The structure and variations in dynamic motions of three polyguanidines possessing different side chains were studied by C-13 CP/MAS NMR. From these results, the structures of the polyguanidines were confirmed, and the C-13 spin-lattice relaxation times in the rotating frame were measured. The polyguanidine backbone mobilities were measured as a function of size and chemical make up (aliphatic vs. aromatic). The main-chain carbon of polyguanidine (II) with aromatic side chains has a higher activation energy, 23.12 KJ/mol, than the polyguanidine (1) with aliphatic side chains, 19.76 KJ/mol. Also, the activation energy of the main-chain carbons of polyguanidine (II) and (LII) with aromatic side chains was found to depend on the size of side chains.