Free Radical Polymerization of N-Vinylformamide and Novel Comb-structure Polyelectrolytes

Abstract

<p>This thesis summarizes experimental and theoretical studies on N-vinylformamide (NVF) free radical polymerization and the synthesis and evaluation of cationic grafted copolymers for flocculation purpose. The free radical polymerization of N-vinylformamide was studied in bulk and aqueous solution processes. The molar heat of reaction for NVF polymerization was measured to be 79.4 kJ/mo!. The polymerization kinetics, including the conversion versus time, molecular weight and molecular weight distribution, was examined systematically. Significant "gel effect" was observed in bulk and solution polymerization when the monomer concentration was over 40wt% (5.63 moI/L). A semi-empirical model based on the free volume theory was proposed to describe both the bulk and solution polymerizations. It was also confirmed that amide group provided crosslinking points that formed PNVF gel during NVF free radical polymerization. The subsequent acidic and basic hydrolysis to convert PNVF to PV Am was also studied systematically. The effect of temperature, polymer/acid (or base) ratios and polymer concentrations were examined. Two methods were used to synthesize cationic grafted copolymers for flocculation purposes. In the first method, NVF monomer was copolymerized with two cationic poly dimethylaminoethyl methacrylate (polyDMAEMA) methyl chloride macromonomers to produce a series of comb-grafted copolymers with different cationic contents. One cationic macro monomer had a molecular weight of 14,100 (50 repeating units) and the other had a molecular weight of 28,200 (100 repeating units). The copolymerization reactivity ratio (r₁) of the comonomer pair (NVF(M₁)-macromonomer(M₂)) was determined to be 3.82 and 6.39 for the two macromonomers with 50 and 100 repeating units. The flocculation performance of the graft copolymers was tested on a TiO₂ aqueous dispersion model system. The graft copolymers were found to be more effective in flocculation than their random linear counterparts. In the second method, poly diallydimethyl ammonium chloride (polyDADMAC) was grafted onto polyacrylamide (PAM) using a mini-mixer in melts with an organic peroxide, Lupersol 130. This process resembled reactive processing usually carried out at a larger scale in extruders to handle highly viscous polymer reactions. The grafting was confirmed and successful. However, severe gelation problem with PAM that occurred during the process hampered the grafting efforts. Parallel to these studies based on experimental data, a theoretical model was derived to describe the bivariate distribution of molecular weight and branching density for comb-grafted copolymers. This model was based on the random grafting of pre-formed side chains onto backbones (grafting-onto) mechanism. However, it is also applicable to the comb polymers synthesized by grafting-from and grafting-through mechanisms as long as the inclusion of the branching point is a random process.</p>