HETEROPHASE WATER-IN-OIL POLYMERIZATION OF ACRYLAMIDE BY A HYBRID INVERSE-EMULSION INVERSE-MICROEMULSION PROCESS/

Heterophase water-in-oil polymerizations of acrylamide have been condu cted in the presence of blends of non-ionic stabilizers at moderate mo nomer concentrations (20%). The initial monomeric system is located ou tside the inverse-microemulsion domain, yet close to the inverse-macro emulsion/inverse-microenulsion phase boundary. A turbid, viscous and u nstable dispersion is produced at the outset and during the intermedia te stages of the polymerization. This evolves to an inviscid and non-s ettling system at high conversions. Transparent inverse latices can al so be produced provided that the polymerizations are conducted semi-ad iabatically. Small angle neutron scattering (SANS) studies of the init ial monomer and reacting systems have found the latices to be particul ate with a particle diameter of 150 nm, independent of conversion. The SANS intensities can be fitted using a polydispese spherical particle s model. Therefore, these heterophase water-in-oil polymerization syst ems seem to follow an inverse-macroemulsion-like mechanism. The 'hybri d inverse-microemulsion/inverse-macroemulsion' polyacrylamides produce d herein have a smaller radius of gyration in aqueous media relative t o those produced by either solution polymerization or a true inverse-m acroemulsion polymerization of the same weight-average molecular weigh t. This is likely due to a large number of intramolecular interactions , such as hydrogen bonds, which are induced by the collapsed nature of the polymer chains in the inverse-microemulsion droplets. The weight- average molecular weight, the radius of gyration and the particle diam eter of the final latex are relatively independent of the polymerizati ons condition a such as initiator level, hydrophilic-lipophilic balanc e (HLB), temperature and physical changes occurring during polymerizat ion. From a kinetic point of view, the molecular weights of these syst ems are controlled by transfer to monomer, while transfer to interfaci al emulsifier is the polymerization rate controling step. A reaction m echanism consisting of a number of elementary reactions has been propo sed for these heterophase-water-in-oil polymerizations. Agreement with the experimental data is found to be good at different levels of init iator, HLBs and temperature. Despite the limitations of this heteropha se water-in-oil polymerizations (the moderate emulsifier levels, low r adius of gyration and its inability to increase the weight-average mol ecular weight beyond 10(6) daltons), this polymerization process can p roduce final latices that are transparent and non-settling with small particles (<150nm). This allows post-reaction chemical modification, e .g. by the Mannich reaction. (C) 1997 Elsevier Science Ltd.