RAFT Emulsion Polymerization Of Methyl Methacrylate And Synthesis And Properties Of Polar Block Copolymers

Methyl methacrylate (MMA) is widely used in industry, and poly(methyl methacrylate) (PMMA) block copolymer holds great promise to be used as compatibilizer or toughening modifier considering that PMMA should be compatible with a large variety of commercial plastics including poly(vinyl chloride), styrene-acrylonitrile, poly(vinylidene fluoride), polycarbonate (PC) and epoxy. Traditionally, it has been difficult to synthesize the block copolymer of PMMA. In the past two decades, the breakthroughs in controlled/living radical polymerization (CLRP) make it possible to use almost all vinyl monomers including MMA to produce high performance polymers with complex chain structures like di-, tri-block, and gradient copolymer. Emulsion polymerization has been considered to be the most promising process to carry out CLRP.In the current thesis, the recently-developed RAFT emulsion polymerization was extended to MMA RAFT emulsion polymerization using amphiphilic poly(methacrylate acid)-b-poly(methyl methacrylate) trithiocarbonate (named as PMAA-b-PMMA-RAFT) as both surfactant and mediator. The often observed colloidal instability and uncontrollable problems were investigated. The synthesis of PMMA-based block copolymers via RAFT emulsion polymerization and their application properties as impact modifier were also discussed. The following results were achieved:1) RAFT ab initio emulsion polymerization of MMA was carried out using a carefully-optimized PMAA41-b-PMMA8-RAFT as both surfactant and mediator. Well-controlled polymerization in terms of negligible coagulum, high polymerization rate, well predicted molecular weight and good colloidal stability could be achieved at 80℃ and low initiator (KPS) concentrations. However, the colloidal stability during RAFT emulsion polymerization of MMA was more sensitive to KPS concentrations and polymerization temperature compared with the styrene (St) system. By direct comparison with the classical St system, it was found that the particle nucleation mode should play significant influence on RAFT emulsion polymerization performance. It was revealed that in the emulsion polymerization of MMA, about 40% acid groups of PMAA41-b-PMMA8-RAFT were buried within the final particles of PMMA due to the coagulation during the homogeneous nucleation stage. The poorer performance of MMA polymerization than that of St polymerization could be well explained by the homogeneous nucleation mode of particles combined with relatively slow transporting rate of macro-RAFT molecules from the partly frozen micelles to the newly-born particles.2) The kinetics and controlled/living features of RAFT ab initio emulsion polymerization of MMA using PMAA41-b-PMMA8-RAFT as both surfactant and mediator were investigated. No inhibition period was observed, and the relatively high final PDIs (～1.5) of PMMA was due to the non-uniform distribution of RAFT agent among different particles resulted from the homogeneous nucleation together with the gel effect in the late stage of the emulsion polymerization. Two approaches were proposed to decrease the PDI of PMMA:a. Both adding small RAFT agent 2-cyanopropan-2-yl benzodithioate (CPDB) as additional mediator and 10wt% toluene based on MMA could decrease the PDI; b. Copolymerization of MMA and St (when St>50wt%) would change the nucleation mode from partial homogeneous nucleation to micellar nucleation and thus result in the PDI as low as 1.3.3) A series of poly(methyl methacrylate)-b-poly(n-butyl acrylate) (PMMA-b-PnBA) diblock copolymers with various compositions was synthesized via RAFT emulsion polymerization. The resulting core (PnBA)-shell (PMMA) particles of PMMA-b-PnBA were found to be very effective impact modifier for PC. The influence of core-shell particle composition, modifier dosage, and core crosslinking on the toughening efficiency was investigated. The results showed that:a. RAFT emulsion polymerization was a simple route to synthesize PMMA-b-PnBA. The diblock copolymers could be achieved within two hours with almost full conversion, and the molecular weights agreed well with theoretical prediction although the PDI was relatively abroad. b. The diblock copolymers could be well dispersed into 100-300nm particles in the PC matrix by melt mixing and the dispersed particle size was highly dependent on the block copolymer compositions. PMMA25o-b-PnBA55o, which could be dispersed uniformly into 100nm particles, presented the best impacting properties. c. Compared with the neat PC, the notched impact strength of PC toughened by 5wt% PMMA25o-b-PnBA550 was increased by four times to 62.81kJ/m2 with the same yield strength, a slightly decreased modulus, and an increased elongation at break. Besides, the solvent resistance was largely improved.4) The partial homogeneous nucleation mode in MMA emulsion polymerization led to very heterogeneous polymer compositions in the synthesis of PMMA-PnBA-PSt triblock copolymer. As a result, the triblock showed poor mechanical properties. Changing the nucleation mode to the micellar nucleation by adding some St to MMA emulsion polymerization (St:MMA=1:1(w/w)) could lead to much better controlled P(MMA-co-St)-PnBA-PSt (30k-70k-30k) triblock copolymer latex. The elastic modulus, ultimate tensile strength, and elongation at break of the P(MMA-co-St)-PnBA-PSt could reach 26.5MPa,9.48MPa and 224%, respectively.