[yellowcat (OP)]

I was discussing this with friends, we have all experienced shoes/boots with crumbling soles, it happens irrespective of the amount of wear that the shoes have had. I realise that it is a chemical reaction but what is happening.

A couple of further points, when it happens it appears to be very quick, one week the shoes will be fine the next they soles will be creaking and crumbling. Is the reaction self catalysing, and if so could it spread by contact, if say someone wearing PU soled shoes was step on part of a disintegrated sole? Could it be caused by bacteria or fungi affecting the sole.
I wondered about contagious crumbling because someone said that her and her husbands walking boot soles started to disintegrate in the same week, even though one pair was a couple of years newer than the other.

[Colin2B]

The problem is due to a process known as hydrolysis where water gets into the sole and a chemical process causes the urethene to degrade.
The problem also seems to affect shoes which are stored for a while, perhaps put away damp, even brand new but stored can be affected. Frequent use appears to help, perhaps by pressure keeping the moisture out?
There are also manufacturing difference. PU is expensive and some manufacturers put talc into the mix to bulk it out. This weakens the PU and, I suspect, as talc is hydrophillic at low humidity this will make matters worse.
It would appear that manufactures of cheaper shoes don't expect you to get more than 4-5 years life whether you wear the shoes of not.

Mmm, I’ve had my Scarpa mountineering boots for 30 yrs and they are still going strong*

*PS added - Vibram rubber soles.

[jeffreyH]

I hear some Beatles songs ...

[Bored chemist]

I hear some Beatles songs ...

Aah! Soles.

[Colin2B]

Aah! Soles.

I thought he meant Polyeurathane Pam

[jeffreyH]

Rubber soul and Beatle boot
https://en.m.wikipedia.org/wiki/Beatle_boot

[Colin2B]

Rubber soul and Beatle boots

I had a pair of Chelsea boots at the time.
But they had leather soles