Microwave-Improved Ring Opening Polymerization Of ε-Caprolactone Initiated By Acids And Alcohols

Poly(ε-caprolactone)(PCL), used as a synthetic aliphatic polyester, was extensively investigated in the biomedicine field due to its high biodegradability, biocompatibility as well as drug permeability. In this paper the developments of PCL are reviewed involving the investigation on its catalyst or initiator system, functionalization and microwave synthesis.To improve in vivo biocompatibility and in vivo safety of PCL, exploring the effective nonmetallic catalyst or initiator system for the ring opening polymerization of ε-caprolactone (ε-CL) is being the hot topics in international research filed. This dissertation is focused on microwave-assisted ring opening polymerization (MROP) of ε-CL initiated by carboxylic acids and alcohols.The MROP of ε-CL initiated by benzoic acid was investigated. The product was confirmed to be PCL and the initiator residue was incorporated into the polymer chain. The weight-average molar mass (Mw) of PCL reached 44800 g/mol with polydispersity index (Mw/Mn) 1.6 when a reaction mixture of e-CL and benzoic acid was irradiated at 680W for 240 min. The influences of microwave power, irradiation time and initiator concentration on the polymerization were studied. The higher the power or the initiator concentration was, the higher Mw of product was in the beginning stage of the polymerization, but the lower maximum of Mw that could be obtained. Lasting irradiation time was beneficial to enhance the Mw. Microwave heating effect markedly influenced the polymerization reaction. The polymer chain propagated fast between 160 to 230C, within which the higher the temperature, the faster the propagation. However, when the temperature was over 230C, the chain propagation was limited and the resultant PCL degraded. The molar mass of microwave-produced PCL was significantly higher than that by conventional heating method. It was due to that microwave irradiation greatly inhibited formation of growing center at the beginning stage of the polymerization and ε-CL was consumed dominantly to lengthen the polymer chain, whereas ε-CL was mainly consumed to form growing centers in thermal polymerization. As a result, the polymer chain propagation was enhanced by microwaves.Three chlorinated acetic acid with different pKa, namely chloroacetic acid, dichloroacetic acid and trichloroacetic acid, were chosen to study the influence of acidity on MROP of ε-CL initiated by acids. Besides the microwave power, irradiation time and initiator concentration, the pKa of initiator obviously influenced the polymerization reaction. The lower the pKa of initiator was, the lower the molar mass ofthe resultant polymer was but faster the reaction proceeded. During the polymerization, the molar mass increased to a maximum first and then decreased. The lower pKa of initiator (higher power or more initiator) was, the earlier the degradation of PCL occurred. Moreover, the decrease in molar mass always began after the monomer was completely consumed. The degradation behavior of PCL in the reaction system was mimicked under microwave irradiation. It was revealed the degradation took place in the absence of the acid, and acid added to the system further favored the degradation. With the stronger or more acid, the extent of degradation was more obvious.A series of poly(?caprolactone)-poly(ethylene glycol)-poly(?caprolactone) triblock copolymers (PCECs) with number-average molar mass (Mn) over 20,000 g/mol were prepared, via the MROP of s-CL initiated by poly(ethylene alcohol) (PEG). The polymerization reaction was influenced by irradiation time, microwave power and the amount of initiator. With the reaction time increasing, both the molar mass and monomer conversion increased gradually. The higher the microwave power was, the higher the monomer conversion and the greater the Mn was. With more initiator, the monomer conversion was higher but lower Mn copolymer was obtained. The Mn and composition of the PCECs were expediently controlled by the amount and chain length of PEG in feed. The in vitro drug release behavi...