Which common material did chemical engineer Otis Ray McIntire make in 1944 leading to a patent for Dow Chemicals?

Polystyrene, aka Styrofoam (commercial name).

Initially discovered by German apothecary Eduard Simon in 1839, there was a lot of further interest in the polymer material in the 19th century. In 1931, German company I.G. Farben began manufacturing polystyrene.

Otis Ray McIntire (1918-1996) rediscovered a Swedish process and Dow purchased the patent. In 1944 Dow themselves patented Styrofoam and began

"producing a lightweight, water-resistant, and buoyant material that seemed perfectly suited for building docks and watercraft and for insulating homes, offices, and chicken sheds."

https://en.wikipedia.org/wiki/Polystyrene