Racemic monomers polymerization

This polymer has also been obtained synthetically via cationic polymerization of D( + )(3-methyl-p-propiolactone 19). The racemic monomer polymerizes predominantly to the isotactic polymer. 

An intermediate case between the polymerization of enantiomerically pure and racemic monomers is the polymerization of a partially resolved mixture of enantiomers with variable optical purity. Two processes are distinguishable, depending on whether polymerization occurs in the presence of an achiral (or racemic) or of a chiral (optically active) catalyst. 

The isoselective polymerization of a racemic mixture of monomers can proceed in two ways depending on initiator, monomer, and reaction conditions. Racemate-forming enantiomer-differentiating polymerization involves both the R and 5 monomers polymerizing at the same rate hut without any cross-propagation [Hatada et al., 2002]. A racemic monomer mixture polymerizes to a racemic mixture of all-5 and all-5 polymer molecules [Pino, 1965 ... 

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],... 

In the next year, Price and his coworkers (6,7) found that the crystalline polymer obtained by Pruitt and Baggett was isotactie. The fact that the crystalline polymer obtained from racemic monomer with the iron catalyst had the same X-ray pattern as the optically active crystalline polymer obtained from the optically active monomer under the same condition showed that these polymers were isotactic, and that the asymmetric carbon atoms in this polymer had the same configuration as in the monomer from which it was derived, i.e., propylene oxide polymerized with retention of configuration of its asymmetric carbon atom. 

Polymerization of non-asymmetric or racemic monomers with the aid of optically active catalysts or initiators ... 

An appreciable optical activity, depending on the degree of polymer stereoregularity, may be expected and, as will be shown below, has been actually observed in vinyl polymers derived from racemic monomers, when one of the two antipodes is preferentially polymerized (104). 

Thus the stereospecific polymerization of a racemic monomer will yield optically active polymers only if it is accomplished stereoelectively, namely in the presence of catalysts capable of polymerizing preferentially one of the two antipodes of the racemic monomer. 

In the field of the stereospecific heterogeneous polymerization of a-olefins, the stereoselective (106,118) and stereoelective (103) polymerization of racemic monomers, having an asymmetric carbon atom in a. 

In this case, in the polymerization of a racemic monomer, the antipode forming the first monomeric unit of a macromolecule should be responsible for the type of antipode prevailingly included in that particular macromolecule. 

Olefin polymerization using heterogeneous catalysts is a very important reaction and stereochemical aspects have been studied extensively. For a review on this topic see Pino et al. [9], Briefly, the origin of stereoregularity in polyolefins (47) is explained by the chiral nature of the acdve site during polymerization. If the absolute configuration of the first intermediate can be controlled by chiral premodification then we should obtain a non-racemic mixture of R - and "S"-chains. This has indeed been observed e.g. with catalyst M4 for the polymerization (partial kinetic resolution) of racemic 3,7-dimethyl-l-octene (ee 37%) and also for the racemic monomer 46 using Cd-tartate M5. 

C 0 chain. The four planar isotactic structures of polypropylene oxide may be designated for convenience as d (up), d (down), l (up) and l (down) isotactic structure 12). The d (up) and d (down) structures are superimpos-able by turning the polymer chain end-over-end so are the l (up) and l (down) structures. In crystallization of isotactic polypropylene oxide obtained from polymerization of racemic monomer, all the four chain structures may be able to fit together in the crystal without a serious packing difficulty because the oxygen and methylene groups are isoelectronic and are of similar size 12). 

An interesting behavior of the polymerization would be the formation of optically active polymers from racemic olefin oxides (81, 153, 154, 162a, 208,212-214,215a, 250,274,275,316,393,500,504,507,508,510, 511). The catalyst systems consist of dialkylzinc and optically active alcohol or amino acid. In the polymerization, one enantiomer of the racemic monomer is selectively introduced in the polymer. 

Although polymerization of the monomer does not proceed to high conversion, this process is most significant because it is the first example of an asymmetric synthesis starting from racemic monomer crystals. 

The coordinate type catalysts are also effective for thiirane polymerizations. The types of systems used are also similar. Thus diethylzinc and in particular diethylzinc/water mixtures have been studied [44]. Other studies made using triethylaluminium and diethylcadmium indicated that these metal alkyls all behave similarly. The reactions seem to be rather complex, and, as also was the case with the epoxides, no well defined kinetic studies have appeared. The polymers produced are of high molecular weight and are often crystalline. Thus stereospecific polysulphides have been reported. Again the bulk of the studies involve PS. Stereoselective polymerization of racemic monomer has been accomplished [45, 46] using a catalyst prepared from diethylzinc and (+) borneol. The marked difference between PO and PS in their polymer-... 

Lundberg and Doty [12] first reported that the initiation of the polymerization of racemic monomer by preformed, optically pure, homo-... 

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]. 

There are several alternative methods for the synthesis of optically active polymers from achiral or racemic monomers that do not involve polymerization catalysts. Optically active polymers have been formed from achiral dienes immobilized in a chiral host lattices [ 106]. In these reactions, the chiral matrix serves as a catalyst and can be recovered following the reaction. For example, 1,3-penta-dienes have been polymerized in perhydrotriphenylene and apochoUc acid hosts, where asymmetric induction occurs via through-space interactions between the chiral host and the monomer [107,108]. The resultant polymers are optically active, and the optical purities of the ozonolysis products are as high as 36%. In addition, achiral monomers have been found to pack in chiral crystals with the orientations necessary for topochemical soHd-state polymerization [109]. In these reactions, the scientist is the enantioselective catalyst who separates the enantiomeric crystals. The oligomers, formed by a [27H-27i] asymmetric photopolymerization, can be obtained in the enantiomeric pure form [110]. 

Dynamic Kinetic Resolution Polymerization of Racemic Monomers 287... 

Hilker et al (44) combined dynamic kinetic resolution with enzymatic polycondensation reactions to synthesize chiral polyesters from dimethyl adipate and racemic secondary diols. The concept offered an efficient route for the one-pot synthesis of chiral polymers from racemic monomers. Palmans at al (18,43) generalized the approach to Iterative Tandem Catalysis (ITC), in which chain growth during polymerization was effected by two or more intrinsically different catalytic processes that were compatible and complementary. 

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