Radical Polymerization of Bio-Renewable Butyrolactone Monomers
In this study, a bio-derived monomer, -methyl--methylene--butyrolactone (MeMBL) was saponified with sodium hydroxide (NaOH) to make the water-soluble monomer sodium 4-hydroxy-4-methyl-2-methylene butanoate (SHMeMB), that was copolymerized via radical polymerization in aqueous solution with acrylamide (AM) and crosslinker to synthesize superabsorbent hydrogels. Absorbency of these hydrogels was shown to be much higher than sodium acrylate hydrogels, with mechanical properties varying with molar composition and crosslinking content. Reactivity ratio of SHMeMB:AM at 50C and 15 wt% were estimated using low conversion data (rSHMeMB=0.12 and rAM=1.10), and the integrated Mayo-Lewis equation (rSHMeMB=0.17 and rAM=0.95). However, in-situ NMR results showed that SHMeMB:AM copolymerizations proceed at a slower rate than of a similar system of AM copolymerized with sodium 4-hydroxy-2-methylene butanoate (SHMB), a similar monomer produced by ring-opening of -methylene--butyrolactone (MBL). Pulsed-laser polymerization coupled with size exclusion chromatography (PLP-SEC) studies were done for both systems at 60°C and 10 wt% monomer concentration. Homopolymerization kp values were estimated to be 25 and 165 L/mols for SHMeMB and SHMB, respectively, confirming that SHMeMB is less reactive than SHMB. Further kinetic studies of SHMeMB:AM copolymerization and homopolymerization of SHMeMB were conducted at elevated temperatures. SHMeMB conversions achieved a limiting value which decreased at higher temperatures, suggesting that polymerization rate was limited by depropagation. Comonomer composition drift also increased with temperature, with more AM incorporated into the polymer while SHMeMB underwent depropagation. Homopolymerization of SHMeMB with added sodium chloride (NaCl) showed a decrease in polymerization rate explained by an increase in propagation rate coefficient (kp) but an even greater increase in termination rate coefficient (kt) as supported by parameter estimation done using PREDICI. Even with added salt, however, depropagation was the dominant mechanism at higher temperatures. Lastly, the kinetic parameters estimated were implemented in a copolymerization model used to estimate the variation of kt with composition in SHMeMB:AM copolymerizations. It was found that the overall termination rate coefficient was dominated by the presence of SHMeMB, with as the estimate for kt,SHMeMB of the same order of magnitude as kt of another ionized water-soluble monomer, sodium methacrylic acid.