Macromolecular Monomer Method
In this method, macromonomers (usually methoxy polyethylene glycol methacrylate) with polymerization activity were prepared by esterification reaction, and then the monomers with certain ratio were mixed together and the product was directly polymerized by solution polymerization. This synthetic process seems simple, but the intermediate separation and purification process is cumbersome and costly. Japanese Catalyst Company used short, long chain methoxy polyethylene glycol methacrylate and methacrylic acid to copolymerize directly into a concrete admixture with good slump retention and polyether side chain.
Macromolecular Reaction Method
In this method, polycarboxylic acid with known relative molecular weight was synthesized first, and then grafted onto polycarboxylic acid by esterification reaction of polyether with known relative molecular weight at higher temperature under the action of catalyst. However, due to the limited types and specifications of polycarboxylic acid products, it is difficult to adjust its composition and relative molecular weight. At the same time, due to the poor compatibility of polycarboxylic acid and polyether, the practical operation of esterification is difficult. With the esterification process, water constantly spills out, and phase separation will occur.
If a polyether monool or diol with good compatibility with polycarboxylic acid can be selected, the problem of phase separation can be solved. He Jing et al. synthesized a kind of Polycarboxylic Water Reducer by using monomer styrene and maleic anhydride to polymerize first, then sulfonate and esterify the copolymer. The water reducer has high dispersibility and good slump retention.
In-situ Polymerization and Grafting
The method integrates polymerization and esterification, that is, polyether is used as reaction medium of unsaturated monomers of carboxylic acid, and esterification occurs at the same time of polymerization of unsaturated monomers of carboxylic acid, thus avoiding the problem of poor compatibility between polycarboxylic acid and polyether. Shawl et al. gradually added the mixed solution of acrylic acid monomer, chain transfer agent and initiator to the aqueous solution of methoxy polyethylene glycol with a relative molecular weight of 2000. After 45 minutes of reaction at 60 ℃, the temperature rose to 120 ℃, and the moisture was continuously removed under the protection of N2 (about 50 min). Then the temperature of catalyst was increased to 165 ℃, and the reaction time was 1 h. The product was obtained by grafting. Although this method can control the relative molecular weight of polymers, the synthesis process is simple and the production cost is low, the monomers containing carboxyl group can only be selected generally, otherwise it is difficult to graft, and this grafting reaction is a reversible equilibrium reaction. There is a large amount of water in the pre-reaction system, and its grafting rate is not high and difficult to control. Molecular design is difficult.
