Alternating copolymers of isobutylene and acrylic ester (designated as ’AIM’ copolymers) were synthesized by complexed copolymerization in the presence of alkylboron halide. Various characteristic properties of the AIM copolymers were revealed. AIM copolymers showed singular properties which could not be expected from those of the corresponding homopolymers. The glass transition temperature, thermal decomposition temperature and tensile strength of AIM copolymers were greater than those of both homopolymers, i.e. additivity was not established. The solubility parameter and refractive index of AIM copolymers showed slightly smaller values than the additive ones of both homopolymers. The hydrolysis resistance of AIM copolymers was excellent compared with that of the corresponding poly(acrylic ester)s.