PURPOSE: To obtain titled polymer in high yields without using an organic initiator, by polymerizing, by ring opening, a low-moisture lactone in the presence of a carbon compound of molybdenum.

CONSTITUTION: A lactone high polymer, average MW>12,000, is prepared by polymerizing, by ring opening, a lactone, moisture content <0.15%, such as δ- valerolactone or -caprolactone in the presence of a carbon compound of molybdenum such as a molybdenum alkoxide represented by the formula Mo(OR)4, wherein R is methyl, ethyl, propyl or butyl, a molybdenum metal chelate compound such as one derived from acetylacetone or benzoylacetone, a molybdenum salt such as molybdenum naphthenate or molybdenum citrate and a molybdenum carbonyl compound such as (C5H5)Mo(CO)3H or Mo(CO)6.