This paper concerns the preparation and characterization of a new graft copolymer comprising polychloroprene backbone and polyisobutylene branches which carry tertiary chlorine termini. Synthesis involved the grafting of isobutylene from polychloroprene by the use of BCI3coinitiator in methylene chloride solvent in the -12 to -78°C range. Conditions have been defined for efficient and convenient graft copolymerizations. Carefully purified grafts (selective solvent extraction) were characterized by a battery of analytical techniques including1H-NMR spectroscopy and GPC. Agreement between calculated and experimental graftMn's indicate absence of disturbing side reactions, i.e., backbone degradation and cross-linking, during synthesis. The presence of tertiary chlorine endgroups has been demonstrated by thermal dehydrochlorination studies. Branch molecular weight, molecular weight distribution, and frequency have been obtained by selective oxidative destruction of the polychloroprene backbone followed by surviving branch analysis. A graft prepared at -20°Carried ∼5 polyisobutylene branches ofMn= 5000 each.