Enhanced Stereocontrol in Disyndiotactic-specific Group Transfer Polymerization of Methyl Crotonate—Stereochemical Evidence of Group Transfer

Abstract

ABSTRACT:Group transfer polymerization (GTP) of methyl crotonate was examined by using a ketene silyl acetal with tert-butyldimethylsilyl group in the presence of HgI 2 and tert-butyldimethylsilyl iodide as a catalyst and a co-catalyst, respectively. Under optimized conditions, the GTP produced disyndiotactic polymers with narrow molecular weight distribution in quantitative yields. The trialkylsilyl group in the initiator components was found to exert control over the stereochemical process of the GTP; the bulky tert-butyldimethylsilyl group leads to the highest disyndiotacticity. The results provide a direct evidence for the transferring silyl group to be involved in the propagation steps in the GTP. [DOI 10.1295/polymj.37.578] KEY WORDS Ditacticity / Stereoregularity / Methyl Crotonate / Ketene Silyl Acetal / 1-Methoxy-1-(tert-butyldimethylsiloxy)-1-propene / Group transfer polymerization (GTP), disclosed in early 1980's by Webster, 1 is one of the versatile living polymerizations for acrylic monomers. The GTP involves ketene silyl acetals as initiators and nucleophilic or Lewis acidic catalysts and has been claimed to proceed through migration or transfer of the silyl group to maintain ketene silyl acetal units at the propagating polymer chain-ends during the polymerization. The proposed mechanism, so-called ''associative mechanism'', assumes active intermediates carrying the silyl group.1-3 On the other hand, a dissociative mechanism has also been proposed, which regards the active intermediate as an enolate as in the case of classical anionic polymerizations. [4][5][6] The above mechanistical argument has urged several researchers to examine the stereochemistry of GTP with the expectation of any specific effects of the ketene silyl acetal ends on the stereochemical aspect of the propagation reaction, if the associative mechanism dominates the GTP. For example, Müller and Sticker reported that syndiotacticity of poly(methyl methacrylate) (PMMA) obtained by the GTP using nucleophilic catalyst was slightly lower than that of PMMA obtained by radical polymerization.7 Mechanism and stereospecificity of the GTP were also discussed for cyclization polymerization of a binaphthyl dimethacrylate by Nakano and Sogah based on the results that the isotacticity of the polymer obtained by the GTP was higher (34%) than that that of the polymer obtained by radical polymerization.8 GTP giving stereoregular polymers was reported for the polymerization of triphenylmethyl methacrylate using a nucleophilic catalyst (mm ¼ 91%).9 Judging from the fact that radical and anionic polymerizations of this monomer also give the isotactic polymers, 10 however, the result is hardly regarded as ''stereocontrol by GTP''.We have reported the GTP of methyl crotonate, a structural isomer of methyl methacrylate, using ketene silyl acetals (1) and (2) (Scheme 1), in the presence of HgI 2 and trialkylsilyl iodides (R 3 SiI) as catalysts. 11Later, the GTP of other alkyl crotonates (alkyl = ethyl, n-propyl, isopropyl, or n-butyl) was also found s...

Stereospecific Group Transfer Polymerization of Methyl Methacrylate with Lewis-Acid Catalysis—Formation of Highly Syndiotactic Poly(methyl methacrylate)

“…However, there had been no distinctive evidence of any special stereocontrol responsible to ''group transfer'', 4-7 until we have reported a Lewisacid catalyzed GTP of methyl crotonate with HgI 2 -iodotrialkylsilanes (R 3 SiI) as catalysts, which produces disyndiotactic poly(methyl crotonate) (Scheme 1); the stereoregularity of the polymer depends on the structure of trialkylsilyl transferring groups. [8][9][10][11] During the investigation of Lewis-acid mediated stereoregulation of GTP, we found a stereospecific GTP of MMA catalyzed by aluminum phenoxide with R 3 SiI as a co-catalyst, which gives PMMA with a bimodal distribution of molecular weight (MW), the high MW fraction of which had high syndiotacticity (Scheme 2). Though the overall products had a lower syndiotacticity, it is evident that there exists highly syndiotactic-specific and highly active species in this GTP system.…”

Production and Polymerization of Biobased Acrylates and Analogs

“…[ 554 ] The poly(MeCr) obtained were found to be disyndiotactic, and the stereocontrol depended on the bulkiness of the SKA. [ 555 ] Other n ‐alkyl crotonates were polymerized by this method and physically characterized. [ 556 ] Notably, the crotonates have T g values 65 to 90 °C higher than their corresponding methacrylates, due to the enhanced stiffness provided by the methyl group in the β position instead of the α position.…”

Organocatalyzed Group Transfer Polymerization

“…Given that the initiation and propagation reactions during a GTP process are in principle rooted in the iteration of the elementary organic reaction, i.e., the Mukaiyama–Michael reaction,22–24 either a Lewis base or a Lewis acid has to be utilized as the catalyst of the GTP. Prior to 2007, the conventional Lewis bases of anionic nucleophiles, such as SiMe 3 ,22,25,26 ,22,25–27 CN − ,22,25,28,29 ,22,28 F − ,26,28,29 oxyanions,30,31 and hydrogen bioxyanions31,32 featuring a sterically hindered bulky counter cation like tris(dialkylamino)sulfonium or tetraalkylammonium, and the Lewis acids of transition‐metal compounds, such as ZnX 2 (X = Cl, Br, and I),27,29,33,34 organoaluminums,27,29,35 HgI 2 + R 3 SiI,24,36–42 CdI 2 + R 3 SiI,34 Yb(OTf) 3 ,43 and Sc(OTf) 3 ,43 dominated. Notably, the anionic nucleophiles associated with bulky non‐metallic cations are actually metal‐free, and can in a sense be viewed as the initial organocatalysts used in the GTP field.…”