ABSTRACT:Stereospecific living polymerization of methyl crotonate was achieved by using ketene trialkylsilyl acetals as initiators in the presence of Hgl 2 and iodotrialkylsi1anes as catalyst and co-catalysts, respectively. Znl 2 and Cdl2 also catalyzed the polymerization_ Disyndiotactic polymers with narrow molecular weight distribution were obtained quantitatively, Livingness and stereospecificity of this polymerization were sensitive to the structure of tria1kylsilyl groups in the initiator and co-catalyst; (C 2 H 5 hSi-and (n-C 3 H 7 lJSi-systems gave the active species with higher thermal stability and higher stereospecificity than (CH 3hSi-system. Configurational determination of the disyndiotactic poly( methyl crotonate)s was made by chromatographic separation of their low molecular weight homologs (tetramer and hexamer), followed by X-ray single crystal analysis. The results of oligomer analysis led to the conclusion that the disyndiotactic polymer chains are formed by the threo addition of monomers to propagating chain-ends and trans opening of monomer C = C double bonds.KEY Two useful pieces of information led to this finding; (1) the GTP of acry1ates using Hgl 2 as a catalyst in toluene gives a good control of molecular weight, 2 • 3 and (2) the use of (CH 3 hSil as a co-catalyst and CH 2Cl 2 /toluene mixture as a solvent drastically accelerates the Hgl 2 -catalyzed GTP of n-butyl acrylate. 4 • 5 Before this finding, direct preparation of poly( methyl crotonate) from its monomer had been considered to be difficult. As a commonly observed feature of crotonates, methyl crotonate gives no polymer by a free radical mechanism. 6 • 7 Typical anionic initiators such as alkyllithiums or Grignard reagents are ineffective for the polymerization of methyl crotonate, 6 -9 whereas branched alkyl crotonates including t-butyl and triphenylmethyl crotonates are polymerized in good yields 6 -16 and in a living manner 17 · 18 by these initiators. The only compound that had been known to polymerize methyl crotonate in a substantial yield was CaZn(C 2 H 5 )4 complex. The polymerization of methyl crotonate with t To whom correspondence should be addressed. tt Present address: Fukui University of Technology, Japan. this complex in toluene at -78oC for 96 h affords a polymer in 28% yield. 8In spite of the difficulty of methyl crotonate polymerization, several kinds of poly(methyl crotonate)s with various stereoregularities have been prepared and identified in the last five years. These poly(methyl crotonate)s were derived from threodiisotactic poly(triphenylmethyl crotonate), 13 diheterotactic poly(t-butyl crotonate),15 and atactic-like poly(t-butyl crotonate) 12 by means of hydrolytic cleavage of the ester functions and subsequent methylation with CH 2 N 2 . The previous paper 1 reported that the poly(methyl crotonate) formed by GTP showed 1 H NMR spectra quite different from those of threodiisotactic, diheterotactic, and atactic-like poly(methyl crotonate)s.The present paper provides details of our studies on the GTP o...
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...
We prepared main chain-type polyrotaxanes by urea bond-forming polyaddition of a diamine axle-containing αcyclodextrin (α-CD)-based pseudo[3]rotaxane to a macromolecular diisocyanate derived from poly(propylene glycol). Bulky isocyanate or aniline used as a comonomer worked as an effective stopper during the key step of the polyrotaxane synthesis. In the polymer preparation, the chain extension method worked effectively to increase the polymerization degree. Polyrotaxane was modified by the main chain extension and the cross-linking of hydroxyl groups of the CD wheels. The morphology and the effect of the rotaxane structure on the thermal and physical properties were evaluated. Some polymers exhibited sufficient toughness for industrial use. Because diamine-type pseudo[3]rotaxane is easily obtained on a large scale (100 g in a 1 L flask) from α-CD and 1,12diaminododecane, this study contributes to the synthesis of various polyrotaxanes for actual use.
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