Carbonyl and Ring-Opening Polymerizations

Several other types of monomers are capable of yielding stereoisomeric polymer structures. Ordered strucmres are possible in the polymerization of carbonyl monomers (RCHO and RCOR ) and the ring-opening polymerizations of certain monomers. Thus, for example, the polymers from acetaldehyde and propylene oxide can have isotactic and syndiotactic strucmres as shown in Figs. 8-3 and 8-4. 

Polyacetaldehyde [lUPAC Poly(oxy[tnethyhnethylene])], like the systems described previously, contains stereocenters that are achirotopic. Both the isotactic and syndiotactic 

The repeat unit (III) in poly(propylene oxide) [lUPAC poly(oxy[l-methylethylene])], on the other hand, possesses stereocenters that are chirotopic since the two chain segements 

The stereochemistry of step polymerization is considered now. Bond formation during step polymerization almost never results in the formation of a stereocenter. For example, neither the ester nor the amide groups in polyesters and polyamides, respectively, possess stereocenters. Stereoregular polymers are possible when there is a chiral stereocenter in the monomer(s) [Oishi and Kawakami, 2000 Orgueira and Varela, 2001 Vanhaecht et al., 2001]. An example would be the polymerization of (R) or (5)-H2NCHRCOOH. Naturally occurring polypeptides are stereoregular polymers formed from optically active a-amino acids. 

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