A-Methylbenzyl-methacrylate

A special case of asymmetric enantiomer-differentiating polymerization is the isoselective copolymerization of optically active 3-methyl-1-pentene with racemic 3,7-dimethyl-1-octene by TiCl4 and diisobutylzinc [Ciardelli et al., 1969]. The copolymer is optically active with respect to both comonomer units as the incorporated optically active 3-methyl-l-pentene directs the preferential entry of only one enantiomer of the racemic monomer. The directing effect of a chiral center in one monomer unit on the second monomer, referred to as asymmetric induction, is also observed in radical and ionic copolymerizations. The radical copolymerization of optically active a-methylbenzyl methacrylate with maleic anhydride yields a copolymer that is optically active even after hydrolytic cleavage of the optically active a-methylbenzyl group from the polymer [Kurokawa and Minoura, 1979]. Similar results were obtained in the copolymerizations of mono- and di-/-menthyl fumarate and (—)-3-(P-styryloxy)menthane with styrene [Kurokawa et al., 1982],... 

Both spectra were almost mirror images of one another as expected from their opposite rotations. The relative intensities of the peaks were nearly proportional to the ratio of the specific rotations. The peaks at 208 and 232 nm may be assigned to the absorptions of the phenyl and ester groups, respectively. The spectral pattern is very similar to that of the copolymer 9 of TrMA with a small amount of (S)-a-methylbenzyl methacrylate illustrated in Figure 5. This also indicates that the large positive rotations of the copolymers 7-9 in Table are attributed to the helical conformation of isotactic TrMA units preferential in one screw sense. 

Figure 7. Difference infrared spectra between the polymethacrylates unexposed and exposed to the electron-beam of 1.6x10 4 C/cm2. (A) poly (methyl methacrylate) (B) poly(a-methylbenzyl methacrylate) (C) atactic poly(a,a-dimethylbenzyl methacrylate) (D) poly(a,a-diphenylethyl methacrylate).

Chirality induction can be achieved in homo- and copolymerization of vinyl monomers based on chiral monomer structure [1,3,8,9]. The first example of this type of polymerization was the copolymerization of (S)-a-methylbenzyl methacrylate with maleic anhydride the polymerization product showed [a]D +23° after removal of the chiral side group [73]. For another example, the copolymerization of an optically active styrene derivative (39) with N-phenylmaleimide (17, R = -Ph) followed by removal of the optically active side group and deboronation gave an optically active N-phenylmaleimide-styrene copolymer [74]. 

On special centres (complexes of Grignard reagents) selectively chosing first the R then the S antipode, two different polymer chains can be obtained from a racemic mixture of a-methylbenzyl-methacrylate [94], The immediate vicinity of the centre is undoubtedly of great importance for monomer addition. This is documented by the effect of solvents. When the centre is sorbed on a polyelectrolyte matrix, the distance of the matrix ions will affect the length and quality of the counter-ion bond of the centre, and thus also the rate of polymerization [95]. 

Several optically active polymers of acrylates and methacrylates have been obtained by enantioselective polymerization of a racemic monomer initiated by a Grignard compound complexed with chiral reagent. Complexing agents for the polymerization of (K,S)-a-methylbenzyl methacrylate include chiral alcohols, such as quinine and cinchonine [63], (— )-sparteine and its derivatives [64-67], and other axially disymmetric biphenyl compounds [68,69]. Other racemic monomers used include (/ ,S)-a-methylbenzyl acrylate [70], (K,S)-l-phenylethyl acrylate, methacrylate and a-ethylacrylate [71], and 1,2-diphenylethylmethacrylate [72]. 

Stereoselective polymerizations and copolymerizations of methacrylates have also been realized recently and are potentially of considerable importance. In the case of (RS)-a-methylbenzyl methacrylate with anionic catalysts and 2,3-epoxypropyl methacrylate with an optically active Grignard catalyst selective propagations appear to occur. Copolymerization of (RS)-a-methyl-benzyl methacrylate and methyl methacrylate also proceed stereoselectively. ... 

Suda has reported the polymerization of rac-a-methylbenzyl methacrylate by a Grignard/binaphthyldiamine (2) initiator where the unreacted monomer at... 

Table 15. Polymerization of a-methylbenzyl methacrylate with diethylmagnesium-chiral alcohol and cyclohexylmagnesium chlotide-(-)-sparteine in toluene at-78°C. Ref. )... 

The asymmetric selectivity arises from the preferential formation of (S)-elective center at the beginning followed by the formation of (R)-elective center after the consumption of most of the (S)-monomer. The copolymerization of the (RS)-mono-mer and methyl methacrylate by this complex yielded a highly isotactic copolymer in which the (S>monomer predominantly incorporated over the (R)-monomer. On the other hand, in the copolymerization with a,a-dimethylbenzyl methacrylate only the homopolymer of a-methylbenzyl methacrylate was obtained with the same as-i mmetric selectivity as in the homopolymerization of this monomer. The results indicate that the steric interaction between the methyl group at the a- rosition of benzyl ester and the (-)-sparteine moiety of the catalyst plays an important role in the stereoelection of the polymerization. 

When (RS)-a-methylbenzyl methaciylate(M2) was copdymerized with methyl methacrylate(Mi) by BuLi at -78 C, the monomer reactivity ratios were obtained to be f] = 1.68, t2 = 0.78 in toluene and r =2.04,r2 = 1.52inTHF. These values were not changed if (S)-a-methylbenzyl methacrylate was employed instead of the racemic monomer. The copoiymeiization of (RS)-af-methylben2yl methacrylate-(Mi) and trityl methacrylateOda) by BuLi in THF also gave the same r and r2... 

The isotacticity is higher in poly[(R)-Q-methylbenzyl methacrylate] prepared by BuLi in toluene than in the polymer of the racemic monomer. However, the stereoregularity of poly[(S)-a-methylbenzyl methacrylate-co-methyl methacrylate] is mostly the same as that of poly((RS)-a-methylbenzyl methacrylate-co-methyl methacrylate], regardless of their compositions except for low methyl methacrylate contents. This indicates that the isotactic placement of the (R)- or (S)-monomer to the growing chain ending in the antipode monomer unit is less favorable than to die anion of the same monomer unit, while the stereospecificity in the addition of a-methylbenzyl methacrylate to methyl methacrylate unit should be the same between the (R)- and (S)-monomers. ... 

S)-a-methylbenzyl methacrylate] was lower than the calculated value and it decreased as the content of alternating sequence increased. 

Electron-withdrawing substituents in anionic polymerizations enhance electron density at the double bonds or stabilize the carbanions by resonance. Anionic copolymerizations in many respects behave similarly to the cationic ones. For some comonomer pairs steric effects give rise to a tendency to altemate. The reactivities of the monomers in copolymerizations and the compositions of the resultant copolymers are subject to solvent polarity and to the effects of the counterions. The two, just as in cationic polymerizations, cannot be considered independently from each other. This, again, is due to the tightness of the ion pairs and to the amount of solvation. Furthermore, only monomers that possess similar polarity can be copolymerized by an anionic mechanism. Thus, for instance, styrene derivatives copolymerize with each other. Styrene, however, is unable to add to a methyl methacrylate anion, though it copolymerizes with butadiene and isoprene. In copolymerizations initiated by w-butyllithium in toluene and in tetrahydrofuran at-78 °C, the following order of reactivity with methyl methacrylate anions was observed. In toluene the order is diphenylmethyl methacrylate > benzyl methacrylate > methyl methacrylate > ethyl methacrylate > a-methylbenzyl methacrylate > isopropyl methacrylate > t-butyl methacrylate > trityl methacrylate > a,a -dimethyl-benzyl methacrylate. In tetrahydrofuran the order changes to trityl methacrylate > benzyl methacrylate > methyl methacrylate > diphenylmethyl methacrylate > ethyl methacrylate > a-methylbenzyl methacrylate > isopropyl methacrylate > a,a -dimethylbenzyl methacrylate > t-butyl methacrylate. 

Optically active 81 partially resolves trans-stilbene oxide and separates several aromatic compounds when used as an HPLC stationary phase. Compound 81 also partially resolved isotactic polymers of (R)-(+)- and (S)-(-)-a-methylbenzyl methacrylate. 

Various types of copolymers of cyclic olefins and other monomers have been prepared by asymmetric synthesis polymerizations using monomers with optically active side groups, ° optically active additives, " cata-lysts, or solvents.Among these, the synthesis of a copolymer of maleic anhydride and (S)-(-)-a-methylbenzyl methacrylate (MBMA, 269) is the first example of preparation of an optically active polymer consisting of a C-C backbone with chiral induction to the main chain. °... 

Liquori et al. [23] first discovered that isotactic and syndiotactic PMMA chains form a crystalline stereocomplex. A number of authors have since studied this phenomenon [24]. Buter et al. [25,26] reported the formation of an in situ complex during stereospecific replica polymerization of methyl methacrylate in the presence of preformed isotactic or syndiotactic PMMA. Hatada et al. [24] reported a detailed study of the complex formation, using highly stereoregular PMMA polymers with narrow molecular weight distribution. The effect of tacticity on the characteristics of Langmuir-Blodgett films of PMMA and the stereocomplex between isotactic and syndiotactic PMMA in such monolayers at the air-water interface have been reported in a series of papers by Brinkhuis and Schouten [27,27a]. Similar to this system, Hatada et al. [28] reported stereocomplex formation in solution and in the bulk between isotactic polymers of / -(+)- and S-(—)-a-methylbenzyl methacrylates. 

A remarkably high stereoelectivity has recently been reported in the polymerization of a-methylbenzyl methacrylate (194,195). For the anionic polymerization of this monomer by a Grignard reagent/ (-) sparteine system in toluene at -78°C, the optical purity of the unreacted monomer reached nearly 100% at about 65% conversion. Additionally, the polymer obtained was shown by NMR to have a very high content of isotactic triads. 

Partially stereoregular copolymers have been obtained by asymmetric polymer synthesis using maleic anhydride as one comonomer and optically active a-methylbenzyl methacrylate (216) or a-methylbenzyl vinyl ether (217) as the other comonomer. These copolymers were optically active even after removal of their a-methylbenzyl groups (Scheme 25). Analogous results have been obtained by Minoura s group (218, 219) on copolymerizing optically active a,3-disubstituted olefins with achiral vinyl monomers. 

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