Online Monitoring of the Final, Divergent Growth Phase in the Step-Growth Polymerization of Polyamines Ray S. Farinato, ‡ Joe Calbick, ‡ Gina A. Sorci, | Fabio H. Florenzano, § and Wayne F. Reed* ,† Physics Department, Tulane University, New Orleans, Louisiana, Cytec Industries, Inc., 1937 W. Main St., Stamford, Connecticut, and Universidade do Vale do Sapucaı ´ - UNIVA Ä S, Pouso Alegre -MG-Brazil - 37550-000 Received May 5, 2004; Revised Manuscript Received November 19, 2004 ABSTRACT: Using automatic, continuous online monitoring of polymerization reactions (ACOMP) the final, divergent growth phase (FDGP) of the condensation polymerization of dimethylamine, epichloro- hydrin, and ethylenediamine was monitored, which produced a highly ramified, polyelectrolytic polyamine. The weight average mass, M w, increased exponentially during the FDGP, whereas weight averaged intrinsic viscosity [η]w increased slowly, reaching a plateau. Multi-detector gel permeation chromatography (GPC) revealed that polymers of mass 20 000 to 10 6 are branched and self-similar, but above this mass, [η] increases less strongly with M. This appears to be due to higher order ramification, a precursor to gelation. The ACOMP trends in Mw and [η]w provide direct online evidence of this process. It is shown computationally that a mere increase in polydispersity cannot explain this behavior. GPC showed the mass distribution becomes highly asymmetric as conversion increases. A plausible kinetic model for the distribution asymmetry is introduced, and a complementary model for the effects of higher order ramification on [η] w. Introduction Polyamines (PA) are used extensively in the water purification industry. While the step growth polymer- ization process to manufacture these was successfully developed in the 1960s, little effort has been devoted to understanding the kinetics and physical properties of these polyelectrolytes. The category of polyelectrolytes referred to as poly- amines encompasses a wide array of chemistries that can result from either free-radical chain growth, step growth, or derivatization reactions on preformed poly- mers. 1,2 The general features of this class of cationic polyelectrolytes include the possibility to synthesize high-charge-density materials from relatively inexpen- sive starting materials and the ability to locate the ionogenic group in the main polymer chain. The amine functionality can be designed to be unquaternized, in which case the charge on the resulting polyion would be pH dependent, or the amine can be quaternized, in which case the polyion charge would be relatively constant over a large range of pH. The materials studied in this paper form a com- mercially important subset of polyamines made by step- growth synthesis from epichlorohydrin (EPI) and di- methylamine (DMA). 3 These materials account for the majority of polyamines used in the water and waste- water treatment industry, where a central objective is the deployment of inexpensive polycationics for use in the coagulation and flocculation of suspended matter. The straightforward step-growth synthesis of polyamines from epichlorohydrin and dimethylamine (EPI-DMA) results in linear polymers of low to medium molecular weight, with a practical upper limit of M w ≈ 10 4 g/mol. This limitation is likely due to competition from side reactions in the step-growth synthesis. 1 Such materials make excellent coagulants and typically operate in wastewater applications via a charge neutralization mechanism. In many applications, however, a greater polymer molecular weight would result in better ef- ficacy. There have been a number of schemes designed to enhance the molecular weight of EPI-DMA polyamines. The approach that currently dominates commercial production, because of its economic viability, is the use of small amounts of amines of functionality greater than two in the synthesis. One example in widespread commercial use is the hexafunctional compound ethyl- enediamine (EDA), whose incorporation into the chain necessarily creates branching of the main chain. Aug- mentation of polymer molecular weight using this scheme is concomitant with a change in the polymer topology. Efforts to commercially produce EPI-DMA and EPI- DMA-EDA polyamines having consistent properties often involve a reliance on bulk solution rheology as a guide to the extent of reaction and as an index of when to quench the step-growth polymerization before an intractable, cross-linked polymer results. In fact, speci- fications of commercial products are often quoted in terms of solution bulk viscosities. More sophisticated analyses of molecular weight distribution and structure are mainly done offline, and do not readily allow for a rapid enough feedback for control purposes during the important final stages of the polymerization. One has only to look at the overall features of a gel permeation chromatography (GPC) trace of an EPI- DMA-EDA polyamine to realize that a single shear viscosity value cannot capture the full suite of features that can be used to describe these materials. As seen below, while the EPI-DMA polyamines in the 10 4 -10 6 mass range display monomodal GPC traces, the EPI- DMA-EDA polyamines often present multimodal, or at * Author to whom correspondence should be addressed. † Tulane University. ‡ Cytec Industries, Inc. § Universidade do Vale do Sapucaı ´. | Current Address: Physics Department, Milsaps College, Jack- son, MS. 1148 Macromolecules 2005, 38, 1148-1158 10.1021/ma049118g CCC: $30.25 © 2005 American Chemical Society Published on Web 01/22/2005