Disproportionation cross termination, copolymerization

Even when only the terminal monomer unit is considered, radical-radical termination in binary copolymerization involves at least seven separate reactions (Scheme 7.12). There are two homoterminalion processes and one cross termination process to consider. In the case of cross termination, there arc two pathways for disproportionation. There are then at least three pieces of information to be gained ... 

The reaction between the PMMA and PS model radicals (4 and 5, generated from the unsymmetrical azo-compound 3) has been studied as a model for cross-termination in MMA-S copolymerization (Scheme 7.13). The value for td tc(90 C) for the cross reaction was 0.56. In disproportionation, transfer of hydrogen from the PS model 5 to the PMMA radical 4 was ca 5.1 times more prevalent than transfer in the reverse direction (from 4 to 5). The value of td/ tc(90°C) is between those of Atd//rtc 90°C) for the self-reaction of these radicals... 

Cross termination n. In free radical copolymerization, termination by reaction of two radicals terminated by monomer units of the opposite type, i.e., termination, by combination or disproportionation with rate constant /cab Crosstermination is often favored over termination by reaction between two like radicals due to polar effects. 

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