Radical cyclopolymerization of difunctional monomers based on 1,6-diene components was performed in one-dimensional channels of porous coordination polymers (PCPs). Although bulk or solution polymerization of the monomers usually gives cross-linked insoluble polymers, the unfavorable interpolymer reactions were effectively suppressed in the narrow nanochannels of PCPs to provide soluble linear polymers. The pore matrices and functionality of PCPs can be readily designed by changing the organic ligands, so that polymerization of the diene monomers in different sized pores was examined. The primary structures of the resulting polymers, such as branching, cyclic structure, and stereoregularity, were changed, depending on the pore characteristics of the PCPs.