Diisotactic racemic polymers

The term "racemo" is introduced here as the logical prefix for the designation of an arrangement that is analogous to racemic, in the sense defined above. It is unfortunate that the meaning of the term racemic current in organic chemistry is not directly applicable to polymers, but the use of the prefix "racemo proposed here should not cause confusion because of the special context. To achieve a full configurational description, it may be necessary to preface the name of a polymer with a compound adjective that combines a term such as erythro, threo, meso or racemo with a term such as diisotactic or disyndiotactic . 

The polymerization of cis oxides gives, in contrast to trans monomers, polymers of different stereochemistry, namely cis-2,3-butene oxide forms racemic (RR, SS) disyndiotactic polymers whereas cis-1,4-dichloro-2,3-butene oxide gives racemic mixtures of (RR, RR) and (SS, SS) diisotactic polymers. 

Analysis of the products obtained after hydrolysis under controlled conditions led to the conclusion that the trans-monomer gave a meso-diisotactic structure, while the cis-monomer produced racemic-diisotactic polymer. A more detailed discussion of the stereochemistry of this class of polymers has already been given in Part I of this review (Adv. Pol. Sci. 37). 

Both the carboxylic acid and aminomethyl substituent groups are in an isotactic arrangement, causing an erythro-diisotactic configuration of the polymer chains. Indeed, the 1,4-addition induces two centers of chirality per monomer unit. However, due to the center of symmetry of the space group the unit cell contains a racemic mixture of the polymer. 

At low temperatures, about -78°C, in triisobutylaluminum/ water-initiated polymerization, which is presumed to be a cationic initiation, an amorphous (elastomeric) polymer is obtained from cis-2,3-epoxybutane, and a crystalline polymer, melting point 100°C, is obtained from the trans-isomer. In a copolymerization of the two isomers, the cis-oxide enters the copolymer at about twice the rate of the trans-isomer. Further, the low-temperature, cationic poly(trans-2,3-epoxybutane) with a crystalline melting point of 100°C was found to consist of diad units with a mesodiisotactic structure, while the crystalline polymer formed by coordinate polymerization of the cis-monomer, melting point 162°C, had diad units that were racemic diisotactic. These results make apparent the importance of the monomer coordination step in polymer chain growth in coordinate polymerizations. 

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