Hi Nova, welcome!
Which exothermic reaction: no idea, there are so many. Do you have more requirements on it? Nontoxic, cheap, self-regulating temperature...? A worry is that most reactions give a quantity of heat, at a poorly predictable pace, and the temperature depends on how well the heat is evacuated, so +80°C could also mean "between +30°C and +120°C" or even "some location see +180°C" and then it isn't the same difficulty at all for the polymer. In that aspect, a solidification would be better than a reaction.
80°C for 5 hours is rather easy. The cheap polypropylene could still work, it's more or less its limit. PVC should work and has all materials and tools to assemble by hot glue, but isn't very flexible. Among the more expensive polymers, nearly everyone accepts +80°C.
Then, friction... It depends brutally on the temperature. (Expensive) PTFE for instance has a low coefficient but over some +50°C it climbs to banal 0.3-0.5. Polypropylene has a low coeff at room temperature.
Maybe you can cover your polymer with a sheet of well-gliding material? Films exist (Du Pont and others) with an adhesive layer and a PTFE or FEP layer for instance. Or try to find a spray.http://www.bpf.co.uk/Plastipedia/Default.aspxhttp://www.matbase.com/http://www.matweb.com/
check the "deflection temperature" or "operating temperature", not the fusion temperature.
If you read German, a fantastic compensium is
Kunststofftabellen, by Bodo Carlowitz