Oxidatively Induced Concurrent Cationic and Radical Polymerization of Isobutylene in the Presence of LiCB₁₁Me₁₂

Abstract

A solution of a mechanistic puzzle is reported: upon initiation with air at 25 °C or with di-tert-butyl peroxide at 80 °C, isobutylene (IB) polymerizes at 1 atm in weakly coordinating solvents containing 10 wt % LiCB(11)(CH(3))(12) to a mixture of highly branched (b-PIB) and linear (l-PIB) polyisobutylene. The former polymer is separable by solvent extraction and is identical with the b-PIB that is produced from IB as a sole product under similar conditions under nonoxidizing radical initiation with azo-tert-butane. The latter polymer differs from standard l-PIB in that it carries a carborane anion attached at the chain end. The molecular weight of b-PIB ranges up to 26,000, and that of l-PIB, up to 50,000. Evidence is presented that the concurrent polymerization of IB to b-PIB and l-PIB is launched by an initial oxidation of the CB(11)(CH(3))(12)(-) anion to a neutral radical CB(11)(CH(3))(12)(•). This radical is proposed to subsequently transfer a methyl radical to IB, thus launching the formation of b-PIB by the radical mechanism while leaving behind the borenium ylide CB(11)(CH(3))(11), which is a strong Lewis acid and induces simultaneously the formation of l-PIB by the cationic mechanism.

Metal Complexes with Very Large Dipole Moments: the Anionic Carborane Nitriles 12-NC–CB11X11– (X = H, F, CH3) as Ligands on Pt(II) and Pd(II)

“…cis-Bis(triethylphosphine)-bis(12-cyano-1-carba-closododecaborate)platinum(II) (16). This was prepared from cisdichlorobis(triethylphosphine)platinum(II) (12) and Cs[12-CN-CB 11 H 11 ] (2) according to GP2.…”

Cationic polymerization of isobutylene at room temperature

“…The most important monomers polymerizing through a cationic mechanism are isobutylene (IB), styrene and its derivatives, vinyl ethers, cyclopentadiene, and N ‐vinylcarbazole 1, 2. Among these, IB is the most studied one as it polymerizes only by cationic mechanism3 and its polymer is the only organic polymer (in comparison with silicones or polyphosphazenes) to exhibit such unique properties as low permeability, good thermal and oxidative stability, chemical resistance, high hysteresis (mechanical dampening), and tack 4, 5. A very important commercial application arising from IB cationic polymerization is the manufacture of high‐ ( M n > 120,000 g mol −1 ), medium‐ ( M n = 40,000–100,000 g mol −1 ) and, especially, low‐molecular‐weight ( M n < 5,000 g mol −1 ) polyisobutylenes (PIB) 5.…”

Carboranes in the chemist's toolbox

“…38 In a somewhat different Li + -catalyzed process, radical polymerization of alkenyl-carborane salts of the type Li + [CH 2 vCH(CH 2 ) n−2 ]CB 11 Me 11 − is initiated by O 2 , azoisobutylnitrile, or di-tert-butylperoxide, and occurs in solution or in the solid state. 39,40 Polymerization of isobutylene in the presence of CB 11 Me 12 −, in air at 25 C or with di-tert-butyl peroxide at 8°C, yields linear and highly branched polyisobutylene having molecular weights up to 50 000 and 26 000 respectively 41,42. While the branched polymer is identical with that obtained under nonoxidizing conditions, the linear polymer has a CB 11 Me 12 − unit appended at the end of the chain.…”

“…While the branched polymer is identical with that obtained under nonoxidizing conditions, the linear polymer has a CB 11 Me 12 − unit appended at the end of the chain. The process is proposed to involve transfer of a methyl radical from a neutral CB 11 Me 12 * radical to isobutylene, thereby initiating radical polymerization, while the remaining CB 11 Me 11 borenium ylide, a strong Lewis acid, induces formation of the linear polymer by a cationic mechanism 41.…”