Free radical copolymerisation of tert-butyl methacrylate (1) with 2-isopropenyl isocyanate (2) has been investigated. Reactivity ratios of r(1) = 3.48 +/- 0.60 for 1 and r(2) = 0.004 +/- 0.04 for 2 were obtained. As 2-propenyl isocyanate has almost no tendency to homopolymerise maximum isocyanate content is reached in an alternating copolymer. The reaction of the isocyanate functionality with morpholine was used to obtain compounds for determination of the composition by NMR-spectroscopy. Glass transition temperature of the copolymers increases with increasing tert-butyl methacrylate content. It was found that thermal cis-elimination of isobutylene from the tert-butyl ester is followed by two further reactions: formation of six-membered anhydride rings by two neighbouring acid functionalities and formation of 3,4-dimethyl-pyrrolidin-2-one from one acid group and an adjacent isocyanate functionality. The reactions could be observed in the solid state and in solution at about 200 degreesC. Molecular weights of the copolymers as determined by size exclusion chromatography decreases with increasing isocyanate content which is due to chain transfer reactions of isopropenyl isocyanate. Conversion per minute decreases with increasing isocyanate content in the monomer feed