Copper-Catalyzed Bimolecular Coupling of α,ω-Dibromide-Functionalized Poly(γ-caprolactone)

alpha,omega-Dibromide-functionalized poly(gamma-caprolactone) was synthesized by reacting alpha,omega-dihydroxy-terminated poly(gamma-caprolactone) with an excess of 2-bromoisobutyryl bromide, leading to above 99% esterification. The resulting dibromide end-functionalized polycaprolactone (Br-PCL-Br) was found to "polymerize" in the presence of an excess of copper(I) chloride catalyst without the addition of vinyl monomers. Size exclusion chromatography (SEC) clearly revealed that the molecular weight of the "polymerized" macroinitiator shifts to higher values, and a broad peak was obtained with the main peak centered at a molecular weight which was twice that of the Br-PCL-Br precursor, suggesting that the main reaction is bimolecular coupling (similar to 80%), while a shoulder at molecular weights about 6-7 times that of the macroinitiator was also observed attributed to multimolecular coupling. The effects of the reaction temperature and the presence of copper(II) catalyst on the radical coupling reactions were also studied. A higher temperature or a lower copper(II) catalyst concentration favored the bimolecular as opposed to the multimolecular coupling, suggesting the possible occurrence of side reactions such as disproportionation and elimination, which hindered further coupling of the free chain ends.