Different Architectural Polymers Via RAFT Heterogeneous Polymerization

Star polymers and intelligent nanogles have attracted wide attention and research inbiomedical field. Core cross-linked star (CCS) polymers represent a uniquearchitecture of polymers, with a large number of arms connected to a small, highlycross-linked core. This unique architecture lends several distinct features of CCSpolymers, including core-shell isolation, multi-valency, manifold end functionalities,reduced solution and bulk viscosity. CCS polymers have been intensively studied forpotential applications in drug delivery, imaging, catalysis and emulsions. Synthetictechniques of CCS polymers typically involve living anionic and cationicpolymerization, metal-catalyzed radical polymerization, ring-opening polymerization,and controlled radical polymerization. Reversible addition-fragmentation chaintransfer (RAFT) polymerization has been one of the major methods used for thesynthesis of CCS polymers on account of its mild reaction conditions, simplicity, anda wide range of applicable monomers. One novel strategy is based on RAFTheterogeneous polymerization, either emulsion or dispersion polymerization, inaqueous dispersed media. The arm-first approach is well accepted for the synthesisof core cross-linked star polymers. The approach typically involves two separatesteps. RAFT arm polymers have to be prepared first, which are then used for thesecond step of cross-lingking polymerization. We report here two novel approachesfor the RAFT synthesis of well-defined CCS polymers in water without theseparation and purification of the arm polymers.The main contents are listed as follows：1. We first developed a one-pot approach for the RAFT synthesis of ahigh-quality CCS in water, which consists of a homogeneous polymerization of(poly(ethylene glycol) methyl ether methacrylate)(PEGMA) to synthesize the armpolymers, polyPEGMA, and a heterogeneous cross-linking polymerization step toassemble the arm polymers into CCS polymers. Various parameters affecting theformation and qulity of CCS polymers were studied, including the type of cross-linker, arm polymer concentration, spacing monomer and arm polymermolecular weight. Cross-linkers of longer length provided the flexibility required toform well-defined CCS polymers. The use of spacing monomers in conjugation withcross-linkers provided stronger driving force and also reduced the stress to producewell-defined CCS polymers. The higher molecular weight arm polymer PPEGMAexhibited increased steric hindrance for CCS formation. Well-defined CCS polymerswere prepared at relatively high polymer concentration. In a word, this one-potapproach is environmentally benign, highly efficient and amenable to a variety ofreaction conditions.2. A one-step approach for the efficient synthesis of CCS polymers wasdeveloped using a commercial macromonomer poly(ethylene glycol methyl ether)methacrylate (PEGMA, Mn=2080) as the arm polymer via RAFT-mediatedemulsion polymerization in PBS solution without using any organic solvent. Variousparameters affecting the formation of CCS were studied, including the concentrationof PBS, the dosage of the cross-linker, macromonomer PEGMA and spacingmonomer. This method is efficient, simple and is performed directly in PBS aqueoussolution. The CCS polymers were effective in the stabilization of emulsions ofstyrene, which were successfully used for the preparation CCS-stabilized particles.Thermosensitive nanogels are a very important kind of intelligent nanogels.Potential applications of these materials have been demonstrated in drug delivery.Here, we used ethylene glycol methyl ether acrylate (MEA) and poly (ethyleneglycol) methyl ether acrylate (PEGA) as the monomers and poly (N,N’-dimethylformamide)(PDMA) as both the RAFT agent and stabilizer to synthesize nanogels.We also investigated their use as emulsifiers for the formation of oil in wateremulsions.The main contents are listed as follows:1. We used MEA and PEGA as the monomers and PDMA as the RAFT agentand stabilizer to synthesize nanogels. The synthesized nanogels with a controllable core-shell structure were monodisperse in wate, with good colloidal stability. Thenanogels were studied as effective emulsifiers in the stabilization of emulsions ofdodecane.