New telechelic polymers and sequential copolymers by polyfunctional initiator-transfer agents (inifers). LIII. Lithiation of 2,4,4-trimethyl-1-pentene and isopropylidene-capped polyisobutylenes by butyllithiums in the presence of complexing agents

Lithiated isopropylidene-telechelic polyisobutylenes (i.e., PIBs capped with end groups) are most interesting novel intermediates for further transformations, e.g., functionalization, polymerization. This report concerns model lithiation experiments of 2,4,4-trimethyl-1-pentene (TM1P) that guided us toward the subsequent quantitative lithiation of isopropylidene-telechelic PIBs. Thus, lithiation of TM1P with, n-, s-, and t-butyllithium, in the presence of various complexing agents (i.e., TMEDA, t-BuOK, 1,2-DPE, CH₃OCH₂CH₂OCH₃, THF, and 12-crown-4) followed by silylation with Me₃SiCl (for the purpose of quantitation) gave three products: 2(trimethylsilylmethyl)-4,4-dimethyl-1-pentene (TM1P-Si), 2(trimethylsilylmethyl)-4,4-dimethyl-2-pentene (TM2P-Si₂). The relative product composition strongly depends on the BuLi/complexing agent ratio and temperature. Among the different butyllithiums and complexing agents the best overall results were obtained with the s-BuLi/TMEDA combination. Complete lithiation of TM1P with minimum dilithiation was obtained using the molar ratio s-BuLi]: TMEDA]: TM1P] = 2 : 2 : 1. The apparent activation energy of lithiation by s-BuLi/TMEDA was found to be 6.7 ± 0.8 kcal/mol. Guided by TM1P model experiments, quantitative monolithiation of isopropylidene-capped polyisobutylene (including ca. 4% chain and isomerization) was achieved using the molar ratio s-BuLi] : TMEDA] : C? C] = 5 : 4 : 1.