a castable thermosetting polyurethane polymer , having improved heat - stability characteristics , comprising a polymeric methylene diisocyanate , a propyleneoxide - extended polyol which is preferably a sugar - based polyol , a polyether diol or triol , and dipropylene glycol or diethylene glycol , in amounts respectively between about 35 - 45 parts by weight , 35 - 45 parts by weight , and 12 - 20 parts by weight , per 100 parts by weight of total diol and triol components , providing an nco / oh ratio between about 1 : 1 and about 1 . 2 : 1 , an effective polyurethane - catalyzing amount of a urethane catalyst , and optionally comprising an effective amount of a drying agent , is disclosed . the cast polyurethane polymer of the invention withstands temperatures of 450 ° f . for 20 minutes without visual polymer degradation , whereas most other castable polyurethane systems degrade between 350 ° f . and 375 ° f ., as a result of which heat - stability the polymer of the present invention is especially suitable for post finishing which involves the employment of higher temperatures , such as painting with thermosetting coatings , powder coating , and anodizing .