Abstract
2-Hydroxyethyl methacrylate (HEMA) is an important component of many acrylic resins used in coatings formulations, as the functionality ensures that the chains participate in the cross-linking reactions required to form the final product. Hence, the knowledge of their radical copolymerization kinetic coefficients is vital for both process and recipe improvements. The pulsed laser polymerization (PLP) technique is paired with size exclusion chromatography (SEC) and nuclear magnetic resonance (NMR) to provide kinetic coefficients for the copolymerization of HEMA with butyl methacrylate (BMA) in various solvents. The choice of solvent has a significant impact on both copolymer composition and on the composition-averaged propagation rate coefficient (kp,cop). Compared to the bulk system, both n-butanol and dimethylformamide reduce the relative reactivity of HEMA during copolymerization, while xylene as a solvent enhances HEMA reactivity. The magnitude of the solvent effect varies with monomer concentration, as shown by a systematic study of monomer/solvent mixtures containing 50 vol%, 20 vol%, and 10 vol% monomer. The observed behavior is related to the influence of hydrogen bonding on monomer reactivity, with the experimental results fit using the terminal model of radical copolymerization to provide estimates of reactivity ratios and kp,HEMA.
Highlights
Acrylic copolymers are commonly used as binder resins in automotive coatings because of their contributions to film properties, including adhesion, strength, appearance, and chemical and water resistances [1]
Calibration for the RI detector was performed using polystyrene standards that ranged from 890–3.55 × 105 g/mol, while the molecular weight (MW) of the copolymers were calculated via universal calibration using a weighted average of the known Mark–Houwink parameters for poly(HEMA) and poly(BMA) homopolymer [14,19]
There is no significant difference in kp,cop values between bulk and BUOH solution for higher hydroxyethyl methacrylate (HEMA) contents, there is some indication that the values in BUOH solution are decreasing to lower than bulk for the higher HEMA fractions of 0.5 and 0.6
Summary
Acrylic copolymers are commonly used as binder resins in automotive coatings because of their contributions to film properties, including adhesion, strength (crack resistance), appearance, and chemical and water resistances [1]. Validated by extensive theoretical and experimental investigations, the PLP technique has been successfully used to determine the propagation coefficients for various monomers, with results summarized by a series of IUPAC-endorsed publications providing benchmark values for styrene [3], methacrylates [4,5,6], acrylates [7,8], and vinyl acetate [9]. A follow-up study determined that solvent choice greatly influences both copolymer composition and the composition-averaged propagation rate coefficient (kp,cop ) for ST/HEMA, but has a negligible influence on the kinetics of ST/BMA copolymerization [15]. Rooney and Hutchinson [18] recently hypothesized that both the type and the amount of solvent significantly impact relative monomer reactivities by influencing the extent of H-bonding interactions between functionalized monomers (HEMA, HEA) and the same units incorporated into the copolymer chain. The ability of the terminal model to represent the dataset is assessed
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