Structural Analyses of Methyl Bicyclobutane-1-carboxylate Oligomers Formed with tert-Butyllithium/Aluminum Bisphenoxide and Mechanistic Aspect of the Polymerization

Abstract

ABSTRACT:Anionic polymerization of methyl bicyclobutane-1-carboxylate (MBCB) with tert-butyllithium (t-BuLi)/bis(2,6-di-tert-butylphenoxy)ethylaluminum [EtAl(ODBP) 2 ] in toluene at À78 C gives transtactic polymers (trans contents > 90%) with high yields, but with low initiator efficiency and broad molecular weight distribution. In order to obtain information on the initiation mechanism, oligomers of MBCB were prepared with t-BuLi/EtAl-(ODBP) 2 in toluene at À78 C, and fractionated into dimer to pentamer by size exclusion chromatography. The dimer fraction was further subjected to recycling SEC and found to consist of two types of dimers of different chemical structures. 1 H NMR analyses of the dimers revealed that one type of the dimer comprises of an initiating tert-butyl (t-Bu) group and two MBCB units while the other contains one t-Bu carbonyl group within the monomeric units in addition to the initial t-Bu group. Based on the NMR spectroscopic information on the oligomers, 1 H NMR spectra of the polymer was investigated with the focus on the end-group structures, that confirms that the carbonyl addition of t-BuLi to MBCB occurred and the ketonic compound thus formed participates in the polymerization. [DOI 10.1295/polymj.37.439] KEY WORDS Transtactic 1,3-Cyclobutane Polymer / Initiation Mechanism / Stereospecific Anionic Polymerization / Chain-end Analysis / NMR spectroscopy / Carbonyl Addition / Bicyclobutanes carrying electron-withdrawing substituents at 1-and/or 3-positions are a remarkable class of reactive monomers.1 Either free radical or anionic polymerization occurs readily across thelike 1,3-bond to give high molecular weight polymers consisting of 1,3-linked cyclobutane rings in the main chain.1 Such polymers have interesting properties including high glass transition temperatures, chemical and thermal stability, superior optical clarity, and resistance to depolymerization.1,3-Disubstituted cyclobutanes exist as either cisor trans-isomers. Accordingly the stereochemistry of the polybicyclobutanes becomes of interest, particularly for its influence on physical properties. Free radical polymerization of dimethyl bicyclobutane-1,3-dicarboxylate gives 95% trans-linkages.2 Similar polymerizations of various 1-monoesters give polymers with trans/cis ratio of about 2/1. 3Anionic polymerization offers potentially better control of stereochemistry of polymers, but early attempts for methyl bicyclobutane-1-carboxylate (MBCB) with n-butyllithium as an initiator gave only low molecular weight products. In the previous paper, 4 we reported the anionic polymerization of MBCB with tert-butyllithium (t-BuLi)/bis(2,6-di-tertbutylphenoxy)ethylaluminum [EtAl(ODBP) 2 ] in toluene at À78 C gave transtactic polymers (trans contents > 90%) in high yields. This kind of favorable effect of aluminum bisphenoxides on the anionic polymerization has been well demonstrated in stereospecific living polymerizations of methacrylates and acrylates.5-14 However, the given poly(MBCB) had broad molecular weight distribution (M w =M...

Bicyclobutanes: from curiosities to versatile reagents and covalent warheads

“…29 Anionic polymerization of methyl bicyclobutane-1-carboxylate itself gives trans-ratios of >90%, when initiated with tert-butyllithium/bis(2,6-di-tert-butylphenoxy)ethylaluminum in toluene at À78 C, albeit with low initiator efficiency and broad molecular weight distribution. 34 A mechanistic study concluded that attack of tert-butyllithium on the ester to incorporate less-reactive ketone unit anions was occurring as a side-reaction but was mostly suppressed by coordination of the ester to the aluminium Lewis acid. 34 Photochemical methods have been employed for synthesis (and degradation) of BCB, and the mechanistic pathways of these reactions have been studied.…”

“…34 A mechanistic study concluded that attack of tert-butyllithium on the ester to incorporate less-reactive ketone unit anions was occurring as a side-reaction but was mostly suppressed by coordination of the ester to the aluminium Lewis acid. 34 Photochemical methods have been employed for synthesis (and degradation) of BCB, and the mechanistic pathways of these reactions have been studied. 37 The 185 nm photolysis of BCB leads to the formation of butadiene and cyclobutene (Scheme 1D).…”

“…In general, polymerization using more highly substituted monomers or anionic polymerization seems to offer higher degrees of stereospecificity. 34 Barfield reported approximately a 75 : 25 ratio of trans - to cis -linkages in anionic poly(bicyclobutane-1-carbonitrile) (PBBC). 35 A subsequent study of PBBC prepared by radical polymerization found a 68 : 32 isomeric ratio, comparable to radical poly(1-methoxycarbonyl)bicyclobutane (68 : 32) and poly(bicyclobutane-1-carboxamide) (73 : 27).…”

Ring-Strain-Enabled Reaction Discovery: New Heterocycles from Bicyclo[1.1.0]butanes

“…Many bicyclo[1.1.0]­butanes, however, can be purified by conventional chromatographic methods and stored over prolonged periods of time. This surprising level of chemical inertia originates from the relatively high kinetic barrier for ring-opening processes. , A major general reactivity pattern, especially for bicyclo[1.1.0]­butanes conjugated to electron-deficient groups (e.g., ester, aldehyde, amide), is radical addition, and this property is useful for the preparation of organic polymers. − Typically, 1,3-disubstituted and sterically shielded bicyclo[1.1.0]­butanes are more robust than less-substituted analogues, and neat samples can be stored at −20 °C for several months with minimal decomposition. Small amounts of radical inhibitors (e.g., BHT) can provide additional stabilization.…”

Asymmetric anionic polymerizations of N-substituted itaconimides having chiral amino-acid esters

“…32,33 In recent years, Nakano et al [34][35][36][37] has reported radical, anionic and cationic polymerizations of 2,7-di-npentyldibenzofulvene and dibenzofulvene, which are 1,1-diphenylethylene analogs and yield p-stacked polymers.…”