Examples of suitable diol components that are employed in the present invention are aromatic and aliphatic diols having the following formulas: ##STR3## wherein Y represents hydrogen, halogen, e.g. Cl or Br, an alkyl or alkoxy group having 1 to 4 carbon atoms, or phenyl, and l is 0 to 2, ##STR4## Particularly, the following diol components are preferred in the instant invention: ##STR5##