Kinetics And Phase Behaviors Of Acrylamide RAFT Polymerization In Polyethylene Glycol Aqueous Solution

Combined the characteristics of aqueous two-phase polymerization (ATPP), reversible addition-fragmentation chain transfer (RAFT) polymerization and aqueous two-phase system (ATPS), a new polymerization method was proposed, in which polyacrylamide (PAM) was synthesized in polyethylene glycol (PEG) aqueous solution by RAFT ATPP with 2,2’-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride (VA-044) as initiator. This new kind of polymerization was organic solution free, stable, and was of low viscosity. After short-time standing, the products synthesized by RAFT ATPP would separate into two phases, one of which had high concentration of narrow distributed PAM and the other phase had high concentration of PEG, which could be recycled. In this work, the kinetics of acrylamide (AM) RAFT ATPP were investigated, and the effects of polymerization parameters on polymerization rate, the molecular weight and its distribution of PAM were discussed. The effects of various factors on the phase behaviors of H2O-PEG-PAM-AM ATPSs were investigated by experimental method and predicted by the extended NRTL model furtherly.Firstly, the properties of AM RAFT ATPP were investigated by several aspects, which include viscosity, gathering effect of polymers, polymerization rate, molecular weight and distribution of PAM. The addition of RAFT agent reduced the viscosity of AM ATPP furtherly. The viscosity decreased with the rise of RAFT agent concentration and increased with the rise of molecular weight and concentration of PEG. In the AM RAFT ATPP, with the rise of AM conversion, PEG and PAM gathered in the two phases respectively. High concentration of initiator and monomer and high temperature could raise the initial polymerization rate, while the polymerization rate became slower with the increase of concentrations of PEG and RAFT agent. The peak polymerization rate increased with the rise of concentrations of initiator and monomer and the temperature of the polymerization system, meanwhile the time of peak polymerization became shorter. The increase of RAFT agent concentration could prolong the time of peak polymerization rate, while the relationship of time of peak polymerization rate and the concentration of RAFT agent was complicated, because RAFT ATPP of AM was the combination of RAFT polymerization and ATPP. With the increase of concentration of RAFT agent, the molecular weight and its distribution of PAM decreased. The increase of concentrations of PEG and AM leaded molecular weight distribution become wider. The concentration of initiator, molecular weight of PEG had little effect on molecular weight distribution of the products.Secondly, phase behaviors were the key of the distribution of components in the two-phases and gathering effect of AM RAFT polymerization, therefore it had great significance to AM polymerization. Phase diagrams of H2O-PEG-PAM ATPSs, consisted of narrow distributed PAM and PEG, were obtained with precipitation weighing method. The rise of molecular weight of PAM and PEG lead the area of two-phase increase. The area of two-phase decreased with the rise of temperature. In H2O-PEG-PAM-AM ATPSs, with the rise of PEG molecular weight, the partition coefficient of AM and PAM decreased slightly, while the partition coefficient of PEG apparently increased. With the rise of PEG concentration, the partition coefficient of AM decreased, the partition coefficient of PAM firstly decreased and then increased, while the partition coefficient of PEG firstly increased and then decreased. When the PEG concentration was 10 wt%, the partition coefficient of PEG was the largest. The influence of PAM molecular weight on the partition coefficients of PAM was not apparent. The partition coefficient of AM increased with the rise of PAM molecular weight, while the partition coefficient of PEG firstly increased and then decreased. When the temperature increased from 30℃to 60 ℃,the partition coefficient of AM and PAM increased, while the partition coefficient of PEG firstly decreased and then increased.Finally, the phase behaviors of AM RAFT ATPP were furtherly described and predicted by thermodynamic model. The influence of molecular weight of PAM and PEG and temperature on the phase diagrams of H2O-PEG-PAM ATPSs could be well predicted by the extended nonrandom two-liquid (NRTL) model; The overall average absolute deviation was lower than 1.5 wt%, and the maximum absolute deviation was lower than 3.0 wt%. For H2O-PEG-PAM-AM ATPSs, the extended NRTL model could well predicted the influence of various factors, such as PAM molecular weight, molecular weight and concentration of PEG and temperature, on the concentrations of all components in the two phases; The overall average absolute deviation was lower than 1.5 wt%, and the maximum absolute deviation was lower than 6.0 wt%; The predicted results of AM concentration in the two phases were especially perfect. When the concentrations of all components in the process of polymerization were predicted by the extended NRTL model, the results were also satisfying, with the overall average absolute deviation lower than 1.5 wt% and the maximum absolute deviation lower than 6.0 wt%.