The Polaroid processes are diffusing processes. Colour dyes migrating in their colour materials (with exception of that grid system); Silver salts migrating in their B&W processes.

The latter is the more modern version of what is called over here Silver-Salt-Diffusion-Processes and employed in several instant processes. The basic ideas were gained in 1940 by Agfa and Gevaert R&D people (Weyde/Rott) independently of each other. (Thus decennias before their merger.) As far as I remember a developed but unfixed paper by accident coming in contact with fixing agent and another plain paper formed the initial cause for further research in on case.

In the Polaroid B&W process a heavy metal plays a significant role in the recipient in reducing the silver migrated as salt from the unexposed parts of the developed negative.

It’s quite annoying doing this APUG thing without sufficient literature at hand. Besides those patents most of it is written in Dutch or German.
But in case you are seriously interested, pm me and the next time I’ll dive into those boxes of mine filled with books, I’ll try to find something of interest to you.