Mechanism Of RAFT Emulsion Polymerization And Synthesis Of Poly(Styrene-b-n-butyl Acrylate-b-styrene)

It has been nearly 20 years since the breakthrough of controlled/living radical polymerization (CLRP). However, the large-scale industrial application of CLRP has not been realized yet. It is generally believed that block copolymer with high molecular weight will be the most important product of CLRP. The emulsion polymerzation is considered to be the most available process to realize CLRP industrial application. This thesis focuses on the problems observed in reversible addition fragmentation chain transfer (RAFT) emulsion polymerization, such as colloidal instability, loss of control on molecular weight, high molecualr weight distribution (PDI). Systematic investigation was done on the mechanism of RAFT emulsion polymerization, polymerization kinetics, and the synthesis of block copolymer. The followings were the innovative achievements:1. Mechanism of RAFT emulsion polymerization:(1) Based on the research, the mechanism of colloidal instability in RAFT emulsion polymerization was:First, in the nucleation period of the emulsion polymerization, when the surfactant with low interfacial tension was used, such as SDS, superswelling theory was proved to be the mechanism of colloidal instability. Early-borne particles were superswollen to 1μm in diameter and were apt to coagulate by shear and buoyancy forces. The surfactant with high interfacial tension such as PAA27-PSt5 and PAA27-PSt5-RAFT could effectively suppress the superswelling; Second, during the synthesis of high molecular weight polymer, the amount of surfactant PAA27-PSt5-RAFT was decreased, respectively. At interval II of the polymerization, due to the insufficient coverage of surfactant on the particle interface, coalescence lead to coagulum formatiom. Post-neutralization could increase the charge repulsion of PAA27-PSts-RAFT, against particle coalescence and increase the colloidal stability of emulsion polymerization.(2) A significant inhibition period was observed in emulsion polymerization mediated by oil soluble small RAFT agent. In the early period of polymerization, small RAFT agents in the micelles converted into oligomeric RAFT agents. The R radicals exited to the aqueous phase and caused inhibition period. At the same time, small RAFT agents in the monomer droplets diffused through the aqueous phase to the polymerization loci to replenish the consumed small RAFT agents. The experiment results showed that with the help of the inhibition period, most polymer chains had been "initiated" at very low conversion, which helped to increase the regularity of polymer chains.(3) In poly(acrylic acid)-b-polystyrene macroRAFT (PAA-PSt-RAFT) emulsion polymerization, initial pH value of the aqueous phase played a critical role for the success of RAFT emulsion polymerization:First, at low initial pH value, a micelle contained large amount of PAA27-PSt5-RAFT chains, and the number of micelles was relatively low. The nucleation period finished at relatively low conversion, and the system was characteristic of controlled polymer molecular weight and low PDI; Second, as the initial pH value increased, the neutralization of poly(acrylic acid) block increased. The required number of PAA27-PSt5-RAFT forming a micelle decreased, and the number of the micelles in the system increased significantly. The particle nucleation lasted the whole polymerization process. Although the good colloidal stability was obtained, loss of controll on molecular weight and high PDI was observed attributed to the continuous initiation of polymer chains.2. Guided by the above research of the mechanism of RAFT emulsion polymerization, the kinetic of macroRAFT emulsion polymerization mediated by PAA.28-PSt5-RAFT and PAA17-PSt3-RAFT was investigated. The system had a obvious inhibition period, followed by a increasing polymerization rate. In Stage II, the polymerization rate Rp=kpn-[M]pNp/NA,in which n-=0.5, Np∝[KPS]0.40 [macroRAFT]0.77 (The KPS concentration was 0.75-2.86x10-3mol/L, and the macroRAFT agent concentration was 0.74-1.58x10-2mol/L). The inhibition period was prolonged with increased initial aqueous pH value, independent of the monomer type. Compared with PAA28-PSt5-RAFT, the prolonged inhibition period was more sensitive for PAA17-PSt3-RAFT with small number of hydrophobic units in R group. This was attributed to the exit of R radicals fragmented from macroRAFT agents and their termination in the aqueous phase.3. A series of well-controlled polystyrene-b-poly(n-butyl acrylate)-b-polystyrene triblock copolymers (PSt-PnBA-PSt, Mn:76,800-338, 100g/mol, the polystyrene content:20.2-71.5%wt) with few percent of dead chains (<6.2%mol) were prepared via macroRAFT emulsion polymerization. During the synthesis of PSt-PnBA diblock copolymers, due to a phase separation occurred inside particles, the PDI increased dramatically when PnBA exceeded 60%wt. The ultimate tensile strength reached 10MPa with an elongation at break of 500% at the polystyrene composition of 40-50%wt.