Chain Transfer and Copolymerization

In the presence of a dissolved polymer, the radical polymerization of a monomer by thermal decomposition of an initiator results in a mixture of homopolymerization and graft polymerization [Brydon et ah, 1973, 1974 Ludwico and Rosen, 1975, 1976 Pham et al., 2000 Russell, 2002]. Polymer radicals (XXX), formed by chain transfer between the propagating radical and polymer, initiate graft polymerization of styrene. The product (XXXI) consists of polystyrene grafts on the 1,4-poly-1,3-butadiene backbone. Polymer radicals are also formed 

For polymers containing double bonds as in 1,4-poly-l,3-butadiene, graft polymerization also involves copolymerization between the pol3mierizing monomer and the double bonds of the polymer in addition to grafting initiated by chain transfer. The relative amounts of the two 

Although this method yields a mixture of homopolymer and graft copolymer, and probably also ungrafted backbone polymer, some of the systems have commercial utility. These are high-impact polystyrene (HIPS) [styrene polymerized in the presence of poly(l,3-buta-diene)], ABS and MBS [styrene-acrylonitrile and methyl methacrylate-styrene, respectively, copolymerized in the presence of either poly(l,3-butadiene) or SBR] (Sec. 6-8a). 

The chain transfer to polymer process that produces long-chain branching is also a graft polymerization process (Sec. 3-6d). 

Polymer radicals can also be produced by the irradiation of a polymer-monomer mixture with ionizing radiation. Thus, the interaction of ionizing radiation with polyethylene-styrene produces radical centers on polyethylene, and these initiate graft polymerization of st5rene to produce poly(ethylene-gra/f-styrene) [Rabie and Odian, 1977], 

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The basic Hammett scheme often does not offer a perfect correlation and a number of variants on this scheme have been proposed to better explain reactivities in radical reactions.-0 However, none of these has achieved widespread acceptance. It should also be noted that linear free energy relationships are the basis of the Q-e and Patterns of Reactivity schemes for understanding reactivities of propagating species in chain transfer and copolymerization. 

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. 

The and e values of the aHyl group in DAP have been estimated as 0.029 and 0.04, respectively, suggesting that DAP acts as a fairly typical unconjugated, bifunctional monomer (42). Cyclization affects copolymerization, since cyclized radicals are less reactive in chain propagation. Thus DAP is less reactive in copolymerization than DAIP or DATP where cyclization is stericaHy hindered. Particular comonomers affect cyclization, chain transfer, and residual unsaturation in the copolymer products. DiaHyl tetrachloro- and tetrabromophthalates are low in reactivity. 

A living cationic polymeriza tion of isobutylene and copolymeriza tion of isobutylene and isoprene has been demonstrated (22,23). Living copolymerizations, which proceed in the absence of chain transfer and termination reactions, yield the random copolymer with narrow mol wt distribution and well-defined stmcture, and possibly at a higher polymerization temperature than the current commercial process. The isobutylene—isoprene copolymers are prepared by using cumyl acetate BCl complex in CH Cl or CH2CI2 at —30 C. The copolymer contains 1 8 mol % trans 1,4-isoprene... 

Chain transfer is kinetically equivalent to copolymerization. The Q-e and Patterns of Reactivity schemes used to predict reactivity ratios in copolymerization (Section 7.3.4) can also be used to predict reactivities (chain transfer constants) in chain transfer and the same limitations apply. Tabulations of the appropriate parameters can be found in the Polymer Handbook 3 ... 

In this copolymerization, most termination is by chain transfer and most chains are initiated by transfer agent-derived radicals. The thiyl radicals generated from the transfer agent react faster with S than they do with acrylate esters (Scheme 7.20). 

Spontaneous copolymerization of cyclopentene (CPT) with sulfur dioxide (SOt) suggests the participation of a charge transfer complex in the initiation and propagation step of the copolymerization. The ESR spectrum together with chain transfer and kinetic studies showed the presence of long lived SOg radical. Terpolymerization with acrylonitrile (AN) was analyzed as a binary copolymerization between CPT-SOt complex and free AN, and the dilution effect proved this mechanism. Moderately high polymers showed enhanced thermal stability, corresponding to the increase of AN content in the terpolymer. 

Metallocene-catalyzed Z-N polymerization is finding increased use on an industrial scale. One application is the production of linear low-density polyethylene (mLLDPE),99 which is a linear polymer with short branches incorporated deliberately at various points along the chain. Short branches are produced by Z-N copolymerization of ethene with 1-butene, 1-pentene, and 1-hexene rather than through radical mechanisms of chain transfer and backbiting. Thus, the process is... 

Olaj et al. [63] expand the Mayo mechanism focusing on the chemistry of the dimer intermediate. They performed UV spectroscopic measurements (315-365 nm) on polymerizing styrene and presented evidence to support the formation of two stereoisomers of the Mayo dimer (DHa and DHb). They suggest that both isomers are consumed during styrene polymerization. Possible consumption pathways are copolymerization, chain transfer, and formation of initiating radicals by MAH with monomer. They believe that only the axial phenyl isomer DHa is capable of generating initiating radicals by reaction with monomer. 

The NHC were chosen as supporting ligands as a result of comparisons with previously described catalytic systems based on dicationic phosphine or sterically encumbered P-diimine. The phosphine and p-diimine systems prevent chain transfer and termination processes. By analogy, it was speculated that mutually c/s-chelating NHC, stabilized by sterically demanding aryl substituents, would help promote the copolymerization of ethylene and carbon monoxide. Unfortunately, mesityl substituents on nitrogen atoms were the only... 

Wang Y., Li A.L., Liang LL., Lu J., Reversible addition-fragmentation chain transfer radical copolymerization of beta-pinene and methyl acrylate, Eur. Polym. J., 42(10), 2006, 2695-2702. 

Copolymerizations involving dienes such as the copolymerization of isobutylene with isoprene are important from the industrial point of view (3,4). Isoprene acts as a strong chain-transfer and terminating agent in the carbocationic polymerization of isobutylene (161), and at high concentration it leads to a cross-linked, insoluble product (180). Because of the limited composition range available for analysis, determination of the reactivity ratios in this system is rather difficult. Reactivity ratios published for the isobutylene/isoprene system are listed in Table 7. 

Living polymerization is a chain polymerization from which chain transfer and chain termination are absent [96IUP]. Also, the rate of chain initiation is much larger than the rate of chain propagation. The polymers have a very low dispersity and the molar masses are predetermined. A further advantage is the control over the end groups, which allow the synthesis of block copolymers (living copolymerization). 

Haddleton, D. M., et al. (1997). Identifying the nature of the active species in the polymerization of methacrylates inhibition of methyl methacrylate homopolymerizations and reactivity ratios for copolymerization of methyl methacrylate/n-butyl methacrylate in classical anionic, alkyUithium/trialkylaluminum-initiated, group transfer polymerization, atom transfer radical polymerization, catalytic chain transfer, and classical free radical polymerization. Macromolecules, 30(14) 3992-3998. 

Using typical parameter values for styrene homopolymerization at 80°C, reaction times of the order of magnitude of 100 h are needed to have = 0.05. Even if it can be shown that low polydispersity values can be achieved also at higher values (>0.2-0.3) [72, 73], these values cannot be accepted in copolymerization. While increased reaction temperatures and thus propagation rates generally should promote smaller dead chain contents [compare Eq (55)], in the case of styrenic copolymers this leads to limited improvements only, since undesired side reactions negatively affecting the polymer quality (such as chain transfer and thermal initiation) become more and more important. 

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