Polymerization of racemic monomers

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. 

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). 

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-... 

Dynamic Kinetic Resolution Polymerization of Racemic Monomers 287... 

Introduction of relatively weak functional groups, such as carbonyl, hydroxyl, nitro, amide, etc., in the nanochannels of PCPs would affect the monomer alignment, which may lead to precision control of stereoselectivity and regioselectivity of the resulting polymers. In particular, PCPs with either helical or chiral structures on the pore surface are of intense interest in chemistry and such porous solids are potentially useful to find applications in enantioselective sorption/separation and catalysis [34, 38 0, 42, 45]. Of considerable interest is the use of the chiral channels to affect asymmetric polymerizations such as asymmetric selective polymerization of racemic monomers as well as asymmetric polymerization of prochiral monomers, which may give helical polymer conformations. 

In the case of a chiral monomer, sequences of the resultant polymer should be directly influenced by the stereoselectivity of these interactive moieties." if the interactive moiety prefers homochiral association, two enantiomeric polymers of individual optical antipodes should be afforded. Contrary to this, when the interactive moiety possesses a strong tendency to adhere another monomer with opposite handedness, an alternative eopolymer of each enantiomer is expected to arise. On the other hand, the monomer with no stereoselectivity will give a random copolymer. Studies on the supramolecular polymerization of racemic monomers are of significant importance, because these three extreme cases are regarded as the simplest models of the molecular arrangements in crystalline states, conglomerates, racemic compounds, and racemic mixtures, respectively. 

In order to elucidate the nature of the growing chain control mechanism, a series of NMR studies was carried out with poly(t-butyloxirane) which was prepared by the polymerization of racemic monomer initiated by t-BuOK (11). Pulsed FT C-NMR spectra of the polder are shown in Fig. 4. By comparison with NMR spectraof poly[(5)-t-buty1oxirane] shown in Fig. 2, it was possible to... 

Stereoselective or stereoelective polymerization of racemic monomers. Stereoregular polymers have been obtained by stereoselective polymerization from many cyclic, racemic monomers (100,121,127-140). Some of these monomers are indicated in Table 2. The identification of the stereoselective character of a polymerization process, which leads to isotactic optically inactive polymers, requires a very accurate characterization of the polymer obtained. For the polymers reported in Table 2, elution chromatography on an optically active support, spectroscopic (IR, or NMR) measurements, comparison of the X-ray pattern with that of the corresponding optically active polymer, and enzymic degradation have been used. In many cases the stereoselectivity of the polymerization process has been confirmed by the stereo-electivity observed when using the catalyst in an optically active form. 

Stereoselective or stereoelective polymerization of racemic monomers. The stereoselective polymerization of racemic monomers was first investigated in the case of racemic a-olefins (184). When the asymmetric carbon atom was in the a-position with respect to the double bond, essentially stereoregular polymers were obtained, the single macromolecules having isotactic main chain and carbon atoms with mostly the seune configuration in the lateral chains. The stereoselectivity decreases when the asymmetric carbon atom of the monomer is in the 3-position with... 

The asymmetric selection stereoelective) polymerization of racemic monomers in the presence of optically active catalysts could be very useful, but the catalytic systems used up to now give a rather poor enantiomeric purity. On the other hand the separation of nonoptically active polymers of racemic monomers into fractions with opposite optical rotation, while possible in several cases, would need an improvement of separation techniques before being employed for preparative purposes. 

---