| Graft copolymerization of styrene (St) and acrylonitrile (AN) in the presence of polypropylene glycol (PPG) is used to prepare graft polyether polyol (POP), which is widely applied in manufacturing high-performance polyurethane foams and elastomers. The present system starts from a homogeneous system and becomes a two-phase system (a PPG-rich continuous phase and a SAN-rich disperse phase) soon after the beginning due to the limitted misicbility between St-AN (SAN) copolymer and PPG. After then, polymerization involves partition behaviors of reactants and two polymerization loci, which makes the polymerization process very complicated. This work is aimed to disclose the polymerization kinetics and the particle growth. Mathematical models are also developed to describe batch and semibatch processes, and to simulate continuous process as well.Equilibrium data of St-AN-SAN-PPG quaternary system were measured. Phase equilibria of the quaternary two-phase systems, St-AN-SAN-PPG, can be treated as phase equilibria between two pseudo-ternary systems, i.e. St-AN-SAN and St-AN-PPG. A pseudo-ternary two phase equilibrium model based on Flory-Huggins (F-H) polymer solution theory is established to describe partition behaviors of St and AN. The temperature-depedent F-H interaction parameters are estimated from Hildebrand-Scatchard solubility parameters and Hansen three-dimensional solubility parameters. The estimated F-H interaction parameters are also fitted by phase equilibrium data. The calculated results agree well with the experimental data under various experimental conditions. Both monomers are prone to exist in PPG-rich phase in most cases. St/AN ratio has a significant role in their partition coefficients, which are defined as the ratio of concentrations in SAN-rich phase to that in PPG-rich phase. The increase of AN amount can increase their partition coefficients, but St has the inverse effect. SAN/PPG ratio and copolymer composition have little impact on partition coefficients of St and AN. Partition coefficients of St and AN are increased slowly with increasing temperature. Two empirical expressions are obtained to calculate partition coefficients of St and AN during polymerization.Basing on the extensive research on graft copolymerization experiments of batch and semibath processes, the following conclusions have been drawn.1) During the polymerization, the main polymerization locus shifts from the continuous process into the disperse phase, leading to sharp increase in polymerization rate and molecular weight (Mw), decrease in graft effciency and broaden in molecular weight distribution (MWD). Increase of monomer concentration, AN fraction in comonomer and reaction temperature can advance the shift of main polymerization locus into the disperse phase.2) Conversion evolution in batch process is similar to that in typical bulk polymerization and gel effect ocurrs soon after the beginning. When the feeds rate of monomer are set to be at a starvation state, the polymerization rate in semi-batch process can be divided into three stages: slow, constant and slow, and the increase of solid content can be also divided into three stages: slow, constant and slow.3) Copolymer composition is only affected by comonomer feed ratio. The apparent azeotropic point is found to be around St/AN=70/30wt/wt. When St/AN ratio changes from75/25to60/40, there is no clear copolymer composition drift.4) Graft efficiency is mainly affect by comonomer ratio. The initial graft efficiency in semi-batch process is higher than that in batch process, so the initial particle size prepared in semi-batch process is smaller than that in batch process. There is no obvious difference between graft efficiency in the later polymerization (ca.0.2-0.3).5) In batch process, average weight molecular weight (Mw) and its polydispersity index (PDI) have no difference at different St/AN ratios. However, Mw and PDI prepared in semi-batch process at St/AN=60/40wt/wt is much higher than those prepraed in batch process, and some cross-linked SAN copolymers are formed at higher solid content under such conditions. At St/AN=70/30or75/25, there is no difference for Mw and PDI obtained between batch and semi-batch process.Heterogeneous free radical polymerization models consisting of initial free radical solution polymerization models and later free radical two-phase polymerization models are developed for batch, semi-batch and continuous graft polymerization using moment method. After phase separation, partition coefficients of monomers are estimated by the phase equilibrium model or the empirical expressions derived from the phase equilibrium model; initiator and CTA are assumed to partition evenly between each phase; partition behaviors of polymer chains are evaluated by the critical chain length assumption. The well-known CCS-AK diffusion controlled model is used to estimate the initiation efficiency, kinetic parameters of termination, propagation and transfer reaction in the disperse phase from its formation. The kinetics data of batch process is used to fit the unknown model parameters.The model describes the polymerization kinetics of batch process well, including monomer conversion, PPG¯omer conversions, copolymer composition, weight average molecular weight, PDI, graft efficiency and graft ratio, and describes the shift of polymerization locus as well. The reactivity ratios of St-AN pair, AN-St pair, St-macromer pair, macromer-St, AN-macromer pair and macromer-AN pair are fitted. In the dispersed phase, the obvious "gel effect" and "cage effect" are found. In most cases, the mathematical model for semi-batch process can describe properties of products well, including:solid content, copolymer composition and graft efficiency, Mw and PDI except for Mw and PDI at St/AN=60/40wt/wt, especially at low feeding rate.Mathematical model for continuous process is used to simulate the effects of reaction conditions on solid content and product structure properties, and to carry out the stability analysis. Solid content increases sharply with increasing monomer concentration. Graft efficiency increases sharply with decreasing St/AN ratio. Mw increases with decreasing St/AN ratio and reaction temperature. Copolymer composition is only affected by comonomer ratio. Graft efficiency as well as Mw and PDI simulated in continuous process is higher than that obtained in batch or semi-batch proces. The reactor can tolerate the disturbances and insure that the transients will move back to the original steady state.Based on "homogeneous aggregation nucleation mechanism", particle formation is cocontributed by absoption of oligomer radicals and entanglement of graft copolymers. Experimental data and simulated results observe that the nucleation occurs at monomer conversion of0.01-0.05in batch process, after then particle number keeps constant. Initial concentrations of macromer and CTA, and St/AN ratio have important roles on particle stability and PSD. Smooth, spherical and well-defined particles can be prepared when St/AN ratio is<80/20wt/wt. Particle size decreases and PSD becomes narrower with increasing CTA concentration and the AN fraction in comonomers. In theory, d∝Wmt02/3·WMo-0.5·[I]0-1/12; actually, d∝Wmt00.58·WM0-0.65·[I]0-0.06.The exponent values of the experimental dependence on monomer intial mass concentration (Wmto), macromer initial mass concentration (WM0) and initiator initial molar concentration ([I]0) is in good accordance with the theoretical values. The calculated conversion-dependent particle sizes under various experimental conditions agree well with the experimental data. Particle size decreases slowly with increasing initiator concentration and reaction temperature. Particle size increases slowly with increasing monomer/PPG ratio and St/AN ratio.PSD prepared via semi-batch process differs much from that prepared by batch process. When all macromer is added into reactor with PPG firstly, the PSD in semi-batch process becomes bimodal PSD, but the PSD remains monodisperse in batch process. When all macromer is added into reactor with monomer mixture continuously, particle variation in semi-batch process is similar to that in batch process, and keeps monodisperse during the whole polymerization process. In semi-batch graft copolymerization process, the following two methods can be used to controll particle size and PSD:optimizing St/AN ratio and optimizing addition manner of macromer. |
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