Synthesis of three-armed poly(trans-4-hydroxy-N-benzyloxycarbonyl-L- proline)-block-poly(ε-caprolactone) copolymers

A series of novel types of three-armed poly(trans-4-hydroxy-N- benzyloxycarbonyl-L-proline)-block-poly(ε-caprolactone) (PHpr-b-PCL) copolymers were successfully synthesized via melt block copolymerization of trans-4-hydroxy-N-benzyloxycarbonyl-L-proline (N-CBz-Hpr) and ε-caprolactone (ε-CL) with a trifunctional initiator trimethylolpropane (TMP) and stannous octoate (SnOct₂) as a catalyst. For the homopolycondensation of N-CBz-Hpr with TMP initiator and SnOct ₂ catalyst, the number-average molecular weight (M_n) of prepolymer increases from 530 to 3540 g mol^-1 with the molar ratio of monomer to initiator (3-30), and the molecular weight distribution (M _w/M_n) is between 1.25 to 1.32. These three-armed prepolymer PHpr were subsequently block copolymerized with ε-caprolactone (ε-CL) in the presence of SnOct₂ as a catalyst. The M _n of the copolymer increased from 2240 to 18,840 g mol^-1 with the molar ratio (0-60) of ε-CL to PHpr. These products were characterized by differential scanning calorimetry (DSC), ¹H NMR, and gel permeation chromatography. According to DSC, the glass-transition temperature (T_g) of the three-armed polymers depended on the molar ratio of monomer/initiator that were added. In vitro degradation of these copolymers was evaluated from weight-loss measurements and the change of M _n and M_w/M_n.