Grafting Copolymerization Of Aliphatic Polyester Via ATRP And Click Chemistry

Due to the good biocompatibility and biodegradability. the aliphatic polyesters have great potential applications for the biomedical materials and the substitution of petroleum-based materials. In the industrial application, it is usually necessary to improve the physicalproperties of aliphatic polyestervia the chemical modification. Because most of the aliphatic polyesters such as poly(ε-caprolactone)(PCL) and poly(lactic acid)(PLA) lack the functional groups along the polymer backbone, their chemical modification is usually difficult. Most researchersstudiedtheterminal functionalization of polyesters by using the functional chemicals as the initiator for the ring-opening polymerization (ROP) of lactone monomer. However, as compared to the terminal functionalization, the pendent functioanlization allows the more homogenous distribution of functionality along the polyesters backbone.In this work, we used PCL as a model polymer and investigated its chemical modification andfunctionalization through the atom transfer radical polymerization (ATRP) and azide-alkyne click chemistrymethods. We prepared the amphipathic graft copolymers bearing the PCL backbone, that is, PCL-g-poly(N-isopropylacrylamide)(PCL-g-PNIPAAm) and PCL-g-poly(ethyleneglycol)(PCL-g-PEG), and also investigated the self-assembly, thermal and thermosensitiveproperties of these graft copolymers.We firstly prepared the chloro-functionalized PCL[P(CL-co-αClεCL)] with different comonomer components by the ROP of chloro-functionalized ε-caprolactone (αClεCL) and unsubstituted ε-caprolactone. The thermal properties of copolymers were analyzed via the differential scanning calorimeter (DSC). The glass transition temperature (Tg)increased while crystallization temperature (Tc)and melting point (Tm) of P(CL-co-αClεCL) decreased as the αClεCL comonomer content in P(CL-co-αClεCL) increased. This indicated that the introduction of αClεCL decreased the crystallization rate and crystallizability of PCL.Secondly.the PCL-g-PNIPAAm amphiphilic graft copolymers were synthesizedvia the atom transfer radical polymerization with activators regenerated by electron transfer (ARGET ATRP) by using the P(CL-co-αClεCL) as macromolecular initiator, Cu(Ⅱ) as catalyst, and the stannous octoateas the reducing agent. The graft density and length of copolymer were controlled by varying the content of chloro functional group in PCL backbone and the P(CL-co-αClεCL)/NIPAAm feed ratio. The presence of PNIPAAm chain destroyed the crystallizability of PCL. No any crystallization and melting peaks were found in the DSC curvesof PCL-g-PNIPAAm. As confirmed by the analyses of nuclear magnetic resonance spectroscopy (NMR), dynamic light scattering (DLS), and transmission electron microscope (TEM), PCL-g-PNIPAAm graft copolymers assembled into core-shell micelles in aqueous solution, which were composed of the biodegradable PCL core and thermoresponsive PNIPAAm corona.The critical micelle concentrations (CMC) of graft copolymers were in the range of6.4-23.4mg/L, which increased with the content of PNIPAAm grafting chians.The mean hydrodynamic diameter (Dh) of micelles depended on the graft density and length, allowing the tuning of particle size within a wide range (26.0-117.3nm).PCL-g-PNIPAAm micelles were thermosensitive and they aggregated when the temperature was increased to above the lower critical solution temperature (LCST).Furthermore, the halidesinP(CL-co-αClεCL) were converted to azides by the nucleophilic substitution with sodium azide. The alkyne-terminated PNIPAAm was synthesized via ATRP using an alkyne-functionalized initiator.The alkyne end-functionalized PEG was synthesized by the esterification reaction between the methoxy polyethylene glycols (mPEG) and the carboxylic acid containing the alkyne group. The amphipathic graft copolymers of PCL-g-PNIPAAm and PCL-g-PEG were then synthesized bythe click chemistry of the azido-functionalized PCLand the alkyne end-functionalized PNIPAAm or PEG.The thermal property, self assembly, and thermoresponsive behavior of these copolymers were studied.Both the PCL-g-PNIPAAm and PCL-g-PEG graft copolymers self-assembled into micelles containing the PCL core in theaqueous solution.The CMC of graft copolymers were in the range of11.2-42.8mg/L and theirDhalues are under the range of65to136nm. The CMC and Dh of graft copolymers increased with the grafting length. The types of graft chains had a significant effect on the micellization and micellestructure. PCL-g-PEG had the larger CMC and Dh values than PCL-g-PNIPAAm.The micelles of PCL-g-PNIPAAm exhibited thermoresponsivebehavior. Because of the hydrophobic nature of PCL, PCL-g-PNIPAAm had the lower LCST than the PNIPAAm homopolymer.