A preparation method for high molar mass poly(dimethyldiallylammonium chloride) (PDMDAAC) is reported in this article. PDMDAAC was prepared by using the high purity industrial grade dimethyldiallylammonium chloride (DMDAAC) monomer from one-step method and ammonium persulphate (APS) as the initiator. The initiator was added all at once and the reaction temperature was increased stepwise to complete the polymerization gradually. The effects of several polymerization condition variables on the intrinsic viscosity value ([eta]) and monomer conversion rate (Cony.) of product PDMDAAC were investigated, respectively. The variables included: T-1 (42.0 to 52.0 degrees C), T-2 (47.5 to 57.5 degrees C), T-3 (55.0 to 75.0 degrees C), m(DMDAAC) (60.0 to 70.0%), m(APS) : m(DMDAAC) (0.25 to 0.45%), m(Na(4)EDTA) : m(DMDAAC) (0 to 0.0071%). Under an optimum condition of T-1 = 46.0 degrees C, T-2 = 52.5 degrees C, T-3 = 65.0 degrees C, m(DMDAAC) = 65.0%, m(APS) : m(DMDAAC) = 0.35%, m(Na(4)EDTA) : m(DMDAAC) = 0.0035%, the maximum [eta] of obtained product PDMDAAC reached 3.43 dL/g, at a Cony. of 100.00%. The M-w of the product measured with GPC-MALLS was 1.034 x 10(6), polydespersity M-w/M-n was 2.421, and the R-g was 60.3 nm. The structure and properties of products were characterized by FTIR and NMR. Thermal decomposition was determined by TGA-DSC. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 118: 1152 1159, 2010