Polymers, crystal structure, heat

The first reaction leads to pol5q)eptides, the second to N, methylglycyl diglycyl glycine. The tetrapeptide will also form amorphous pol3mier when heated to temperatures close to its melting point of 205° C (77). The loss of the methyl alcohol is thus not the reason for the loss of order. A period of mobility before polsmierization is also unlikely, since on reaction in solution only piperazine-2,5-dione is the major product (20). The reactions of the crystalline tri- and tetrapeptide are thus topo-chemical, but no transition from the monomer to the polymer crystal structure seems possible under the chosen reaction condition. 

SN) c is much more stable than its precursor S2N2. When heated in air it decomposes explosively at about 240°C but it sublimes readily in vacuum at about 135°. The crystal structure reveals an almost planar chain polymer with the dimensions shown in Fig. 15.36. The S and N atoms deviate by about 17 pm from the mean plane. The structure should be compared... 

Some polymer is also formed but this can be converted into the bicyclic S11N2 by refluxing in CS2. The X-ray crystal structure (Fig. 15.37b) shows that the 2 N atoms are planar.This has been interpreted in terms of sp hybridization at N, with some delocalization of the p. lone-pair of electrons into S-based orbitals, thus explaining the considerably diminished donor power of the molecule. S11N2 is stable at room temperature but begins to decompose when heated above 145°. 

This polymer has a crystal structure different from the normal hexagonal poly(thioformaldehyde). At 153° C it changes to the hexagonal configuration. Credali and Russo (8) state that this new form ran be heat-stabilized by acetylation of its end groups and that the acetylated polymer does not degrade at 250-260° C, a temperature range at which ordinary poly(thioformaldehyde) is very unstable. 

As mentioned earlier, poly(thioformaldehyde) is a highly crystalline polymer, and it has been shown to have a hexagonal crystal structure (14, 24). Thus, poly-(thioformaldehyde) obtained by irradiation of trithiane followed by heating has a hexagonal unit cell with a = 5.07 A and c=36.52 A crossed by one helical chain parallel to the c axis. The chain has an identity period of 36.52 A that comprises 17 —CH2S— units in 9 turns of the helix (14). Poly(thioformaldehyde) made by other methods crystallizes similarly (24). 

Differential Scanning Calorimetry. Some structural information is provided by the thermal behavior of the polymer. The homopolymer of DPP crystallizes when heated above the glass transition temperature. A crystallization exotherm at the appropriate temperature therefore indicates the presence of DPP blocks, either as the homopolymer or in a block copolymer. 

Crystalline trithiane, the sulfur containing analog of trioxane polymerizes in a topotactic reaction after irradiation on subsequent heating to 180° C 93). Again the crystal structure is twinned and differential thermal analysis has shown a higher melting point for polymers produced by solid state polymerization than for solution polymerized trithiane... 

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