Aldehydes, stereoregular polymerization

Pu and co-workers incorporated atropisomeric binaphthols in polymer matrixes constituted of binaphthyl units, the macromolecular chiral ligands obtained being successfully used in numerous enantioselective metal-catalyzed reactions,97-99 such as asymmetric addition of dialkylzinc reagents to aldehydes.99 Recently, they also synthesized a stereoregular polymeric BINAP ligand by a Suzuki coupling of the (R)-BINAP oxide, followed by a reduction with trichlorosilane (Figure 10).100... 

The stereoregular polymerizations discussed thus far have dealt with monomers commonly used in polymerizations. Recently, carbonyl compounds, such as the aldehydes and ketones, have been successfully converted into high molecular weight polymers. It seems appropriate, therefore, to examine here the principles governing the polymerizability of unsaturated compounds. 

Table IV gives an example of our own work on the polymerization of a number of higher aldehydes. Potassium triphenylmethoxide—a soluble initiator—polymerized a number of higher aldehydes to crystalline isotactic poly aldehydes. Table V lists a number of alkali alkoxides and other related compounds used as initiators for the n-butyraldehyde polymerization. Neither the type of the alkoxide nor the cation is of any great importance for the polymerization rate, the polymer yield, and stereoregularity of the resulting polyaldehyde as long as the initiator is adequately soluble in the reaction mixture.

To determine whether the solvent itself can bring about crystallinity or stereoregularity in the free-radical polymerization of vinyl chloride, a large number of polymerizations in a variety of solvents was carried out. Among the solvents studied were nitro compounds, amines, amides, nitriles, ethers, esters, ketones, aldehydes, carboxylic acids, anhydrides, and heterocyclic compounds. In all, 38 solvents were evaluated. The experiments were carried out in sealed glass ampoules with 0.25-2 moles of solvent per mole of VCM, under nitrogen, at temperatures between 0 and 60°C for 18 hr with 0.0008 mole of AIBN per mole of monomer. 

In general, it was found that the majority of these solvents did not lead to stereoregular PVC. The resins produced in some aliphatic acids did exhibit a somewhat enhanced stereoregularity. Only in aliphatic aldehydes was crystalline PVC formed. Considering that aldehydes are chain-transfer agents for VCM polymerizations, it was not entirely surprising that yields and molecular weights were low [174]. 

Haloaldehyde polymerization provides a new and quite different approach for the study of the polymerization behavior of aldehydes. The properties of per-haloaldehyde polymers are also substantially different from those of polyformaldehyde or of higher aliphatic polyaldehydes. We are discussing here the results of our work on the preparation and polymerization of nine (fluoro, chloro, and bromosubstituted) perhaloacetal-dehydes with special emphasis on the stereoregularity of the polmers obtained. ( 7 )... 

The study of the perhaloacetaldehyde polymerization provides an ideal example for the investigation of the polymerizability of the aldehydes, the relationship of the polymer stereoregularity as a function of substituent size and shape. 

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