I thought it was urethane. I thought I read it on a package.
from http://eclecticproducts.com/downloads/sds-shoegoo-clear-english.pdf they say it's Styrene Butadiene Copolymer, which I don't think is urethane
urethane (McNett Seam Grip) and shoe goo are both synthetic organic polymers. They both have Toluene as solvent, which is probably the predominant smell.

