Ring-Opening Polymerization of Trimethylene Carbonate with Phosphazene Organocatalyst

Abstract

Aliphatic polycarbonate (APC) compounds are an important class of biodegradable materials with excellent biocompatibility, good biodegradability, and low toxicity, and the study of these compounds and their modification products aims to obtain biodegradable materials with better performance. In this context, the ring-opening polymerization (ROP) of trimethylene carbonate (TMC) from a low nucleophilic organic superbase of phosphazene (t-BuP₄) as a catalyst and benzyl alcohol (BnOH) as an initiator at room temperature was carefully studied to prepare poly(trimethylene carbonate) (PTMC) which is one of the most studied APC. ¹H NMR and SEC measurements clearly demonstrate the presence of a benzyloxy group at the α-terminus of the obtained PTMC homopolymers while investigation of the polymerization kinetics confirms the controlled/living nature of t-BuP₄-catalyzed ROP of TMC. On the basis of this, the block copolymerization of TMC and δ-valerolactone (VL)/ε-caprolactone (CL) was successfully carried out to give PTMC-b-PCL and PTMC-b-PVL copolymers. Furthermore, PTMC with terminal functionality was also prepared with the organocatalytic ROP of TMC through functional initiators. We believe that the present ROP system is a robust, highly efficient, and practical strategy for producing excellent biocompatible and biodegradable PTMC-based materials.

Keywords: PTMC; controlled/living nature; end functionality; t-BuP4.

Figure 1

¹H NMR spectrum of the reaction mixture (Table 1 and S3, 30 min.) in CDCl₃.

Scheme 1

General scheme for the ring-opening polymerization of TMC using benzyl alcohols as initiators and t-BuP₄ as catalyst, as well as the synthesis of block copolymers.

Figure 2

MALDI-TOF MS spectra of the obtained PTMC. The theoretical values were calculated using the following equation: 107.13 + n × 102.09 + 1.01 + 22.99, where n is the degree of polymerization and the mass values correspond to the segments and end groups comprising the PTMC chain as shown in the scheme.

Figure 3

(A) SEC curves of PTMC obtained with different initial monomer/initiator ratios: a 10, b 30, c 50, d 100, (B) Dependence of monomer conversion on reaction time.

Figure 4

(a) Dependence of M_n,SEC (○), M_n,theo (●) and Ð (▲) of the obtained PTMC on the conversion of TMC and (b) kinetic plot for the t-BuP₄ catalyzed ROP of TMC initiated with BnOH.

Figure 5

SEC curves of (a) PTMC-b-PVL and (b) PTMC-b-PCL copolymers.

Scheme 2

A possible initiator/chain-end mechanism for t-BuP_4-catalyzed ROP of TMC.

Figure 6

¹H NMR spectra of PTMC initiated by (a) 6-azido-1-hexanol (AHA) (N₃-PTMC) and (b) methoxy poly(ethylene glycol) (mPEG-b-PTMC).

Figure 7

The DSC (a) and TGA (b) curves of PTMC.