The polymerization of phenylisocyanate in bulk and in solution of THF by complex base (CB), NaNH2/(CH3)(3)CONa catalyst under vacuum was previously reported. The polymer obtained was fractionated by washing and extracted with different solvents. Polymer fractions were shown to be highly crystalline by X-ray analysis. Different fractions have similar crystalline structure with some modifications in cell parameters of primitive monoclinic unit cell. Mass spectral analysis gave valuable information for the chain structure and polymerization mechanism of polyphenylisocyanate. The evolution of NH2- and NH3 at low temperature and early stage by pyrolysis showed the chain-ending with -NH2. This is an indication of initiation by NH2- in complex base catalyst. The polymer does not contain large amounts of dimer and trimer when polymerized by this catalyst.