"Beyond a thousand years from now humans are not quite recognizably human, and I have trouble finding characters."
- Larry Niven

A similar scenario forms the basis for the 1971 novel Mutant 59: The Plastic Eaters by Gerry Davis and Kit Pedler:

"In the shaft leading to the [ventilation] grille a mindless, groping mass of malodorous corruption was thrusting its way silently towards the surface. Buoyed up by bubbling foam it steadily rose. Single units in an obscene abrogation of normal order divided and made two. Two became four and four, eight. Endlessly supplied with food, each unit absorbed nutrient and in a soft, ancient certainty fulfilled its only purpose - to multiply, to extend and to multiply...

"In the Coburg Street control room of the London Underground system,there was a full emergency... In a dozen tunnels, trains ground down to a halt. Hordes of terrified commuters made their way anxiously along dark, musty tunnels to the lights and safety of the next station. There were minor explosions, fires, and the failure of a million wires andcables. As the dissolution of plastic proceeded and accelerated in rate,the elegant order of the system gradually turned into complete chaos.

"On the surface, in the freezing December air, the smell of the rotting plastic began to hang permanently in the air. A cloying, wet, rotting smell similar to the smell of long-dead flesh. It filled streets , basements and factories. Traffic lights failed, causing irresolvable jams.... The breakdown of plastic spread into BroadcastingHouse.... A gas main with polypropylene seals on its pressure regulatorserupted into flame.... Plastic cold-water pipes softened, ballooned, andburst, flooding into shops, homes, and restaurants.

"Slowly and inexorably, the rate of dissolution increased; failures occurred in increasing succession until, within forty-eight hours, the centre of London had become a freezing chaos without light, heat, or communication."
(Thanks to Winchell Chung for mentioning this.)