Study of the Kinetic Hydrate Inhibitor Performance of Poly(N-vinylcaprolactam) and poly(N-isopropylmethacrylamide) with Varying End Caps

Poly(N-vinylcaprolactam) (PVCap) is an effective kinetic hydrate inhibitor (KHI) that has been widely used in both industry applications and laboratory studies. Industrially, organic peroxides are often used as the N-vinylcaprolactam (VCap) polymerization initiator. In our own past studies, we usually make PVCap by polymerizing VCap with azoisobutyronitrile (AIBN), giving a 2-cyanoprop-2-yl polymer end cap. In this study, VCap was polymerized with AIBN and a series of mercaptocarboxylic acid chain transfer agents (CTAs) to modify the end groups of PVCap. These end-group-modified PVCaps were investigated as KHIs in high-pressure rocking cells. Some of them showed better KHI performance than PVCap made with AIBN alone. The mercaptoacetic-acid-group-modified PVCap (PVCapSCH(2)COOH) had significantly better inhibition performance than normal PVCap on both structure I (sI) and structure II (sII) gas-hydrate-forming systems in aqueous solution as well as a sII hydrate-forming system containing added decane. However, the most powerful sII gas hydrate inhibitor among these end-group-modified PVCaps in this study was found to be the mercaptosuccinic-acid-group-modified PVCap [PVCapSCH(COOH)CH2COOH]. For example, in slow constant cooling tests at 76 bar with the sII gas-hydrate forming gas mixture, 2500 ppm of PVCapSCH(COOH)CH2COOH in deionized water gave the first detectable hydrate formation at an average temperature [T-o(av)] of 8.5 degrees C. In contrast, PVCap made with AIBN alone gave a statistically significant and higher T-o(av) value of 9.7 degrees C. In contrast, 4-mercaptobenzoic-acid-group-modified PVCap (PVCapSC(6)H(4)COOH) in deionized water increased the T-o(av) value to 10.5 degrees C. We also introduced the mercaptoacetic acid and mercaptosuccinic acid groups into poly(N-isopropylmethacrylamide) (PNIPMAA) polymers, again using AIBN as an initiator. Both of the end-group-modified PNIPMAAs gave superior KHI performance than PNIPMAA made using AIBN alone.