Aliphatic hyperbranched copolyesters by combination of ROP and AB2-polycondensation

Hyperbranched aliphatic copolyesters have been prepared by the copolymerization of epsilon-caprolactone and 2,2-bis(hydroxymethyl)butyric acid (AB(2)-monomer), catalyzed by (i) HfCl4(THF)(2) and (ii) diphenylammonium trifluoromethanesulfonate (DPAT), respectively. In both cases, copolymerization by combined ROP/AB(2)-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 (M) over bar (n) = 22000-166000 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 epsilon-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 AB(2) Polycondensation. However, the applicability of this system is restricted due to side reactions that can lead to crosslinking.