Kinetic hydrate inhibitors (KHIs) are water-soluble polymers designed to delay gas hydrate formation in upstream gas and oilfield operations. Most classes of KHI polymers that have been discovered are vinylic polymers made by radical polymerization of vinyl monomers, giving atactic polymer structures. Atactic polymers may not be optimal for KHI performance. However, metallocene-mediated stereospecific coordination polymerization of polar vinyl monomers to stereoregular polymers can be carried out to give syndiotactic and isotactic polymers of certain monomers with molecular-weight control. This was carried out for several poly(N,N-dialkylacrylamide)s. We have carried out structure II gas hydrate KHI ramping tests in high-pressure autoclaves on the poly(N,N-dialkylacrylamide)s. Clearly, polymer tacticity does affect the performance of this class of polymers. Isotactic polyacryloylpyrrolidine (PAP) gave shorter induction times than atactic PAP. Assuming that the primary nucleation inhibition mechanism is perturbation of the water structure, these observations can be rationalized in that the side groups in isotactic PAP are more crowded together than in atactic PAP, which results in less water perturbation for the isotactic PAP and, therefore, less effect on disrupting hydrate nucleation. However, isotactic PAP gave longer periods of slow hydrate crystal growth than atactic PAP. This could be rationalized by considering that the isotactic PAP has a higher concentration of side groups along one side of the polyvinyl backbone than atactic PAP, which may give it better adsorption onto the growing hydrate crystal surfaces. All three different tacticity poly(N,N-dimethylacrylamide)s (PDMAA)s were poorer nucleation inhibitors than the PAPs. Syndiotactic PDMAA gave, on average, longer induction times than isotactic or atactic PDMAA. This can be rationalized by the fact that the syndiotactic polymer has alternating up and down dimethylamide side groups, giving the best surface/volume ratio for this polymer tacticity and, therefore, the best perturbation of the water structure. Again, this is assuming that is the dominant nucleation inhibition mechanism. The isotactic PDMAA clearly gave shorter slow crystal growth periods than the syndiotactic polymer, while the atactic polymer gave the longest times. For the isotactic polymer, this is the opposite of the results that we obtained with the PAPs, in which the isotactic polymer gave the longest hydrate slow growth periods.
