Styrene–maleic anhydride copolymerization through polar transition states in polar solvents

Radical copolymerization of styrene (M 1 ) and maleic anhydride (M 2 ) was performed in highly polar, oxygen lone-pair donor solvents (N,N-dimethylformamide, N-methylformamide) at high temperatures (135 and 145°C) and in strong π-electron donor solvents (1,3-dimethoxybenzene and 1,2,4-trimethoxybenzene) at 60-145°C. The corresponding reactivity ratios were evaluated by Fineman-Ross, Kelen-Tudos, and error-in-variables model (RREVM) methods. The reactivity ratios obtained (r 1 r 2 120°C, random copolymers were obtained in decalin. Thus, in decalin, styrenemaleic anhydride charge-transfer complexes are present at lower temperatures where they might play a role in alternation. In decalin the solvation of the polar transition states (relative to the macroradicals and monomers) does not favor copolymerization through such dipolar structures. Thus, at low temperatures growing chains could add to charge-transfer complexes. However, at temperatures above 120°C the concentration of charge-transfer complexes becomes so small that they can no longer be kinetically important contributions to the copolymerization. Thus, random copolymers with high styrene mole ratios are produced in decalin. The equilibrium constants for complex formation between maleic anhydride and both 1,3-dimethoxybenzene and 1,2,4-trimethoxybenzene were determined in 1 : 1 (v/v) CCI 4 /CDCl 3 using a technique based on changes in the NMR chemical shifts of the maleic anhydride protons. © 1995 John Wiley & Sons, Inc.