Isotactic polyaldehydes

Isomorphous replacement in isotactic polyaldehydes was shown by A. Tanake, Y. Hozumi, K. Hatada, S. Endo, and R. Fujishige (42). These authors studied the binary polymer systems formed by acetaldehyde, propionaldehyde, n-butyraldehyde, iso-butyraldehyde and w-heptanal. All the copolymers are crystalline over the whole range of compositions. In the case of binary copolymers of acetaldehyde, propionaldehyde and K-butyraldehyde the unit cells have the same tetragonal space group UJa, with the same chain axis (4.8 A), while the dimensions of the a axis change continuously as a function of the copolymer composition. In the case of copolymers of isobutyraldehyde with other aldehydes, the continuous variation of the lattice constants a and c were observed. 

Statistical substitution between chains having the same helix handedness, but different orientations of the side groups ("up" and "down chains) was proved or at least suggested as probable for a variety of isotactic vinyl polymers. In particular we will mention here the cases of isotactic polypropylene (53), isotactic poly-w-butene-1 (54), isotactic polyaldehydes (55) and isotactic-1,2-poly-4-methylpentadiene (56). 

Like atactic higher polyaldehydes, the isotactic polyaldehydes are also unstable even at room temperature. However, end-capping rendered them stable to higher temperatures and permitted extensive purification and characterization of the polymers (50, 51,52). 

Most isotactic polyaldehydes are insoluble at room temperatures but are soluble at elevated temperatures in some solvents. We have studied the gel point, which is a measure of polymer solubility. A number of poly aldehydes were investigated in tetrahydronaphthalene polyformaldehyde has the highest gel point observed—namely, about 200° C. For the series of unbranched polyaldehydes the gel point decreases with the increasing length of the aliphatic side chain (Table VI) and parallels the melting points of the polymer. 

Isotactic polyaldehydes melt (with some decomposition) at a somewhat higher temperature than the corresponding olefin polymers (Table VIII) (7), particularly in the higher members of the homologous series. Like isotactic polyolefins, branched isotacting polyaldehydes melt substantially higher than the corresponding unbranched polymers. 

Table VIII. Melting Points of Crystalline Isotactic Polyaldehydes...

In all of the above discussions we have treated only the monomers with carbon-carbon double-bonds. It is probable that polymers of non-oleftnic monomers such as polypropylene oxide (103, 104, 105) poly-ethylidene (106, 107, 108, 109) and polyaldehydes (110, 111) polymerize to isotactic structures by the same mechanism. The same correlation of ionicity of the catalysts with the isotactic structures and syndiotactic structures should also be possible. 

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.

Natta and co-workers (35) have shown by careful x-ray investigation that crystalline polyaldehydes are isotactic polymers crystallizing in a four-fold helix with an identity period of 4.8 A. (Figure 4). 

Solution viscosities were determined at elevated temperatures on purified polyaldehyde samples, and inherent viscosities as high as 0.85 have been obtained. Isotactic poly-ft-butyraldehyde solutions in purified tetrahydro-naphthalene are stable in the viscometer at 140° C. for at least 2 hours. 

The polymerization of the higher aliphatic aldehydes has many similarities with formaldehyde polymerization. Notable differences are a lower ceilii temperature and the possibility of different steric configurations due to the substituted carbon atom. Especially anionic catalysts such as alkali metal alkoxides, soluble hydrides, and organo metal compounds lead to polymerizations during which crystalline isotactic polymer is produced 96). Little is known about the morphology and the detailed crystallization mechanism of the polyaldehydes. 

---