Short poly(ethylene glycol) block initiation of poly(l-lactide) di-block copolymers: a strategy for tuning the degradation of resorbable devices

30 Apr 2018

The current range of medical applications of resorbable polyesters could be hugely expanded if more effective strategies for tailoring degradation rate were available. Block copolymerisation with PEG has been shown to reduce degradation times, however, to date, this has relied on the addition of PEG to short lengths of polyester. This results in copolymers with high fractions of PEG and low molecular weights, reducing the potential range of applications. Furthermore, there has been no systematic study on the relative lengths of the blocks. In this work, we employed short hydroxyl-functionalised methoxy-terminated mPEG to initiate the synthesis of PLLA, resulting in controlled di- block copolymers with short mPEG blocks and long PLLA blocks. A controlled series of polymers was made with PLLA lengths (60 < Mn (kg/mol) < 200) and mPEG lengths (550 < Mn (g/mol) < 5000) giving very low mPEG wt% (0.1 < wt% < 1.5). We found, that, despite the low fraction of mPEG, water uptake and the rate of hydrolytic degradation, k, increased. Significantly, k, for the polymers was dependent only on the presence of mPEG, and was little affected by mPEG length or PLLA length in the ranges studied. Moreover, mass loss began in all polymers when the Mn of the polymer fell below a threshold of about 20 kg/mol and depended on both the initial molecular weight of PLLA and the presence (but not the length) of mPEG. Short chain mPEG therefore provides a new route for targeted, temporal control of resorbable polyesters for biomedical devices.