Weak-link analogue

There is much evidence that weak links are present in the chains of most polymer species. These weak points may be at a terminal position and arise from the specific mechanism of chain termination or may be non-terminal and arise from a momentary aberration in the modus operandi of the polymerisation reaction. Because of these weak points it is found that polyethylene, polytetrafluoroethylene and poly(vinyl chloride), to take just three well-known examples, have a much lower resistance to thermal degradation than low molecular weight analogues. For similar reasons polyacrylonitrile and natural rubber may degrade whilst being dissolved in suitable solvents. 

Thermal stability is largely concerned with chemical reactivity which may involve oxygen, u.v. radiation or depolymerisation reactions. The presence of weak links and the possibility of chain reactions involving polymer chains may lead to polymers having lower thermal stability than predicted from studies of low molecular weight analogues. 

Formycin A (LXXIV, R = NH2) and B, (LXXIV, R = OH), and pyra-zomycin (LXXV) are natural products having weak antiviral activity. The first two are active against influenza in vitro [277] and the last has activity against vaccinia in mice [278]. -They are unique in being nucleoside analogues with the rare C-riboside link. 

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