Acidic Aqueous-Phase Copolymerization of AA and HPEG Macromonomer: Influence of Monomer Concentration on Reactivity Ratios

Abstract

Poly(acrylic acid-co-polyethylene glycol 2-methyl-2-propenyl ether) copolymers are comb-like water-soluble copolymers produced by the copolymerization of acrylic acid (AA) and polyethylene glycol 2-methyl-2-propenyl ether (HPEG). The main application of these copolymers is as superplasticizers for cementitious materials, also known as polycarboxylate ethers (PCE's). The kinetics of the water-soluble monomers is substantially more complex than that of non-water-soluble monomers as their kinetics depend on various parameters such as monomer concentration, pH, and ionic strength. In this work, aqueous in situ H-1 NMR copolymerizations of AA and HPEG at different initial overall monomer weight fractions and comonomer molar ratios under acidic media were carried out. The gathered kinetic data were used for the estimation of the reactivity ratios of AA-HPEG. A nonlinear least-squares (NLLSQ) method based on the Mayo-Lewis composition equation was used to estimate the reactivity ratios by fitting the cumulative copolymer composition or free monomer molar fraction as a function of overall monomer conversion. The estimation indicates that the reactivity ratio of acrylic acid is higher than that of the HPEG monomer, which is estimated as close to zero. In addition, the reactivity ratio of AA depends on the overall monomer weight fraction; the higher the initial overall monomer concentration, the higher the reactivity ratio is. An empirical expression is derived that describes the dependency of the reactivity ratio of AA on the overall monomer mass fraction (r(AA) = 1.76 (+/- 0.062) + 0.0275 (+/- 4.37 x 10(-3)) w(M) (%); r(HPEG) = 2.3 x 10(-14) (+/- 6.02 x 10(-3))).