We report for the first time the synthesis and kinetic gas hydrate inhibition of a series of pseudo-polypeptides, poly(N-alkylglycine)s, with varying length alkyl side chains. The polymers have similar molecular weights for performance comparison. Tests were carried out in a multi-cell rocking rig at high pressure using a structure-II-forming natural gas blend. The best polymer tested was poly(N-propylglycine), with the performance being dependent upon the polymer molecular weight and water solubility.

A series of polymers containing the 2-isopropenyl-2-oxazoline (iPOx) monomer have been synthesized by either anionic polymerization or RAFT polymerization. Both homopolymers of varying molecular weight were prepared as well as copolymers with methyl methacrylate (MMA) and N-isopropylmethacrylamide (NIPMAM) comonomers. The iPOx homopolymers were shown to be very poor Structure II tetrahydrofuran hydrate crystal growth inhibitors, indicating weak interaction of the oxazoline group with the hydrate crystal surface. They were also tested for their performance as kinetic hydrate inhibitors on a Structure-II-forming natural gas mixture. The homopolymers showed significant and increasing performance as the molecular weight decreased to 2000 Da. Copolymerization of iPOx with MMA did not improve the KHI performance, but copolymers of iPOx with NIPMAM did show an improvement, although not as good performance as polyNIPMAM homopolymer of similar molecular weight. However, using a 35:20 molar ratio iPOx:NIPMAM copolymer it is possible to obtain good KHI performance and a high cloud point of 70 °C in water, which is useful for injection into hot fluids. These results demonstrate that polymers containing predominantly non-amide-based monomers such as iPOx can be useful, high cloud point KHIs. In the case of iPOx polymers the primary inhibition mechanism appears to be nucleation inhibition since hydrate crystal growth inhibition is poor.

A series of poly(β-peptoid)s, specifically poly(N-alkyl-β-alanine) homopolymers and copolymers with various Nalkyl substituents, have been synthesized. The activity of poly(β-peptoid)s as kinetic hydrate inhibitors (KHIs) was studied for the first time. Gas hydrate inhibition was studied in high-pressure rocking cells using a synthetic natural gas blend to promote the formation of gas hydrate structure II. The best gas hydrate kinetic inhibition was observed with poly(N-ethyl-β-alanine)-co-Npropyl-β-alanine) with a molecular weight of 4075 Da. The structure−activity relationship (SAR) observed in this investigation confirms the notion that, for water-soluble polymers, the presence of larger aliphatic side chains leads to improved kinetic inhibition. Two isomeric forms of poly(N-ethyl-β-alanine)-co-N-propyl-β-alanine) were studied, a random copolymer and a block copolymer. The random isomeric form performed better than the block analogue. This fact suggests that the correct molecular spacing of the monomeric units is required for best kinetic hydrate inhibition. To our knowledge, this is the first report comparing the KHI performance of random and block copolymers of the same molecular weight and empirical formula.

A series of copolymers of N-vinylazacyclooctanone (VACO) with more hydrophilic monomers, including the 5−7 ring N-vinyl lactams, N-vinyl-N-methyl acetamide (VIMA), and N-vinyl acetamide (NVA), have been synthesized. Their performance as kinetic hydrate inhibitors (KHIs) was compared to poly(N-vinylazacyclooctanone) (PVACO) homopolymer in high pressure rocking cells using a Structure II hydrate-forming natural gas mixture. The best 1:1 copolymer between any two of the four N-vinyl lactams was found to be for VP/VACO copolymer. 1:1 VACO/VIMA copolymers were also shown to be a superior KHI to PVACO, whereas copolymers of other ratios of these two monomers were less effective KHIs. 2-Butoxyethanol (BGE) was also found to be a good solvent and synergist for 1:1 VACO/VIMA copolymers. Finally, 1:1 N-vinyl lactam/NVA copolymers were shown to have worse KHI performance than the equivalent 1:1 N-vinyl lactam/VIMA copolymers at similar molecular weights.

A series of water-soluble homopolymers and copolymers of 1-alkyl-vinyl pyrrolidones (1-alkyl-VPs) with alkyl groups, including methyl, ethyl, isopropyl, and n-propyl, has been synthesized. Homopolymers with larger alkyl groups had cloud points in water lower than 0 °C. The comonomers used in the copolymers were N-vinyl pyrrolidone (VP) and N-methyl-N-vinyl acetamide (VIMA). The performance of the water-soluble 1-alkyl-VP polymers as kinetic hydrate inhibitors was investigated for the first time. Tests were conducted in high-pressure rocking cells using a structure II hydrate-forming natural gas mixture and compared to poly(N-vinyl lactam)s of similar molecular weight. The best copolymer of this series was the 1:1 nPr-VP/VIMA copolymer. 1-dimethyl-VP and 1-ethyl-VP homopolymers and the 1:1 n-propyl-VP/VP copolymer also showed better performance than poly(N-vinyl caprolactam) and poly(N-vinyl azocyclooctanone) of similar molecular weights. The structure− performance analysis in this study suggests that a more lipophilic ring than PVP gives better structure II gas hydrate kinetic inhibition than PVP itself. Second, alkylation of PVP gives a better kinetic hydrate inhibitor (KHI) than expanding the PVP lactam ring with the same number of carbon atoms as used in the alkylation.