Chain transfer, to monomer and

Chain transfer to monomer and to other small molecules leads to lower molecular weight products, but when polymerisation occurs ia the relative absence of monomer and other transfer agents, such as solvents, chain transfer to polymer becomes more important. As a result, toward the end of batch-suspension or batch-emulsion polymerisation reactions, branched polymer chains tend to form. In suspension and emulsion processes where monomer is fed continuously, the products tend to be more branched than when polymerisations are carried out ia the presence of a plentiful supply of monomer. 

Assuming that the number average degree of polymerization (DP ) is determined by chain transfer to monomer and assuming unimolecular termination relative to propagation (i.e., chain breaking due to solvent, polymer, impurities are absent), the simple Mayo equation55 ... 

Living" carbocationic polymerizations are most difficult to achieve mainly because of chain transfer to monomer and termination processes both of which frequently occur in carbocationic polymerizations. It has recently been demonstrated (JL) that "quasiliving" polymerization of a-methylstyrene (aMeSt) can be achieved by slow and continuous monomer addition and that the number-average molecular weight (Mn) of PaMeSt increases linearly with the weight of added monomer. A theory for quasiliving polymerizations has been developed (2). 

Bryant has calculated the changes in free energy for various reaction steps of the polymerization of tetrafluoroethylene. He concluded (1) that the initiation and propagation are about twice as favorable for tetrafluoroethylene as the analogous reactions for ethylene, (2) that termination by combination is more favorable than disproportionation, and (3) that chain-transfer to monomer and to polymer are less likely than the combination of radicals. 

One of the most useful features of living polymerizations, which proceed in the absence of chain transfer to monomer and irreversible termination, is the ability to prepare block copolymers. Compared with living anionic polymerization the development of living cationic polymerization is rather re-... 

For example, the number average degree of polymerization resulting from a chain polymerization which involves both chain transfer to monomer and chain transfer to a chain transfer agent. A, is described by Eq. (7). 

This expression for contains two unknown rate constants, k and ks. The Arrhenius coefficients for these rate constants were determined using the 140 °C conversion data from Figure 7.6. The parameters were estimated for both the case of chain transfer to monomer, and again for chain transfer to DH. Given that the two chain transfer models differ only in their predictions of Mw and that the fit was against conversion data, the optimum Arrhenius constants for both cases were the same ... 

The rate of radical production by chain transfer to monomer and transfer agent can be expressed as... 

Vinyl groups are formed by chain transfer to monomer and j8—hydride transfer, that is, the equivalent of Eqs. (9.9)-(9.11). The formation of vinylidene and trans-vinylidene end groups is less clear. One possibility is participation of vinyl end groups in chain transfer analogous to Eq. (9.10). Transfer to the methylene and methine protons of vinyl end groups would generate frans-vinylene and vinylidene end groups, respectively. 

The effect of unwanted reactions may be compensatory. For example, both chain transfer to monomer and initiation by H20 increase the number of macromolecules (N) and give rise to lower than theoretical molecular weights whereas slow initiation broadens MWD, since N is smaller than the initiator concentration I0 and not constant. 

Section 4.6.2 illustrates the experimental procedures that have recently been applied toward the study of high-pressure free-radical polymerization processes. Section 4.6.3 presents results of propagation, termination, chain-transfer (to monomer and to polymer), and P-scission rate coefficients for ethene homopolymerization. Recent results from experiments and modeling investigations into high-pressure copolymerizations (with ethene being one of the monomers) are reported in Section 4.6.4, together with information on homopolymerization rate coefficients of the comonomer species. 

Where Cat, M, A , P are the catalyst, monomer, growing chain (active centre), and inactive macromolecule respectively k , and k, are rate constants of the initiation reactions, chain transfer to monomer, and chain termination respectively. 

Two reaction loci are considered, the polymer-rich dispersed phase and the C02-rich continuous phase. A kinetic scheme typical of free-radical reactions and including initiation, propagation, terminations, and chain transfer to monomer and to polymer is applied to each phase. 

In general, the free-radical polymerization of vinyl monomers includes chain initiation, propagation, chain transfer to monomer and bimolecular termination reactions. However, there is strong evidence that, in the free-radical polymerization of VCM, some reactions (e.g., chain transfer to monomer, formation of short- and long-chain branches, etc.) involve complex kinetic mechanisms [44]. In fact, the presence of chloromethyl and ethyl short-chain branches in PVC validates the conclusion that the propagation reactions involve several types of radicals [46]. Figures 4.8—4.10 show in detail the mechanisms leading... 

A comprehensive kinetic mechanism is proposed to describe the combined chemical and thermal free-radical polymerization of styrene. Thus, besides the commonly employed reactions (e.g., chemical initiation, propagation and termination), thermal initiation and chain transfer to monomer and to Diels-Alder adduct reactions are included. In particular, the so-called AH thermal initiation mechanism of Mayo comprises a reversible Diels-Alder dimerization of styrene to form l-phenyl-1,2,3,9-tetrahydronaphtalene (AH), the formation of a styryl (m) and a 1-phenyltetralyl radical... 

To identify the mechanism(s) responsible for loss of surface-tethered radicals in SI-PMP, several irreversible termination reactions that can result into the permanent loss of surface-tethered radicals, including (a) bimolecular termination, (b) chain transfer to monomer, (c) chain transfer to dithiocarbamyl radical, (d) chain transfer to an adjacent polymer chain, and (e) chain transfer to solvent, were considered by Rahane et al. in their development of a kinetic model to describe SI-IMP [81]. The decrease in the concentration of surface-tethered radicals by chain transfer to monomer and by bimolecular termination reactions is captured by Equation 12.2 ... 

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