Living and Highly Isotactic Polymerization of Methyl Methacrylate by t-C4H9MgBr in Toluene

Abstract

ABSTRACT:Highly isotactic PMMAs with narrow molecular weight distribution (Mwl M. < 1.2) have been prepared by the polymerization of MMA in toluene at -78oC initiated by t-C 4 H 9 MgBr prepared in diethyl ether. The polymerization proceeded in a "living" manner, in which the initiation reaction was fast and quantitative whereas the propagation was slow. The ether solution of t-C4 H9MgBr used as the initiator contained almost equal amounts of MgBr2 produced by an Wurtz type coupling during the preparation. It has been found from 1 H NMR analyses that the excess amount of MgBr 2 contributes to form the initiating species of homogeneous activity and stereospecificity. Polymerizations of MMA by n-, iso-, and s-C4 H9 MgBr were also investigated in toluene at -78°C. With an increase in the bulkiness of the alkyl group the isotacticity of the polymer increased and the molecular weight distribution became narrower. The addition of MgBr 2 also .increased the isotacticity of the polymer. The results of block copolymerizations of MMA and MMA-d8 or ethyl methacrylate are also described.KEY WORDS Methyl Methacrylate I t-Butylmagnesium Bromide I Isotactic PMMA I Tacticity I Narrow Molecular Weight Distribution I Living Polymerization I Grignard Reagent I Schlenk Equilibrium I Block Copolymerization I 1 H and 13 C NMR I understanding of the reaction mechanism. Isotacti<:: poly(methyl methacrylate) (PMMA) is usually prepared by an anionic initiator such as alkyllithium or Grignard reagent in a nonpolar solvent. Such a polymerization system often involves multiple active species and side reactions, 1 -11 making the molecular weight distribution (MWD) of the resulting PMMA broad. For example, in the polymerization of alkyl methacrylate by n-C 4 H 9 Li 8 • 10 or CH 3 CH 2 CH 2 CD 2 Li 12 the initiator reacts with the carbonyl double bond during the initiation process, and this side reaction brings about the formation of propagating species of different tacticities and activities. Moreover, in the polymerization of methacrylate by Grignard reagent, Schlenk equilibrium also makes the reaction complex, 2 • 6 and these have prevented a clear In the previous communication, 13 we reported that t-C 4 H 9 MgBr prepared in diethyl ether caused no side reaction in the polymerization of MMA in toluene at -78°C and formed highly isotactic PMMA with narrow MWD. That is the first example of the direct preparation of highly isotactic PMMA with narrow MWD, although it was derived from the isotactic poly( triphenylmethyl methacrylate) prepared in THF with an organolithium compound. 14 • 15 In this work we studied the polymerization of MMA by t-C 4 H 9 MgBr under several reaction conditions and clarified the mechanism of formation of highly isotactic PMMA with narrow MWD. 1037

Anionic Polymerization of (Meth)acrylates with Trialkylsilyl-protected Lithium N-Benzylamide

“…12,13 Following this finding, we also reported several stereospecific living polymerizations of methacrylates; syndiotactic-specific one with tert-butyllithium (tBuLi)/tributylaluminum (n-Bu 3 Al), 14,15 and heterotactic-specific one with t-BuLi and bis(2,6-di-tert-butylphenoxy)methylaluminum [MeAl(ODBP) 2 ]. [16][17][18][19][20][21][22][23][24] More recently, the authors have reported that t-BuLi/bis(2,6-di-tert-butylphenoxy)ethylaluminum [EtAl(ODBP) 2 ] initiating system was effective for stereospecific living polymerization of methacrylates, 20 acrylates 25 and methyl bicyclobutane-1-carboxylate.…”

“…In order to confirm this assumption, 13 C NMR experiments of these initiator solutions were carried out in toluene-d 8 at À78 C (Figure 1). In the 13 C NMR spectrum of BnTMSNLi itself showed two sets of distinguishable signals.…”

“…Identification of a polymerization by-product, N-benzylmethacrylamide, was made by analysis of 1 H NMR, 13 2 (1/3) was conducted in toluene at À78 C. The results were summarized in Table I. In the first series of experiments, the reagents were added to toluene in the following order (addition order [A]); (1) aluminum compounds, (2) lithium amides, and (3) monomer, as reported for the polymerization with t-BuLi/ R 3 Al.…”

“…Though almost no difference was found among the polymers obtained with these lithium amides, some of the 1 H NMR spectra of the crude polymerization mixtures with BnTMSNLi/ n-Bu 3 Al, recovered without precipitation, indicate several characteristic sharp signals, assignable to low molecular weight compounds, particularly in the olefinic region (Figure 4a), which were not seen in the spectra of the polymers recovered after precipitation in hexane. From the analyses of the hexanesoluble part of the crude mixture by 1 H NMR ( Figure 5), 13 C NMR and mass spectrometry (see EXPERIMETAL section), the signals were assigned to N-benzylmethacrylamide, which should be produced through the carbonyl addition of the lithium amide to MMA. In the initiation process, BnTMSNLi attacked to the carbonyl function of MMA to form Ntrimethylsilyl-N-benzylmethacrylamide 35 with release of lithium methoxide (Scheme 3).…”

The Organometallic Polymerization of (Meth)acrylates: An Overview

“…terr-Butylmagnesium bromide may be used to synthesize highly isotactic PMMA in noncoordinating solvents at low temperatures (44)(45)(46). TTie unusual control imparted by this reagent is thought to arise from an interaction between the initiating species, which has been postulated to exist as a complex aggregate bearing multiple tert-butyl groups due to the Schlenk equilibrium, and the active endgroup (47).…”