AbstractPoly‐L‐proline in the molecular weight range 50,000–90,000 has been synthesized fromL‐proline‐N‐carboxyanhydride. The molecular weight obtained is dependent on the initiator concentration. The rate constants for the polymerization have been determined by measuring the CO2evolution. By observation of rotation during the polymerization, it is possible to deduce the conformation of the polymer in the polymerization solution. It is concluded that the immediate product of the polymerization is neither pure form I nor pure form II. The differences between form I and form II of poly‐L‐proline have been further characterized by measurements of their ultraviolet rotatory dispersion and their ultr