Question

Producing polymers with high molecular weights is difficult in step polymerizations. Why is it difficult and which techniques exist to overcome this difficulty?

Answer 1

Practical use of polymers
is the temperature at which thermal breakdown of the polymer chains
occurs. Decomposition temperatures obviously will be sensitive to impurities,
such as oxygen, and will be influenced strongly by the presence of inhibitors,
antioxidants, and so on. Nonetheless, there will be a temperature (usually
rather high, 200" to 400") at which uncntalyzed scission of the bonds in a chain
will take place at an appreciable rate and, in general, one cannot expect to
prevent this type of reaction from causing degradation of the polymer. Clearly,
if this degradation temperature is comparable to T,, as it is for polypropenenitrile
(polyacrylonitrile), difficulties are to be expected in simple thermal
molding of the plastic. This difficulty is overcome in making polypropenenitrile
(Orlon) fibers by dissolving the polymer in N,N-dimethylmethanamide
and forcing the solution through fine holes into a heated air space where the
solvent evaporates.