SYNTHETIC POLYMER MICROGELS FROM PRECIPITATION POLYMERIZATION

Abstract

Surfactant-free, micron-sized particles with reproducible diameters, controlled crosslink densities and pH responsiveness are useful in a range of applications, including waste-water treatment, crop protection, and tissue engineering. This thesis focuses on the synthesis of crosslinked polymer microgels by free-radical precipitation copolymerization of maleic anhydride (MAn) with electron rich monomers (e.g. styrenes, vinyl ethers) in marginal solvent mixtures. By matching comonomer reactivities, monomer-polymer solubility parameters and degree of crosslinking, the synthesis of narrow-dispersed, functional microgels can be achieved. Here, their properties are explored by optical and electron microscopy, and their mechanical properties by real-time deformability cytometry (RT-DC). The copolymerization of MAn with 4-methyl styrene (4MS), styrene (Sty), and divinylbenzene (DVB) affords polymer particles that are 3 to 20 µm in diameter, depending on reaction solvent composition and crosslinker loading. Hydrolysis of backbone anhydride units yields carboxylic acid groups that render pH-responsiveness to the microgels, where they exhibit reversible swelling as a function of pH. The rapid, alternating polymerization between electron-poor and electron-rich monomers affords high polymer conversions and moderate to high isolated particle yields even in conditions of low total monomer loading. In the presence of 3 to 10 mol% DVB, discrete microgels are formed when the MEK/heptane solvent mixtures have a total Hansen solubility parameter of 18 ≤ δt ≤ 22 (MPa^1/2). The mechanical properties of a small series of poly(4MS-co-MAn-co-DVB) particles measured by real-time deformability cytometry (RT-DC) showed an inverse correlation between Young’s modulus and vol% MEK in the reaction solvent. Replacement of styrenic monomers with alcohol-containing vinyl ethers such as hydroxyethyl vinyl ether (HEVE) resulted in the formation of mono and narrow-disperse microgels with reduced swelling ratios due to lack of hydrophobic association upon collapse of the polymer network at pH < pKa1 of succinic acid. These microgels also demonstrated an alternative crosslinking reaction attributed to intermolecular ester formation between HEVE hydroxyl groups and MAn units on neighbouring polymer chains, which was shown to eliminate the need for added divinyl crossinkers. In preliminary experiments, vinyl ether-containing microgels showed resistance to protein adsorption compared to poly(4MS-co-MAn-co-DVB) microgels of similar size. The work presented in this thesis emphasizes the versatility of precipitation polymerization for the development of functional microgels in the 3 to 20 µm range, synthesized from readily available styrenic, acrylic, and vinyl ether monomers.