412 034 and 4 046 729 further the processes to prepare such polyurethanes are well recognized as exemplified by the aforementioned patents in accordance with the present invention aromatic aliphatic and cyclo aliphatic diisocyanates or mixtures thereof can be used in forming the polymer such diisocyanates for example are tolyene 2 4 diisocyanate tolylene 2 6 diisocyanate meta phenylene diisocyanate biphenylene 4 4 39 diisocyanate methylene bis 4 phenyl isocyanate 4 chloro 1 3 phenylene diisocyanate naphthylene 1 5 diisocyanate tetramethylene 1 4 diisocyanate hexamethylene 1 6 diisocyanate decamethylene 1 10 diisocyanate cyclohexylene 1 4 diisocyanate methylene bis 4 cyclohexyl isocyanate tetrahydronaphthylene diisocyanate isophorone diisocyanate and the like preferably the arylene and cyclo aliphatic diisocyanates are used most advantageously in the practice of the invention characterisitcally the arylene diisocyanates encompass those in which the isocyanate group is attached to the aromatic ring the most preferred isocyanates are the 2 4 and 2 6 isomers of tolylene diisocyanate and mixtures thereof due to their ready availability and their reactivity further the cyclo aliphatic diisocyanates used most advantageously in the practice of the present invention are 4 4 39 methylene bis cyclohexyl isocyanate and isophorone diisocyanate selection of the aromatic or aliphatic diisocyanates is predicated upon the final end use of the particular material as is well recognized by those skilled in the art the aromatic isocyanates may be used where the final product is not excessively exposed to ultraviolet radiation which tends to yellow such polymeric compositions whereas the aliphatic diisocyanates may be more advantageously used in exterior applications and have less tendency to yellow upon exposure to ultraviolet radiation although these principles form.