Synthesis Of Polycarprolactone Related Polymers Via Living/Controlled Radical Ring-Opening Polymerization

Polycaprolactone(PCL)is an important polymeric material with good biocompatibility and biodegradability.It is widely used in packaging materials,tissue engineering materials and drug delivery,etc.Most of the existing PCL-based polymeric materials are prepared by ring-opening polymerization of caprolactone(?-CL)which has ineluctable short parts such as harsh reaction conditions and poor adaptability of the monomer to the catalyst.Besides,due to the limitation of polymerization method,it is difficult to adjust the polymerization performance by copolymerized with functional vinyl monomers.Fortunately,the monomer 2-methylene-1,3-dioxane(MDO)provides a more flexible approach in the preparation of such materials by prepareing a PCL-like polymer via simple free radical ring-opening polymerization.Furthermore,utilizeing the "living" radical polymerization technology to the ring opening polymerization of MDO can realize the regulation of the obtained polymer's structure.It also provided the opptunity for realizing copolymerization with various vinyl monomers to obtain random and block copolymers.Those obtained copolymers are reactive to be further post-modified,and completely degradable becauseful of the ester bond introduced into the polymer backbone.In this work,the reversible addition fragmentation chain transfers radical polymerization(RAFT)polymerization of MDO was carried out,including the design of topological structure,polymerization behavior,design and synthesis of amphiphilic polymers and their assembly performance.The details are as follows:(1).The controllable hyperbranched PCL was synthesized by reversible addition-fragmentation chain transfer radical ring-opening polymerization(RAFT)along with self-condensed vinyl polymerization(SCVP),using 2-methylene-1,3-dioxepane(MDO)as the mian monomer and vinyl acetate(VAc)as second monomer.Living polymerization behavior was proved via pseudo linear kinetics,the molecular weight of polymers increasing with conversion and successful chain extension by the addition of monomers such as MDO and VAc.The structure of polymers was characterized by nuclear magnetic resonance spectroscopy(1H NMR),tripe detection gel permeation chromatography(tripe-GPC)and differential scanning calorimetry(DSC).The results show that the degree of branching,molecular weight and composition of polymers can be tuned by changing the polymerization condition,and thus achieved the controllable hyperbranched polymers containing PCL.Further,realized the design and preparation of polymers with different crystallinity and degradation rate through the design and synthsis of the structure.(2).The selection of ring-opening/ring-retaining competitive reaction of MDO under different polymerization mechainsims and conditions was investaged by characterizing the obtained polymer's structure through 1H NMR and GPC.The results showed that ring-opening reaction portion of MDO during polymerization was increased with temperature and the initial charge of MDO in cases of both cationic and radical polymerizations.However,MDO preferred to polymerize through ring-retaining way in case of cationic polymerization,while it showed high tendency of ring-opening way in case of radical polymerization.(3).The copolymerization of MDO and PEGA was investigated using TPO as photo-initiator and 2-dodecylsulfanylthiocarbonylsulfanyl-2-methyl-propionic acid(DTMPA)as a chain transfer agent under blue LED(?max=450-460 nm)at 25?.The obtained copolymers showed tunable thermos-responsive behavior due to the introducing of PEGA segament.The LCST can be obtained by changing the molar charge ratio.Moreover,the reactivity ratio values of these two monomers 0.02,rPEGA=0.86)under current polymerization condition has been determined.In addition,amphiphilic block copolymer containing PCL segment in both blocks was synthesized by copolymerization MDO with PEGA using hydrophobic macromolecule(poly(VAc-co-MDO))as a macro-RAFT agent.The block copolymerization was futher used for self-assembled and obtained nanomicelles containing PCL segments in the both core and shell.Finally,the hydrolysis behavior of the polymer micelles was examined.According to the tracking test results of dynamic light scattering(DLS)and transmission electron microscopy(TEM),it can be found that the particle size of the micelle gradually decreases as the hydrolysis time progresses.