Polyurethane is a stepwise polymerization where careful control of the reactant stoichiometery is needed to achieve a suitably high molecular weight of the resulting polymer. In other words, one isocyanate group reacts with one hydroxyl group. Polyurethane polymerizations generally are going to utilize equal or near-equal amounts of isocyanate groups and hydroxyl groups. One way to add more wt.% polyol while keeping equal amounts of isocyanate and hydroxyl groups is to utilize a higher molecular weight (or higher viscosity) grade of polyol. If one simply added an excess of polyol without controlling for reactant stoichiometry the resulting polymer would become less and less mechanically rigid or would not even form a polymer at all. The book "polymer chemistry" by himenez and Lodge can probably be checked out at your university library and has all the fundamental description of stepwise polymerization stiochiometry mathematics.

Polyurethane is a stepwise polymerization where careful control of the reactant stoichiometery is needed to achieve a suitably high molecular weight of the resulting polymer. In other words, one isocyanate group reacts with one hydroxyl group. Polyurethane polymerizations generally are going to utilize equal or near-equal amounts of isocyanate groups and hydroxyl groups. One way to add more wt.% polyol while keeping equal amounts of isocyanate and hydroxyl groups is to utilize a higher molecular weight (or higher viscosity) grade of polyol. If one simply added an excess of polyol without controlling for reactant stoichiometry the resulting polymer would become less and less mechanically rigid or would not even form a polymer at all. The book "polymer chemistry" by himenez and Lodge can probably be checked out at your university library and has all the fundamental description of stepwise polymerization stiochiometry mathematics.

Formation of polyurethane foam is usually done by reacting a diol with a diisocyanate in the presence of moisture (or little water) or a blowing reagent. If diisocyanate is to be reacted with a polymer under the name of polyol (polyvinyl alcohol is an example of this), then this is another story: you will get polyol macro molecules that are cross-linked by urethane linkages that may be called polyurethane in an approximate sense. Of course, this polymer analogous conversion requires a neutral medium.

While inter-chain cross-linking will decrease the overall polymeric volume (thereby increasing density), the opposite effect will be partial or total loss of the spongy or foamy character of the product causing the formation of a semi-rigid or rigid product.