Anionic polymerization of methyl methacrylate by difunctional lithium amide initiators with trialkylsilyl protection

Abstract

Dilithium diamides derived from silyl-protected diamines (that is, N,N′-bis(trimethylsilyl)-1,3-bis(aminomethyl)benzene and N,N′-bis(triisopropylsilyl)-1,3-bis(aminomethyl)benzene, 1a and 1b, respectively) were examined as difunctional initiators for the anionic polymerization of methyl methacrylate (MMA) in toluene with the aid of organoaluminum mediators. While the dilithium diamides in combination with tributylaluminum afforded the polymers in quantitative yields, the less sterically demanding silyl amide (1a) underwent 1,2-addition (carbonyl addition) during the initial stage of polymerization to form poly(MMA) (PMMA) containing the initiator fragment, with the methacryloyl group at the chain end as a minor component (26%). Dilithium diamide 1b, which contains triisopropylsilyl groups that prevent the 1,2-addition due to their steric bulkiness, afforded PMMA with a 1,3-xylylenediamine unit (initiation fragment) in the middle of the chain. H2/Pd-C-mediated cleavage of the diamine unit yielded PMMA with a half molar mass in a narrow molar-mass distribution, which provided proof of the equivalent reactivity of the two amide anions in 1b. Dilithium diamides of Me3Si- and i-Pr3Si-protection were examined as difunctional initiators for polymerization of methyl methacrylate (MMA) in toluene in combination with tributylaluminum. The less sterically demanding Me3Si-derivative underwent 1,2-addition (carbonyl addition) in the initial stage of polymerization to form type B poly(MMA) (PMMA) as a minor component (26%). The i-Pr3Si-derivative gave type A PMMA exclusively with the 1,3-xylylenediamine unit in the middle of the chain. H2/Pd-C-mediated cleavage of the diamine unit yielded PMMA with a half molar mass and narrow molar-mass distribution.