Hyperbranched aliphatic copolyesters have been prepared by copolymerization of ε‐caprolactone with 2,2‐bis(hydroxymethyl)butyric acid, catalyzed by immobilized Lipase B from Candida antarctica (Novozyme 435) under mild conditions. Via this novel combination of ring‐opening AB polymerization and AB2 polycondensation, the degree of branching (DB) and, consequently, the density of functional end groups can be controlled by the comonomer ratio in the feed (0 < DB < 0.33).

Summary: Hyperbranched aliphatic copolyesters have been prepared by the copolymerization of ε‐caprolactone and 2,2‐bis(hydroxymethyl)butyric acid (AB2‐monomer), catalyzed by (i) HfCl4(THF)2 and (ii) diphenylammonium trifluoromethanesulfonate (DPAT), respectively. In both cases, copolymerization by combined ROP/AB2‐polycondensation was achieved. The degree of branching (DB) and consequently the density of functional groups of the resulting copolyesters were controlled by the comonomer ratio in the feed. Molecular weights in the range $\overline M _{\rm n}$ = 22 000–166 000 g · mol−1 (GPC, PS standards) were obtained, with apparent polydispersity indices of 1.20 to 1.95. The DB was in the range 0.03–0.35. Remarkably, HfCl4(THF)2 appeared to cause no transesterification of the ester bonds in the hyperbranched polymer formed. Further esterification or functionalization of the hydroxyl end groups of the hyperbranched polymers is therefore possible in a convenient two step/one pot process. The prepared hyperbranched polycaprolactones can be used as multifunctional initiators for the ROP of ε‐caprolactone, which is also catalyzed by HfCl4(THF)2, resulting in multi‐arm star polymers. Diphenylammonium trifluoromethanesulfonate (DPAT) was also found to catalyze the combination of ROP and AB2 polycondensation. However, the applicability of this system is restricted due to side reactions that can lead to crosslinking.Synthesis of hyperbranched copolyesters by combined ROP/polycondensation.magnified imageSynthesis of hyperbranched copolyesters by combined ROP/polycondensation.

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