Full Idea
Kant made the materially motivated determinations of the will of the French bourgeois into pure self-determinations of the "free will", of the will in and for itself.
Clarification
The 'bourgois' are the middle classes
Gist of Idea
Kant made the political will into a pure self-determined "free" will
Source
report of Immanuel Kant (Groundwork of the Metaphysic of Morals [1785]) by K Marx / F Engels - The German Ideology §II
Book Reference
Marx,K./Engels,F.: 'The German Ideology', ed/tr. Arthur,C.J. [Lawrence and Wishart 1985], p.99
A Reaction
This is the social determinism of Marx and Engels. Most commentators would say that Kant was taking the idea of "free will" from religion rather than politics, but presumably Marx would merely reply "same thing!"