolymerization. Free-radical polymerization consists of initiation, termination, and chain transfer. Polymerization is initiated by the attack of free radicals that are formed by thermal or photochemical decomposition by initiators. When an organic peroxide or azo compound free-radical initiator is used, such as i-butyl peroxide, benzoyl peroxide, azo(bis)isobutylonitrile, or diazo- compounds, the monomer s double bonds break and form reactive free-radical sites with free electrons. Free radicals are also created by UV exposure, irradiation, or redox initiation in aqueous solution, which break the double bonds [3]. 

This chapter discusses the reaction engineering of chain-growth polymerization. In order to form polymers of specified properties, we observe that reactor temperature is a very important variable. To find this, the energy balance equation must be solved, along with mole balance relations of various species. In the study of copolymers, the quantities of practical interest are the relative distributions of the monomers on polymer chains and the overall rates of copolymerization. With these, it is possible to carry out the reactor design. 

An example of a commercial semibatch polymerization process is the early Union Carbide process for Dynel, one of the first flame-retardant modacryhc fibers (23,24). Dynel, a staple fiber that was wet spun from acetone, was introduced in 1951. The polymer is made up of 40% acrylonitrile and 60% vinyl chloride. The reactivity ratios for this monomer pair are 3.7 and 0.074 for acrylonitrile and vinyl chloride in solution at 60°C. Thus acrylonitrile is much more reactive than vinyl chloride in this copolymerization. In addition, vinyl chloride is a strong chain-transfer agent. To make the Dynel composition of 60% vinyl chloride, the monomer composition must be maintained at 82% vinyl chloride. Since acrylonitrile is consumed much more rapidly than vinyl chloride, if no control is exercised over the monomer composition, the acrylonitrile content of the monomer decreases to approximately 1% after only 25% conversion. The low acrylonitrile content of the monomer required for this process introduces yet another problem. That is, with an acrylonitrile weight fraction of only 0.18 in the unreacted monomer mixture, the low concentration of acrylonitrile becomes a rate-limiting reaction step. Therefore, the overall rate of chain growth is low and under normal conditions, with chain transfer and radical recombination, the molecular weight of the polymer is very low. 

Polylactides, 18 Poly lactones, 18, 43 Poly(L-lactic acid) (PLLA), 22, 41, 42 preparation of, 99-100 Polymer age, 1 Polymer architecture, 6-9 Polymer chains, nonmesogenic units in, 52 Polymer Chemistry (Stevens), 5 Polymeric chiral catalysts, 473-474 Polymeric materials, history of, 1-2 Polymeric MDI (PMDI), 201, 210, 238 Polymerizations. See also Copolymerization Depolymerization Polyesterification Polymers Prepolymerization Repolymerization Ring-opening polymerization Solid-state polymerization Solution polymerization Solvent-free polymerization Step-grown polymerization processes Vapor-phase deposition polymerization acid chloride, 155-157 ADMET, 4, 10, 431-461 anionic, 149, 174, 177-178 batch, 167 bulk, 166, 331 chain-growth, 4 continuous, 167, 548 coupling, 467 Friedel-Crafts, 332-334 Hoechst, 548 hydrolytic, 150-153 influence of water content on, 151-152, 154... 

The most industrially significant polymerizations involving the cationic chain growth mechanism are the various polymerizations and copolymerizations of isobutylene. In fact, about 500 million pounds of butyl rubber, a copolymer of isobutylene with small amounts of isoprene, are produced annually in the United States via cationic polymerization [126]. The necessity of using toxic chlorinated hydrocarbon solvents such as dichloromethane or methyl chloride as well as the need to conduct these polymerizations at very low temperatures constitute two major drawbacks to the current industrial method for polymerizing isobutylene which may be solved through the use of C02 as the continuous phase. 

Block copolymer synthesis from living polymerization is typically carried out in batch or semi-batch processes. In the simplest case, one monomer is added, and polymerization is carried out to complete conversion, then the process is repeated with a second monomer. In batch copolymerizations, simultaneous polymerization of two or more monomers is often complicated by the different reactivities of the two monomers. This preferential monomer consumption can create a composition drift during chain growth and therefore a tapered copolymer composition. 

Nearly all synthetic polymers are synthesized by the polymerization or copolymerization of different "monomers." The chain growth process may involve the addition chain reactions of unsaturated small molecules, condensation reactions, or ringopening chain-coupling processes. In conventional polymer chemistry, the synthesis of a new polymer requires the use of a new monomer. This approach is often unsatisfactory for Inorganic systems, where relatively few monomers or cyclic oligomers can be Induced to polymerize, at least under conditions that have been studied to date. The main exception to this rule is the condensation-type growth that occurs with inorganic dl-hydroxy acids. 

An unsaturated polyester resin consists of a linear polyester whose chain contains double bonds and an unsaturated monomer such as styrene that copolymerizes with the polyester to provide a cross-linked product. The most common unsaturated polyester is made by step growth polymerization of propylene glycol with phthalic and maleic anhydrides. Subsequent treatment with styrene and a peroxide catalyst leads to a solid, infusible thermoset. 

Direct evidence for a mechanism involving a single mode of chain growth as depicted in Scheme 1 came from our studies on the copolymerization reaction in alcoholic solvents35). The mechanism, as outlined in Scheme 1, involves the formation of Pd-acyl species as intermediates at every other step in the propagation sequence. Since the formation of esters through the reaction of transition metal-acyls with alcohols is well-precedented, it should be possible to intercept the propagation sequence if the polymerization was carried out in the presence of alcohols, Eq. (13). 

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