Synthesis Of Branched Polyacrylamides By Semi-Batch RAFT Polymerization

Polyacrylamide (PAM) is one of most important water-soluble polymers. It had been widely used in wastewater treatment, papermaking, oil recovery and so on. The domestic PAM consumption in2010already exceeded450k t/a. As a flocculant, the PAMs are greatly dependent on their cationic degrees, charge distribution, and chain topologies. At present, the commercial PAM flocculants are linear random copolymers produced by conventional free radical polymerization, resulting in copolymer composition drifting during the course of copolymerization, low efficiency of charge utilization in flocculation, and no control on chain topologies. Moreover, high molecular weights make the PAMs have a long dissolution time with high viscosity in aqueous.Hyperbranched polymers have large quantities of branches and terminal functional groups, and possess lower intrinsic viscosities and shorter dissolution times than their linear counterparts. They are usually prepared by polycondenzation of ABn-type monomers or self-condensing vinyl polymerization (SCVP), with difficulties to prepare the monomers. The advent of controlled/"living" racial polymerization (CLRP) provides a new approach for the synthesis of hyperbranched polymers. Reversible addition-fragmentation chain transfer (RAFT) polymerization is one of the most promising CLRP techniques, and has been used to synthesize polymers with block, star and hyperbranched structures, owning to its applicabilities to various monomers and solvents and relative mild polymerization conditions.In this work, RAFT copolymerization of acrylamide (AM) and a divinyl monomer, N,N’-Methylenebisacrylamide (BisAM), was conducted using a semi-batch technique. Star (s-PAM) and hyperbranched polyacrylamides (b-PAM) were synthesized using different BisAM feeding policies. The b-PAMs were then used as a multi-functional macro-RAFT to mediate the polymerization of a cationic monomer, methacryloxyethyltrimethyl ammonium chloride (DMC) to produce hyperbranched cationic polyacrylamides (b-CPAM) with all the cationic units concentrating on the chain terminals. The paper covers,1) Aqueous RAFT polymerization of AM using3-(((benzylthio) carbonothioyl)thio)propanoic acid (BCPA) as a RAFT chain transfer agent was investigated. The RAFT polymerization of AM was well controlled in addition to high polymerization rate.2) Using BisAM as a divinyl branching/cross-linking agent, star polyacrylamides (s-PAM) were synthesized by arm-first or core-first RAFT polymerization, while b-PAMs were produced by constant feeding or batch RAFT polymerization. Although the core-first policy and batch RAFT polymerizations were found to be efficient to produce the s-PAMs having more arms and the b-PAMs with high branching densities, respectively, the control of gelation in these two approaches was very challenging. The semi-batch RAFT polymerization with constant feeding of BisAM was more feasible to synthesize the b-PAMs with high molecular weights and branching densities, while the arm-first policy was more applicable to produce the s-PAMs having more arm numbers.3) The effect of BisAM content, feeding rate, initiator concentration, and solid content on the b-PAM synthesis was investigated. The kinetics of semi-batch RAFT copolymerization of AM and BisAM with a constant BisAM feeding rate were also studied. It required much less RAFT chain transfer agent to suppress the gelation and tailor the b-PAMs in the semi-batch RAFT polymerization than that in batch polymerization, less than1/15of RAFT chain transfer agent in batch polymerization. Cyclization played an important role in suppressing gelation during the polymerization.4) In combination of the RAFT polymerization mechanism with a semi-batch reactor model, a semi-batch RAFT copolymerization kinetic model was developed to predict monomer conversions, molecular weights and distribution, branching densities (BD), and cyclization densities. The theoretical simulation agreed well with the experimental data. The model provided a guide for designing and tailoring the b-PAMs.5) The RAFT polymerization of DMC was conducted using the b-PAMs as a multi-functional macro-RAFT. The b-CPAMs with all the cationic blocks locating at the chain end of branches were synthesized for the first time. The DMC polymerization mediated by the b-PAMs showed controlled/living characteristics.6) Flocculations of TiO2suspensions using the b-CPAM samples having controlled MWs, cationic degrees and BDs were carried out and compared with3commercial linear cationic polyacrylamide flocculants. The b-CPAMs had comparable flocculation performance to the commercial flocculants although they possessed much lower molecular weight and cationic degree, which has a highly potential for industrial applications of high performance flocculants.