Propylene oxide, stereoregular polymerization

The stereochemistry of ring-opening polymerizations has been studied for epoxides, episul-fides, lactones, cycloalkenes (Sec. 8-6a), and other cyclic monomers [Pasquon et al., 1989 Tsuruta and Kawakami, 1989]. Epoxides have been studied more than any other type of monomer. A chiral cyclic monomer such as propylene oxide is capable of yielding stereoregular polymers. Polymerization of either of the two pure enantiomers yields the isotactic polymer when the reaction proceeds in a regioselective manner with bond cleavage at bond 1. 

Outside of vinyl addition polymerizations some crystallizable stereoregular polymers also have been prepared. An example is isotactic polymer from propylene oxide made by using ferric chloride com-... 

Cationic polymerizations of oxiranes are much less isospecific and regiospecific than are anionic polymerizations. In anionic and coordinated anionic polymerizations, only chiral epoxides, like propylene oxide, yield stereoregular polymers. Both pure enantiomers yield isotactic polymers when the reaction proceeds in a regiospecific manner with the bond cleavage taking place at the primary carbon. 

Propylene oxide, O—CH2—CH(CH3), exists as two antipodes. Thus, stereoregular products can be formed during the polymerization of one of the antipodes. On the other hand, polymerization of the racemic monomers often produces atactic products with many head-head linkages. 

The polymerization of 2,3-epoxybutane with the same initiator as used with propylene oxide shows that the oxygen/substituted-carbon atom bond can be cleaved and, hence, a mechanism can be logically proposed to account for the head-to-head, tail-to-tail structures identified by Price and Vandenberg. With 2,3-epoxybutane, it was found that amorphous polymer could also be as pure disyndiotactic as the crystalline forms. Amorphous polymer could arise from short sequences of stereoregularity that were too short to form crystallizable segments. These could arise when coordination of monomer temporarily displaced alkoxide, interrupting chain growth, which, when resumed, could be selective for the antipode monomer. 

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