synopsisThe radical-induced grafting of styrene onto polybutadiene (PBD) in benzene solution a t 6OoC has been studied. Provided the PBD concentration is kept below about 1.0 monomermole/l. the polymerization of styrene shows normal kinetic behavior. The proportion of polystyrene incorporated as graft is independent of the initiator (benzoyl peroxide) concentration but increases as the ratio of PBD to styrene in the reaction medium increases.It appears that the reaction leading to graft formation is direct attack of initiator radicals on the rubber, probably by a hydrogen-abstraction reaction.Azobisisobutyronitrile produces no graft copolymer in this system.
A kinetic analysis of the grafting reaction has shown that, provided rubber radicals are not involved in termination reactions, the observed normal kinetics of styrene polymerization are to be expected. An expression relating the graft fraction with the rubber and monomer concentrations has been derived and its validity verified from the results reported in Part I. The observation that the molecular weight of the ungrafted PBD falls during the reaction has been explained on a theoretical basis.
The free radical polymerization of styrene in various solvents has been studied. Using the rotating sector technique at 25°C rate coefficients for termination kt and propagation kp were determined. The linear decrease of the former with decreasing medium fluidity shows that termination is diffusion-controlled. Values of kp decrease as the solvents used are increased in concentration. A similar trend in kp is observed when polymerization is initiated thermally at 60°C. The solvent effect on kt is similar to that previously recorded for methyl methacrylate, but solvents such as benzonitrile and diethyl malonate diminish k, of styrene while they enhance kp for methyl methacrylate.
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