Peroxide-Curable Macromonomer Derivatives of Isobutylene-Rich Elastomers

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Dakin, Jackson McGuire
Macromonomer , Peroxide-Curable , Butyl Rubber , Ionomer
Macromonomers bearing oligomerizable C=C functionality have been prepared by the nucleophilic displacement of allylic bromide functionality on brominated poly(isobutylene-co-isoprene) (BIIR). Whereas commercial grades of isobutylene-rich elastomers do not cure under the action of peroxides, these materials undergo simultaneous cross-linking and degradation when activated by radical initiators, with the competitive balance dictated by the reactivity of the oligomerizable group. Vinyl benzoate, vinyl imidazolium, and acrylate functionalities cure rapidly to high cross-link density whereas the maleimide graft is too reactive and unstable for any utility. Methacrylate and itaconate macromonomers cure to moderate extent while maleate esters and unactivated terminally unsaturated groups are unable to significantly counteract the degradation mechanism and do not afford any appreciable cross-link density to BIIR. The most reactive macromonomers display the potential for scorch, an effect that is efficiently mitigated with the introduction of (2,2,6,6-tetramethyl-piperidin-1-yl)oxyl (TEMPO) to quench free radical cure activity. Furthermore, an acrylated adduct, AOTEMPO, is able to recover more of the cross-link density that would otherwise be lost to irreversible free radical coupling. These nitroxyls display longer than expected induction times, likely due to the catalytic nature of TEMPO when alkoxyamine decomposition is significant. A suite of elastomeric ionomers bearing N-functional imidazolium bromide functionality have been prepared in order to investigate the N-alkylation dynamics with brominated poly(isobutylene-co-para-methylstyrene) (BIMS) as well as the subsequent peroxide cure activity of the reactive ionomer. A functional imidazole bearing a methacrylate group displayed moderate alkylation rate and good cure activity whereas a 4-vinylbenzyl analogue provides fast alkylation at the expense of storage stability. N-Allylimidazole is rapidly alkylated by BIMS in both solution and solvent free processes and the resulting ionomer displays unique cure dynamics. This phenomenon is investigated by model compound polymerization and is likely due to the unique free radical reactivity of allyl imidazolium moieties. The cross-linked ionomer displays many of the beneficial physical properties associated with a hybrid ionic/covalent network including good resistance to stress relaxation and thermal stability.
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