Copolymerization Systems Involving Reversible Propagation Steps

The influence of reversal of propagation reactions on copolymer composition has been studied for three systems: (1) anionic copolymerization of vinyl mesitylene (M1) and α-methylstyrene (M2) in tetrahydrofuran (THF), (2) radical copolymerization of styrene (M1) and methyl methacrylate (M2) in o-dichlorobenzene (ODCB), and (3) cationic copolymerization of styrene (M1) and α-methylstyrene (M2) in methylene chloride and in bulk. 14C-labeled M2 was used in systems (1) and (3). In each case a transition from “normal” behavior to “abnormal” behavior was observed as the conditions were adjusted to favor depropagation reactions involving the shedding of M2 units. For systems (1) and (2), Lowry's mechanism II, in which only active species terminating in three M2 units are assumed to depropagate, provides a reasonably satisfactory interpretation of the variation of copolymer composition with [M2] at constant feed composition. The following reactivity ratios were determined: System (1) at −78°C in THF: r 1 = 0.29 ± 0.03, r 2 = 0.49 ± 0.02; at 0°C: r 1 = 0.2 ± 0.1, r 2 = 0.72 ± 0.07. System (2) at 132°C in ODCB: r 1 = 0.545 ± 0.006, r 2 = 0.588 ± 0.007. System (3) at −20°C in bulk: r 1 = 0.14 ± 0.06, r 2 = 10.1 ± 1.5.