Polymerization of gaseous formaldehyde in hexane using various initiators including tetravalent organo-tin compounds was investigated. The end group of the obtained polymer was examined and it was concluded that the chain-transfer agents (such as methanol, etc.) were almost completely consumed during the polymerization of gaseous formaldehyde. Only two series of initiators, quaternary ammonium salt and dialkyltin dimethoxide compounds, produced a polymer with a narrow-molecular-weight distribution (M-w/M-n = 2). The polymer with the narrow-molecular-weight distribution (M-w/M-n = 2) produced a product with high Dart impact strength. The polymer with the broad-molecular-weight distribution (M-w/M-n > 2.9) gave the product with low Dart impact strength.