Pecularities of Solid-state Polymerization

This picture is slightly oversimplified, however, because it has been found that polymer formed during solid-state polymerization of TXN crystallizes as a helix with 1.81 —CH20— units per turn, while the TXN molecule has 3 such units per ring 11J). In the TXN crystals the distance between adjacent molecules is equal to 4.18 A while three CH20 units in a polymer helix occupy 5.77 A 116). 

Also the cross section of a TXN molecule is much larger than the cross section of polyoxymethylene helix which is only 17.3 A 116 1,7 . 

polymer formation requires rearrangements in all a-, b-, and c-axis directions which in turn requires a relatively high mobility under the polymerization conditions, 15 . 

Indeed, a rather straightforward correlation was found between the rate of polymerization and mobility of the monomer within a crystal lattice by broad-line NMR 118). TXN polymerizes rapidly only above 40 °C and at this temperature a very narrow line is present in the centre of the H-NMR spectrum, indicating considerable molecular motion. It was found also, that the maximum polymerization rate is reached at a temperature just below the melting temperature of TXN, implying again that extensive molecular mobility is required 119 ,20  

These results indicate that RISS1 polymerization of TXN is not a simple topochemi-cal process, in which monomer molecules are converted into polymer units without leaving their lattice positions. It was suggested that polymerization may proceed according to a solid- gas- solid scheme, i.e. TXN sublimes from the monomer crystal lattice into cavities formed as a result of crystal lattice rearrangement and then adds to the growing end from the gas phase120). 

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