Polymethyl methacrylate (PMMA) or poly (methyl 2-methylpropanoate)
Plexiglas is the trade name for Rohm and Haas Company acrylic.
AKA Limacryl, R-Cast, Perspex, Plazcryl, Acrylex, Acrylite, Acrylplast, Altuglas, Polycast, Lucite & a few others.
Thank you Wikipedia.