Isobutylene‐rich macromonomers: Dynamics and yields of peroxide‐initiated crosslinking

Abstract

New isobutylene-rich elastomers bearing multiple pendant styrenic, acrylic, maleimidic, vinylic, and allylic functional groups have been prepared and examined in the context of peroxide-initiated crosslinking. Halide displacement from brominated poly(isobutylene-co-isoprene) (BIIR) by the requisite carboxylate nucleophiles in homogeneous toluene solutions provide the desired esters in quantitative yield without complications from dehydrohalogenation or premature crosslinking. Heating the resulting macromonomers with dicumyl peroxide to 160 C under solvent-free conditions gives thermoset derivatives, with reaction rates and yields depending markedly on functional group structure. In general, high cure extents can only be achieved using highly reactive pendant functional groups, owing to the competitive balance between crosslinking through C@C oligomerization, and degradation through b-scission of backbone macroradical intermediates. Independent control of crosslinking rates and cure extents is gained through the use of nitroxyl radical traps bearing acrylate functionality.

Post‐polymerization modification by nitroxide radical coupling

“…In the case of butadiene‐rich polymers, such as cis ‐BR and vinyl‐BR, a higher number of allylic macroradicals is formed because these polymers are good hydrogen atom donors owing to the low bond dissociation energy of allylic C–H bonds and they respond to a DCP‐initiated reaction by a fast hydrogen abstraction. Both cis ‐BR and vinyl‐BR show very high G ′ values due to crosslink formation by allylic macroradical coupling, but for these polymers also C=C oligomerization can generate crosslinks through a radical addition process . Rheology data showed that both TEMPO and AOTEMPO delayed the onset of crosslinking for cis ‐BR and vinyl‐BR, but this effect occurred with a significant drop in crosslink density even for a low trapping ratio ([TEMPO]/2[DCP] = 0.25) (Fig.…”

Design of functional macromonomer derivatives of poly(isobutylene‐co‐isoprene)

“…The average contact angle from ten trials was used in a Zisman plot of the cosine of the contact angle against the surface tension of the corresponding test liquid. [12] Analysis of extrudate quality involved extruding elastomer (1 g) from a Tinius Olsen Melt Flow Index apparatus at 140 8C with a 10.0 kg load. The resulting extrudates were photographed at 10 times magnification with an Olympus microscope equipped with a Polaroid digital camera.…”

“…[12] Maleic anhydride and itaconic anhydride are leading candidates for a functional IIR macromonomer, since both reagents are relatively inexpensive and are amenable to a ring-opening þ BIIR esterification sequence that promises good atom economy. Ideally, the polymer modification can be conducted without solvent in a standard elastomer processing device.…”

“…Of course, reaching a successful compromise between functional group content and crosslink density is contingent on the requirements of a particular end use. [12] Isobutylene-rich graft copolymers with water-soluble PEO side chains have already attracted considerable interest, [28,29] since their amphiphilic character can be tuned by adjusting the length and number of pendant groups. The first reported synthesis of IIR-g-PEO involved direct etherification of CIIR, [30] yielding a phase-separated morphology for copolymers containing as little as 10 wt% of 750 MW PEO side chains.…”

“…[9,10] A straightforward adaptation of this approach produced acrylate and vinylbenzoate esters from BIIR by halide displacement with appropriate tetrabutylammonium carboxylate salts (Scheme 1). [11,12] Conversion of the 0.15 mmol of allylic bromide functionality per gram of BIIR to the desired esters provided macromonomers that crosslink to a high covalent network density when heated with a small amount of peroxide initiator.…”