Thermal polymerizations of alkali 4-(2-bromoethyl)benzoates (2-BEBAs) were investigated. The polymerization of the lithium salt at 220 degrees C for 2 h under reduced pressure in bulk, followed by esterification, produced poly(methyl 4-vinylbenzoate), having a number-average molecular weight ((M) over bar(n)) of 9500 in a 54% yield. Thus, elimination of hydrogen bromide to form a double bond occurred, followed by vinyl polymerization. In contrast, polymerization of the potassium salt at 200 degrees C for 2 h afforded poly(oxycarbonyl-1,4-phenylene-ethylene) (polyester 1), having an inherent viscosity of 0.19 dL g(-1) in a 95% yield: i.e., polycondensation proceeded to afford the polyester. Reaction of the sodium salt at 220 degrees C for 2 h produced polyester 1 having (M) over bar(n) of 4000 in a 28% yield as well as 4-vinylbenzoic acid in a 9% yield. In the reaction of the sodium salt, both polycondensation and double bond formation occurred. Thus, these polymerizations depended on the counter cations of 2-BEBAs.