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 degrees C) and in strong pi-electron donor solvents (1,3-dimethoxybenzene and 1,2,4-trimethoxybenzene) at 60-145 degrees 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) much less than 1) show that alternating copolymers were produced. These results are best explained if the copolymerizations proceed through highly polarized radical transition states rather than styrene-maleic anhydride charge-transfer complexes. Polar solvents stabilize highly polarized radical transition states thereby promoting alternating polymerization at higher temperatures than can be observed in nonpolar solvents. Donor solvents might also stabilize the electrophilic maleic anhydride radical more than the nucleophilic styryl radical thereby tending to depress k(21) relative to k(12). The copolymerizations were also studied in decalin (a nonpolar, nondonor solvent) at 60-145 degrees C. At lower temperatures alternating copolymers were combined. However, at temperatures >120 degrees C, random copolymers were obtained in decalin. Thus, in decalin, styrene-maleic 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 degrees 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) CCl4/CDCl3 using a technique based on changes in the NMR chemical shifts of the maleic anhydride protons.