Polyurethane formation template

The alcohol oxygen of butane-1,4-diol attacks the central carbon of hexamethylene diisocyanate, the N=C π electrons move onto nitrogen and a proton transfer gives the urethane –NH–CO–O– link; with water the isocyanate gives a carbamic acid that loses CO₂, the blowing reaction of polyurethane foams.

Mechanism step by step

A diisocyanate and a diol add to each other: each O–H adds across N=C to give a carbamate (urethane) link

1. Addition – The alcohol O attacks the isocyanate carbon; the N=C π electrons go to N
2. Proton transfer – N takes the proton from O (in practice through another alcohol or the amine catalyst): the urethane link
3. Foaming with water – Water adds to –NCO the same way, giving a carbamic acid; its zwitterion loses CO₂ gas

Key points

• Step-growth polyaddition: no small molecule is lost, so the repeat unit is diisocyanate + diol.
• The urethane (carbamate) link –NH–CO–O– forms N–H···O=C hydrogen bonds between chains, giving hard segments in elastomers such as spandex.
• Water + isocyanate → carbamic acid → amine + CO₂, which blows foams; the amine reacts with more –NCO to give urea links.
• Industrial monomers: MDI or TDI with polyether or polyester polyols; triols give cross-linked foams. Tertiary amines (DABCO) and tin carboxylates catalyse the addition.

Drawn with 73 atoms, 10 arrows, 11 shapes – every one editable in MolDraw.

Sources

• Odian, Principles of Polymerization, 4th ed., Wiley 2004, ch. 2 Textbook
• Wade, Organic Chemistry, 8th ed., Pearson 2013, ch. 26 Textbook