Asymmetric selective (or stereoelective) polymerization of various racemic methacrylates with cyclohexylmagnesium bromide (c‐HexMgBr)‐(‐)sparteine (1/1.2) catalyst was studied in toluene at −78°C. The methacrylates of α‐ethylbenzyl (EBMA), α‐isopropylbenzyl (i‐PBMA), α‐tert‐butylbenzyl (t‐BBMA), sec‐butyl (s‐BuMA), 1‐methylallyl (1‐MAMA), 2,3‐epoxypropyl (2,3‐EPMA), 2‐phenylpropyl (2‐PPMA), and menthyl (MentMA) alcohols were used as racemic monomers. In the polymerization of EBMA and i‐PBMA (S) enantiomers were consumed preferentially and the optical purity of initially polymerized i‐PBMA was as high as 97%. Optically pure (R) monomers were recovered at about 60% conversion for i‐PBMA and 80% for EBMA. In t‐BBMA, however, the (R) monomer was consumed preferentially over the (S) isomer. In the polymerization of s‐BuMA and 1‐MAMA (S) monomers were consumed in excess and the optical purity of the polymers formed in the early stage was about 30%. In 2,3‐EPMA and 2‐PPMA, which have asymmetric centers at the β position from the ester oxygen, (R) antipodes were more reactive. MentMA did not polymerize at −78°C. Enantiomer selectivity ratios rS and rR were determined in the polymerization of EBMA, i‐PBMA, and 1‐MAMA. All polymers except poly(t‐BBMA) were highly isotactic, but the tacticity of poly(t‐BBMA) could not be estimated. Circular dichroism spectra of optically active polymers of α‐substituted benzyl esters were measured.
ABSTRACT:Ethylmagnesium alkoxides (EtMgOR) resulting from an equimolar reaction of diethylmagnesium (Et2Mg) with a variety of alcohols were employed as catalysts for the stereospecific polymerization of methyl methacrylate (MMA) in toluene at -78°C. The catalysts obtained from normal alcohols did not show a clear stereospecific tendency, but the alkoxides of 2-monosubstituted primary alcohols such as 2-methyl-1-propanol (iso-BuOH), 2-butanol, cyclohexylmethanol, and ( -)-cis-myrtanol, worked as isospecific catalysts. Secondary and tertiary alkoxides formed syndiotactic polymers. In the polymerization with Et2Mg-isoBuOH system, the activity of the catalyst and the isotacticity of the obtained polymer were highest at [iso-BuOH]/[Et2Mg] = 1; the isotacticity decreased with an increase in temperature. The stereospecificity of Et2Mg-2-methyl-2-propanol (t-BuOH) system did not depend on the [t-BuOH]/[Et2Mg] ratio; the temperature effect was not pronounced. The stereospecific polymerization of ethyl, tert-butyl, a-methylbenzyl, and a, a-dimethylbenzyl methacrylates was also investigated with iso-BuOMgEt and t-BuOMgEt in toluene at -78°C. The structure of the catalysts was investigated by means of the 1 H NMR method.KEY WORDS Stereospecific Polymerization / Diethylmagnesium / Ethylmagnesium Alkoxide / Methyl Methacrylate / Methacrylate / Tacticity / Anionic Polymerization / Many organomagnesium compounds such as dialkylmagnesium (RMgR), 1 ' 2 alkylmagnesium amide (RMgNR'R"), 3 and Grignard reagent (RMgX) 1 ' 2 have been used as stereospecific catalysts for the polymerization of methacrylic esters. These compounds possess Mg-carbon, Mgnitrogen, and Mg-halide bonds, respectively, in addition to the Mg-carbon bond. However alkylmagnesium alkoxide (RMgOR') has scarcely been used for the polymerization of methacrylates. Coates and Ridley have reported that alkylmagnesium alkoxide can be prepared from dialkylmagnesium and alcohol, and forms dimer, trimer, and higher oligomers in benzene depending on both the alkyl group and alkoxy group. 4Such an association of alkylmagnesium alkoxides has also been observed even in diethyl ether.5 group bound to a magnesium cation always stays at a position close to a polymer-chain anion and the structure of the alkoxy group may have an influence on the stereoregulation of the polymerization.The polymerization of methacrylates catalyzed by alkylmagnesium alkoxides is initiated by an insertion reaction of a monomer molecule into the Mg-carbon bond. Consequently, the alkoxy In order to ascertain this point, we undertook the polymerization of methyl (MMA), ethyl (EtMA), tert-butyl (t-BuMA), a-methylbenzyl (MBMA), and a, a-dimethylbenzyl (DMBMA) methacrylates by using various ethylmagnesium alkoxides (EtMgOR) obtained from equimolar reaction of diethylmagnesium and alcohols in toluene. In particular, the polymerization of MMA with Et2Mg-2-methyl-l-propanol (isoBuOH) and Et2Mg-2-methyl-2-propanol (tBuOH) systems was investigated in detail. EXPERIMENTAL MaterialsCommercial MMA, EtMA, and t-BuMA we...
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