Chain Transfer Methods

The relative amounts of the two processes (a) and (b) depend on the nature of the double bond. For example, the process (b) predominates when the double bond is more reactive. Although the method yields graft copolymer mixed with homopolymer, it has found successful industrial applications, as in the production of HIPS mentioned above and ABS copolymer made by copolymerizing styrene and acrylonitrile in the presence of poly( 1,3-butadiene). 

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The hydrogen abstraction from —SH groups is faster than from —OH groups. Hebeish et al. [9] and Misra et al. [10,11] reported the chain-transfer method of initiation of graft copolymerization onto cellulosic substrates with azobisisobutyronitrile (AIBN) and benzoyl peroxide (BPO) as initiators. 

Chiefari, J. andE. Rizzardo, Control ofFree Radical Polymerizationby Chain Transfer Methods, Chap. 12 in Handbook of Radical Polymerization, K. Matyjaszewski and T. P. Davis, eds., Wiley, New York, 2002. 

It is evident that the values of the transfer constants are dependent on the nature both of the attacking radicals and of the transfer agent itself, and that similar effects should be expected during the synthesis of graft copolymers by chain transfer methods. For example, with respect to toluene the chain transfer constant is a little greater for methyl methacrylate radicals than for styrene radicals on the contrary, with respect to halogenated solvents (CC14) the polystyrene radical is much more effective in the removal of a chlorine atom. Vinyl acetate chains are far more effective than either of the other two polymer radicals. 

Chiefari, J. Rizzardo, E. Control of Free-Radical Polymerization by Chain Transfer Methods. In Handbook of Radical Polymerization-, Matyjaszewski, K. Davis, T. P., Eds, Wiley Interscience New York, 2002 pp 629. 

A study on chain transfer grafting of butyl acrylate onto PVC in emulsion following the synthesis of graft copolymers showed that for the chain transfer method, the grafting efficiency does not increase as Flory s chain transfer theory predicted. 

The molecular weight of the polymers obtained depends upon the phosphine used, and the addition of a-olefins can be used to induce chain transfer nevertheless, the system is less susceptible to the a-olefin chain transfer method than the aforementioned Ni catalysts. A major advantage of this catalyst system is its tolerance toward polar groups, enabling its use in the copolymerization of functional derivatives of norbornene and for polymerization in suspension or emulsion. 

Chain transfer to solvent is an important factor in controlling the molecular weight of polymers prepared by this method. The chain-transfer constants for poly(methyl methacrylate) in various common solvents (C) and for various chain-transfer agents are Hsted in Table 10. 

Molecular Weight. PE mol wt (melt index) is usually controlled by reaction temperature or chain-transfer agents. Reaction temperature is the principal control method in polymerization processes with Phillips catalysts. On the other hand, special chemical agents for chain transfer are requited for... 

VEs do not readily enter into copolymerization by simple cationic polymerization techniques instead, they can be mixed randomly or in blocks with the aid of living polymerization methods. This is on account of the differences in reactivity, resulting in significant rate differentials. Consequendy, reactivity ratios must be taken into account if random copolymers, instead of mixtures of homopolymers, are to be obtained by standard cationic polymeriza tion (50,51). Table 5 illustrates this situation for butyl vinyl ether (BVE) copolymerized with other VEs. The rate constants of polymerization (kp) can differ by one or two orders of magnitude, resulting in homopolymerization of each monomer or incorporation of the faster monomer, followed by the slower (assuming no chain transfer). 

Solution Polymerization. This method is not commercially important, although it is convenient and practical, because it provides viscous cements that are difficult to handle. Also, the choice of the solvent is a key parameter due to the high solvent chain-transfer constants for acrylates. 

One chain-end is typically unsaturated due to chain transfer and termination mechanisms. Mol wts can range from several hundred to several million. There is no long-chain branching unless special synthesis methods ate employed. The mol wt distribution is commonly the most probable,... 

The termination of radical polymerization cannot be prevented under normal conditions. This would be possible only in a polymerization initiated in rigid media, assuming that no chain transfer occurs, or if the radicals are trapped, for instance, by precipitation of the polymer during the process of its formation. Both methods have been used, and indeed the termination was considerably slowed down or even prevented permanently. However, such systems are of little value for synthesizing polymers according to a preconceived pattern. 

It has been shown that equivalent information can be obtained by analysis of logfnumbcr chain length distribution) plots (the log CLD method)."4" 7 For the case where termination is wholly by disproportionation or chain transfer, it is possible to show that (eq. 6) applies ... 

Methods used for evaluating transfer constants are the same as for conventional chain transfer. 

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