Chain transfer with monomer

Nonlinear structures may arise in vinyl polymerizations through chain transfer with monomer or with previously formed polymer molecules, but such processes usually occur to an extent which is scarcely significant. A more common source of nonlinearity in the polymerization of a 1,3-diene is the incorporation in a growing chain of one of the units of a previously formed polymer molecule. The importance of both branching by chain transfer and cross-linking by addition of a polymer unit increases with the degree of conversion of monomer to polymer. 

Molecular weight is regulated to some degree by control of the chain transfer with monomer and with the cocatalyst, plus internal hydride transfer. However, hydrogen is added in the commercial processes to terminate the reaction because many systems tend to form longer chains beyond the acceptable balance between desired processing conditions and chain size. 

Operating at low temperature (<80°), ki can be neglected with respect to k Pc st Therefore, the relationship (29) is equal to the ratio between the number of polymeric chains interrupted by the chain transfer processes, depending on the catalyst concentration, and the number of polymeric chains interrupted by the chain transfer with monomer. 

The similarity between polymers and homologous low molecular weight compounds may be sometimes disturbed on account of the presence of some highly reactive sites, e.g. unsaturated end groups due to chain termination by disproportionation (a) or to chain transfer with monomer (6) ... 

Our kinetic work (10) showed that the small molecule radical produced by chain transfer with monomer had to be a stable radical. This was confirmed in the present paper by analysis of the isotope effect on the bulk polymerization rates. The isotope effect on molecular weights and rates unequivocally showed that almost 100% of the chain transfer involved the vinyl hydrogen. There is some evidence in the literature to support the idea of a stable vinyl radical. Phenyl acetylene acts as a retarder when copolymerized with styrene or methyl methacrylate (25). Thus the phenyl vinyl radical is very stable compared to the growing styryl or methacrylyl radical. 

For all catalysts studied under the usual conditions of slurry polymerization of ethylene (75-80 °C, [C HJ = 0.1-0.6 mol/1) chain transfer with monomer predominates. This is directly illustrated by the independence of the polyethylene molecular mass ofmonomer concentration in the case of TiCIj and TiCl4/MgCl2 - - AlEtg... 

Unlike many other polymerization reactions, chain transfer with monomer cannot occur. However, many compounds (proton donors in particular) function as chain-transfer agents when added to the system. In general, the mechanism of the chain-transfer reaction can be depicted this way ... 

Chain Transfer In the absence of any externally added transfer agent, three transfer reactions [Eq. (9.9)-(9.12)] may be considered, viz., chain transfer with monomer, chain transfer with aUcylaluminum, and spontaneous chain transfer. 

Since chemisorbed ethylene disappears by initiation reaction with adsorbed hydrogen, pro-pagation reaction with surface-bound polymer chain, and polymer chain transfer with monomer (Fig. 9.8), one may write (Friedlander and Oita, 1957) ... 

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