Novel Initiators for Ring-opening Polymerisation of Cyclic Esters for Use as Absorbable Surgical Sutures

Abstract

The main aim of this research has been to design, synthesise and test the effectiveness of various tin(II) alkoxide initiators in the bulk ring-opening polymerisation (ROP) of cyclic esters for intended use as absorbable surgical sutures. Tin(II) alkoxides in solid form were synthesised using anhydrous tin(II) chloride, triethylamine and various glycols (diols), namely: diethylene glycol (DEG), polyethylene glycol 300 (PEG300), polypropylene glycol 400 (PPG400) and polypropylene glycol 1200 (PPG1200). In addition, some liquid tin(II) alkoxides were synthesied via the reaction between tin(II) octoate (Sn(Oct)2) and either diethylene glycol (DEG) or ethylene glycol (EG). Structural and thermal characterisation of the initiators was carried out by Fourier-Transform Infrared Spectroscopy (FT-IR), Proton-Nuclear Magnetic Resonance Spectroscopy (1H-NMR), Liquid Chromatography Mass Spectroscopy (LC-MS), Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA).

Kinetic studies of the bulk ROP of -caprolactone (CL) using the liquid tin(II) alkoxides were carried out at 130 oC by dilatometry. The initiators used were bis(tin(II) octoate) diethylene glycol ([Sn(Oct)]2DEG) and bis(tin(II) octoate) ethylene glycol ([Sn(Oct)]2EG) and the results compared with those from the conventional tin(II) octoate and tin(II) octoate/diol systems. The experimental data used for the kinetic calculations was taken from the 20-70 % conversion range for constructing both first-order and zero-order rate plots. From the results, it was found that the rate data fitted more closely to the expected first-order kinetics with respect to the monomer concentration for the pre-prepared liquid initiators used and gave higher rate constants than for the corresponding tin(II) octoate / diol mixtures. However, in some cases, the data fitted a zero-order rate plot more closely, such as for tin(II) octoate, where there were limiting factors involved which caused deviations from first-order kinetics.

When comparing the solid and liquid initiators in the ROP of CL, whereas the liquid initiators were completely soluble in the CL monomer, the solid initiators were only partially soluble. Furthermore, the undissolved solid initiator was clearly visible in the solid polymer as an impurity and needed to be removed by careful purification. Therefore, in terms of initiator solubility, polymer molecular weight and polymer purity, the liquid initiators were more efficient and tended to give more reproducible results. The highest molecular weight PCL ( > 40,000) was obtained from the liquid [Sn(Oct)]2EG initiator.

Poly(L-lactide-co-ε-caprolactone) 75:25 mol % (PLC) copolymers were synthesised using both solid and liquid tin(II) alkoxide initiators at 130 oC for 48 hrs. The copolymers were all obtained in near-quantitative (> 90 %) yields with compositions close to the initial comonomer feed ratio (from 1H-NMR). Their monomer sequencing was partly random and partly blocky (from 13C-NMR) due to the differing monomer reactivity ratios. From the GPC results, the PLC copolymers using the liquid synthesised tin(II) alkoxides were found to give higher molecular weights and lower polydispersities (PD) ( = 40,000-100,000; PD = 1.30-1.87) than the solid initiators ( = 30,000-52,000; PD = 1.73-2.24) due to their complete solubility in the monomers.

The PLC copolymers synthesised from both the solid and liquid initiators were melt-spun into monofilament fibres at 135-160 oC with minimal draw and quench cooling. The as-spun fibres were very weak and highly extensible due to their largely unoriented and limited crystalline morphology. Molecular orientation and further crystallization were then gradually built into the fibres through a series of hot-drawing and annealing steps, both of which were temperature and time-dependent. Tensile testing and DSC results indicated that the best quality fibres were those obtained from PLC synthesised using the liquid initiators which had yielded the highest copolymer molecular weights. The processed fibre from PLC synthesised using the [Sn(Oct)]2DEG initiator showed the highest tensile strength of > 300 MPa which is comparable with commercial monofilament sutures. In overall conclusion, the novel liquid tin(II) alkoxide initiators have been shown to be more effective than conventional initiators such as Sn(Oct)2 and have been able to produce PLC copolymers which have potential for further development as absorbable monofilament surgical sutures.