Monomer Structure and Solvent Effects on Copolymer Composition in (Meth)acrylate Radical Copolymerization

Abstract

Solvent effects on reactivity ratios (ri ) and overall composition-averaged copolymer propagation rate coefficients (k p,cop) are generally not observed during methacrylic ester radical copolymerization. However, hydroxyl-bearing comonomers, such as 2-hydroxyethyl methacrylate (HEMA), lead to significant deviations from expectation for both ri and k p,cop, an effect that is highly dependent on solvent choice and rooted in hydrogen bond interactions. The current understanding of the influence of hydrogen bonding on organic solution (meth)­acrylic ester radical (co)­polymerization kinetics is reviewed by summarizing trends in structure/reactivity for methacrylate homopropagation rate coefficients (k p) and methacrylate macromonomer relative reactivity during copolymerization. In addition, the peculiarities that characterize the apparent enhanced reactivity of hydroxyl-bearing monomers during copolymerization are outlined. Finally, a modeling framework to systematically capture the effects of solvent and hydrogen bonding on copolymer composition, through specific intramolecular hydrogen bond associations between monomer and growing chain, is presented for several methacrylate, acrylate, and styrene copolymerizations.

Solvent Effects on Radical Copolymerization Kinetics of 2-Hydroxyethyl Methacrylate and Butyl Methacrylate

“…In line with previous studies, the polar HEMA has higher incorporation into the copolymer than the non-polar BMA. While this enhanced reactivity is explained by H-bonding between HEMA molecules, it is somewhat surprising, as discussed by Rooney and Hutchinson [18], that HEMA does not also enhance BMA reactivity to the same extent through H-bonding with the BMA carbonyl group; previous work has shown that BUOH as a solvent enhances BMA reactivity both during homopolymerization [12] and when copolymerized with styrene [15]. The result suggests that the intermolecular bonding between HEMA units is stronger than that between HEMA and BMA.…”

“…Thus, it was concluded that the bootstrap effect alone cannot explain the magnitude of the increased HEMA incorporation. Rooney and Hutchinson [18] propose that the increased reactivity of HEMA results not from monomer aggregation, but rather from HEMA hydrogen bonding between monomer and a HEMA unit located on the growing chain close to the radical end in the non-polar solvent, increasing the incorporation rate of the HEMA into the copolymer relative to the bulk system.…”

“…While all studies relate the behavior to the hydroxyl functional group, the literature has been unable to reach a consensus on the cause of the increased reactivity. It has been hypothesized that a donor–acceptor complex between the growing polymer radical and the aromatic solvent affects reactivity [28,31], and that the non-polar solvent may lead to partial solubility that results in localization of the monomer species and preferential solvation [23,26] or to enhanced H-bonding involving HEMA units in the polymer chain [18]. As discussed for the copolymer composition results, perhaps all of these factors contribute to the behavior observed.…”

“…Polymerizations were performed to low conversions in a pulsed laser setup following previously established procedures [14,15,16,17,18]. A Coherent (Santa Clara, CA, USA) Excimer Xantos XS Laser (XeF as the reactive gas) capable of producing a 351 nm laser pulse was used as the radiation source, with 5 ns pulse duration, 500 Hz pulse repetition capability, and 6 mJ maximum energy per pulse.…”

“…While similar behavior has been observed when HEMA or 2-hydroxyethyl acrylate (HEA) are copolymerized with other acrylates and methacrylates [16,17], the influence of solvent choice on the system reactivity ratios has yet to be generalized. 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. However, more data are required to determine whether this representation can be applied across a broad range of conditions.…”