Synthesis Of Silica Particles Grafted With Well-defined Complex Macromolecular Architectures

Synthesis and properties of inorganic-polymer hybrid materials have attracted much attention recently due to their wide applications in many fields. This research is aimed at synthesis and characterization of silica particles grafted with homopolymers, block copolymers and V-shaped copolymers via facile synthetic methodology. In this study, polymerization techniques such as reversible addition-fragmentation chain transfer (RAFT) polymerization, atom transfer radical polymerization (ATRP) and ring-opening polymerization (ROP) and highly efficient coupling reactions such as radical induced chain exchange reaction and alkyne-azide cycloaddition reaction were combined to perform surface modification of silica particles ranging from nanometer to micron scale. In addition to development of a novel method to synthesize suprapure block copolymers, a few approaches to well-defined polymers grafted onto the surface of silica particles were attained as well. The main contents are listed as follows.In part 1, radical induced addition-fragmentation chain transfer reaction was successfully applied to surface modification of silica particles, suprapure block copolymers were achieved by de-grafting reaction, and the recovered solid chain transfer agents (CTAs) were efficiently reused. This methodology primarily involves four types of reactions: (a) well-defined homopolymers and block copolymers up to triblock were synthesized by RAFT polymerization of vinyl monomers and followed by sequential chain extension polymerization; (b) Fumed silica supported S-Benzyl S￠-trimethoxysilylpropyltrithiocarbonate (Si-BTPT) as a Z-supported solid CTA was synthesized by coupling reaction; (c) Thermal initiator (such as AIBN), macro CTA and solid CTA were mixed to perform radical induced chain exchange reaction at 60 oC in toluene, and silica-polymer hybrids were obtained; (d) Most of graft polymers were cleaved from the surface of silica particles by de-grafting reaction at higher temperature (such as 80 oC) in the presence of large excess of thermal initiator, and solid CTA was also recycled. Reactions c and d form a cycle, and the cycles could be performed for many times due to their high efficiency, evident from similar grafting and de-grafting efficiencies were quite similar in various cycles. This methodology has at least the following advantages: 1) Molecular weights of graft polymers and their precursors (macro CTAs) were usually similar, whilst the polydispersity of graft polymers was slightly lower than that of macro CTAs due to lack of dead chains; 2) This method was suitable for any Z-supported solid CTAs and free RAFT agents or MADIX agents; 3)ω-Functionalized orα,ω-difunctionalized polymers could be obtained when the raw RAFT agents to synthesize macro CTA or thermal initiators used for de-grafting reaction possessed exact functionalities such as hydroxyl, carboxyl and protected amino group. Radical induced chain exchange reaction is a highly efficient method for surface modification of solid supports, and it is more general than Z-supported RAFT graft polymerization or other coupling reactions.In part 2, a trifunctional initiator 3-hydroxymethyl-5-prop-2-ynyloxybenzyl 2-bromo-2-methylpropionate (HPB) bearing alkynyl, hydroxyl and alkyl bromide moieties was synthesized and used for the synthesis of silica particles grafted with two-component polymers via stepwise, two-step or tandem one-pot method, and their physico-chemical properties were investigated. The stepwise involves three-step reactions. First, PCL macroinitiator with alkyne and bromine was synthesized by ROP of caprolactone (CL) with HPB initiator and Sn(Oct)2 catalyst. Second, well-defined PCL-b-PMMA, PCL-b-PBA, PCL-b-PtBA and PCL-b-PSt diblock copolymers were synthesized by ATRP of methyl methacrylates (MMA), butyl acrylate (BA), tert-butyl acrylate (tBA) and styrene (St) using PCL macroinitiator. Last, silica nanoparticles or mesoporous silica particles grafted with V-shaped copolymers were obtained by alkyne-azide cycloaddition reaction. When SiO₂-g-(PCL-b-PtBA) nanohybrids were subjected to hydrolysis with trifluoroacetic acid, pH-sensitive SiO₂-g-(PCL-b-PAA) hybrid materials were obtained, whose surface charge numbers and sizes of aggregates at different pH values were quite different. Meanwhile, two-step and one-pot method were also efficiently used to synthesize silica particles grafted with two-component polymer segments, which could afford high grafting density and possess advantages in case of cost and time although the grafted chains may lack uniform chain length. Preliminary results from DSC analyses indicated the glass transition temperatures of PMMA and PSt chains before and after graft reaction were quite different. The silica-V-shaped copolymer hybrids usually exhibited decreased Tg values corresponding to relaxation of PMMA or PSt chains as compared with those of macro CTAs (graft polymer precursors), whilst hybrid materials comprising PSt segments possessed significantly enhanced Tg values as compared with those of free polymers produced in solution during tandem polymerization and graft reaction. These results may be ascribed to different confined degree at the surface of silica particles and distinct role of interactions among the target segment, PCL and solid supports.In summary, synthetic method of silica-polymer hybrids was further developed in this study, silica particles grafted with polymers up to triblock copolymers, V-shaped copolymers were achieved, and physico-chemical properties of the resultant hybrid materials were investigated. These nano- to micron-scale hybrid materials possess potential applications in smart materials and biomedical materials. Moreover, these methods given in this thesis are suitable for surface modification of other solid supports such as silica wafer, magnetic Fe3O4, carbon nanotube and graphene.