Effect of Polymer Tacticity on the Performance of Poly(N,N

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Energy Fuels 2010, 24, 2554–2562 Published on Web 03/24/2010

: DOI:10.1021/ef901609b

Effect of Polymer Tacticity on the Performance of Poly(N,N-dialkylacrylamide)s as Kinetic Hydrate Inhibitors Luca Del Villano,† Malcolm A. Kelland,*,† Garret M. Miyake,‡ and Eugene Y.-X. Chen‡ †

Department of Mathematics and Natural Science, Faculty of Science and Technology, University of Stavanger, N-4036 Stavanger, Norway, and ‡Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872 Received December 29, 2009. Revised Manuscript Received March 7, 2010

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. thereof,6-11 poly(N,N-(di)alkyl(meth)acrylamide)s,12-15 polyvinyloxazolines,16 polymaleimides,17 polymaleamides,18 poly(Nvinylalkanamide)s,19 modified acrylamidopropylsulfonic acid (AMPS) polymers,20 and polyallylamides.21

Introduction Kinetic hydrate inhibitors (KHIs) are water-soluble polymers designed to delay gas hydrate formation in upstream gas and oilfield operations.1-3 Most classes of KHI polymers that have been discovered are vinylic polymers. Examples are polyvinylpyrrolidone (PVP),4,5 polyvinylcaprolactam and co-polymers

(11) Fu, B.; Neff, S.; Mathur, A.; Bakeev, K. SPE Prod. Facil. 2002, August, SPE 78823. (12) Colle, K. S.; Costello, C. A.; Talley, L. D.; Longo, J. M.; Oelfke, R. H.; Berluche, E. International Patent Application WO96/08672, 1996. (13) Namba, T.; Fujii, Y.; Saeki, T.; Kobayashi, H. International Patent Application WO96/38492, 1996. (14) Kelland, M. A.; Svartaas, T. M.; Ovsthus, J. A New class of kinetic hydrate inhibitor. Proceedings of the 3rd International Conference on Gas Hydrates. Ann. N. Y. Acad. Sci. 2000, 912, 281. (15) Kelland, M. A.; Rodger, P. M.; Namba, T. International Patent Application WO98/53007, 1998. (16) Colle, K. S.; Talley, L. D.; Oelfke, R. H.; Berluche, E. International Patent Application WO96/08673, 1996. (17) Colle, K. S.; Costello, C. A.; Talley, L. D. Canadian Patent Application 96/2178371, 1996. (18) Kelland, M. A.; Klug, P. International Patent Application WO98/23843, 1998. (19) Colle, K. S.; Talley, L. D.; Oelfke, R. H.; Berluche, E. International Patent Application 96/41784, 1996. (20) Pfeiffer, D. G.; Costello, C. A.; Talley, L. D.; Wright, P. J. International Patent Application WO99/64718, 1999. (21) Colle, K. S.; Costello, C. A.; Talley, L. D.; Oelfke, R. H.; Berluche, E. International Patent Application WO96/41834, 1996.

*To whom correspondence should be addressed. Telephone: þ4751831823. Fax: þ47-51831750. E-mail: [email protected]. (1) Kelland, M. A. Production Chemicals for the Oil and Gas Industry; CRC Press: Boca Raton, FL, June 2009. (2) Kelland, M. A. Energy Fuels 2006, 20, 825. (3) Sloan, E. D., Jr.; Koh, C. A. Clathrate Hydrates of Natural Gases, 3rd ed.; CRC Press: Boca Raton, FL, 2008. (4) Sloan, E. D. U.S. Patent 5,420,370, May 30, 1995. (5) Long, J.; Lederhos, J.; Sum, A.; Christiansen, R. L.; Sloan, E. D. Proceedings of the 73rd Annual GPA Convention, New Orleans, LA, March 7-9, 1994. (6) Sloan, E. D. U.S. Patent 5,432,292, 1995. (7) Bakeev, K.; Myers, R.; Chuang, J.-C.; Winkler, T.; Krauss, A. U.S. Patent 6,242,518, 2001. (8) Cohen, J. M.; Wolf, P. F.; Young, W. D. U.S. Patent Application 5,723,524, 1998. (9) Angel, M.; Stein, S.; Neubecker, K. International Patent Application WO/2001/066602, 2001. (10) Kolle, K. S.; Oelfke, R. H.; Kelland, M. A. U.S. Patent 5,874,660, 1999. r 2010 American Chemical Society

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Figure 1. Structures of isotactic (I), syndiotactic (S), and atactic (A) polyvinyl polymers.

All of these polymers are most easily made on a commercial scale by radical polymerization of vinyl monomers using azo or peroxy catalysts, such as 2,20 -azobis(2-methylpropiononitrile) (AIBN) or potassium persulphate, respectively. This means that the resulting polymers will always have an atactic polymer structure, in which the pendant groups on the polyvinyl backbone are randomly orientated up or down along the chain. The size of the pendant groups can affect the tacticity in some cases, but generally atactic polymers are formed. There are two tacticities of polyvinyl polymers with more ordered structures; these are syndiotactic polymers with alternating up and down pendant groups and isotactic polymers with pendant groups all on the same side (either all up or all down) (Figure 1). It occurred to us that atactic polymers might not be structurally optimal for KHI performance and that the more ordered syndiotactic or isotactic polymers might show greater performance. However, special metal-based catalysts are needed to produce highly stereoregular polyvinyl polymers. For example, transition-metal catalysts of metals, such as titanium and zirconium, are well-known Ziegler-Natta catalysts for polymerization of alkenes to polyalkenes.22-25 However, most of these catalysts are not suitable for the polymerization of vinylic monomers with polar groups, which limits the range of monomers that can be polymerized.26 However, one of the two groups authoring this paper, at the Colorado State University, has developed more robust catalysts for the asymmetric coordination polymerization of polar vinyl monomers of four different classes using a chiral ansazirconocenium ester enolate catalyst.27 Other groups have reported stereo-controlled polymerization of isopropylacrylamide by reversible addition-fragmentation chain transfer

(RAFT) polymerization with yttrium-based catalysts to produce isotactic polymers but not syndiotactic polymers.28-31 These catalysts are unfortunately not able to polymerize vinyl lactam polymers, which are the most commercially used class of KHI polymers, but atactic acrylamide-based KHIs have been well-researched. In fact, a KHI based on poly(N-isopropylmethacrylamide) has recently become commercially available.1 The new catalysts are metallocene-based incorporating η5cooordinated cyclopentadienyl or indenyl rings, with an example being rac-(EBI)Zrþ(THF)[OC(OiPr)dCMe2][MeB(C6F5)3]-. This is used to make isotactic polymers. This catalyst was used for the first successful coordination-addition polymerization of N,N-dialkylmethacrylamides and the first example of kinetic resolution of a racemic methacrylamide by enantiomeric metallocene catalysts.32 Figure 2 illustrates the stereospecific polymerization of N,N-dialkylacrylamides with two of these catalysts. To form atactic polymers, standard radical polymerization with AIBN was carried out. For this project, we wanted to investigate poly(N,N-dialkylacrylamide)s that would have good effect as KHIs. Therefore, we chose to polymerize N,N-dimethylacrylamide, N,Ndiethylacrylamide, and acryloylpyrrolidine to form polymers of all three tacticities. To compare their performances, it was also necessary that polymers in the same class had approximately the same molecular weight because this is a factor that affects KHI polymer performance.1,14 The polymers that were synthesized are summarized in Table 1. Syndiotactic polyacryloylpyrrolidine (PAP) with a Mn value in the range of 5000-10 000 was not available, as well as syndiotactic poly(N,N-diethylacrylamide). In fact, this may not have been a loss to this investigation because we discovered that GMM2a (syndiotactic PAP with Mn = 1420) was not soluble in water, even after sonication or after first dissolving it in a polar organic solvent, such as acetone. Therefore, the comparison of the KHI performance to a syndiotactic polymer was limited to using GMM9, poly(N,N-dimethylacrylamide) (PDMAA), in comparison to other tacticity PDMAAs. The aim was to synthesize polymers of all three tacticities with similar molecular weight because it is known that molecular weight affects KHI performance.2 For atactic PAPs, it is known that molecular weights (weight-average molecular weight, Mw) of about 1500-2000 give optimum KHI performance and the performance drops slowly as the average

(22) Pino, P.; Cioni, P.; Wei, J. J. Am. Chem. Soc. 1987, 109, 6189. (23) Kaminsky, W.; Ahlers, A.; M€ oller-Lindenhof, N. Angew. Chem., Int. Ed. 1989, 28, 1216. (24) Pino, P.; Galimberti, M.; Prada, P.; Consiglio, G. Makromol. Chem. 1990, 191, 1677. (25) Kaminsky, W. Angew. Makromol. Chem. 1986, 145/146, 149. (26) Chen, E. Y.-X. Dalton Trans. 2009, 8784. (27) (a) Miyake, G. M.; Mariott, W. R.; Chen, E.Y.-X. J. Am. Chem. Soc. 2007, 129, 6724. (b) Miyake, G. M.; Chen, E. Y.-X. Macromolecules 2008, 41 (10), 3405. (28) Ray, B.; Isobe, Y.; Morioka, K.; Habaue, S.; Okamoto, Y.; Kamigaito, M.; Sawamoto, M. Macromolecules 2003, 36 (3), 543. (29) Ray, B.; Isobe, Y.; Matsumoto, K.; Habaue, S.; Okamoto, Y.; Kamigaito, M.; Sawamoto, M. Macromolecules 2004, 37, 1702. (30) Nuopponen, M.; Kalliom€aki, K.; Laukkanen, A.; Hietala, S.; Tenhu, H. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 38. (31) Lutz, J.-F.; Neugebauer, D.; Matyjaszewski, K. J. Am. Chem. Soc. 2003, 125, 6986.

(32) Miyake, G.; Caporaso, L.; Cavallo, L.; Chen, E. Y.-X. Macromolecules 2009, 42, 1462.

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Figure 2. Stereospecific coordination-addition polymerization of N,N-dialkylacrylamides with rac-(EBI)Zrþ(THF)[OC(OiPr)dCMe2][MeB(C6F5)3]- catalyst to form isotactic polymers (above) and Ph2C(Cp)(Flu)Zrþ(THF)[OC(OiPr)dCMe2][MeB(C6F5)3] to form syndiotactic polymers (below).

Figure 3. Sapphire cell high-pressure test equipment. Table 2. Composition of SNG

Table 1. Poly(N,N-dialkylacrylamide) Samples Synthesised for This Studya sample ID

polymer

tacticity

molecular weight (Mn)

PDI

PAP1 PAP2 GMM1 GMM2 GMM2a GMM3 GMM4 GMM5 GMM6 GMM7 GMM8 GMM9

PAP PAP PAP PAP PAP PDEAA PDEAA PDMAA PDMAA PDMAA PDMAA PDMAA

isotactic atactic atactic isotactic syndiotactic atactic isotactic atactic atactic isotactic isotactic syndiotactic

5330 10400 1570 1950 1420 8590 2180 9570 21150 5660 23200 25140

1.65 1.19 1.04 1.07 1.07 1.29 1.08 1.15 1.33 1.15 1.08 1.13

component

mol %

methane ethane propane iso-butane n-butane N2 CO2

80.67 10.20 4.90 1.53 0.76 0.10 1.84

especially when the alkyl group is isopropyl.14 However, isotactic poly(N-isopropylacrylamide) is known to be insoluble in water at all temperatures and, therefore, could not be used in KHI experiments.29 Polymer Synthesis and Characterization. Atactic,36 isotactic,37 and syndiotactic38 poly(N,N-dialkylacrylamide)s were synthesized using literature procedures. Gel permeation chromatography (GPC) analyses of the polymers were carried out at 40 °C at a flow rate of 1.0 mL/min, with DMF as the eluent, on a Waters University 1500 GPC instrument equipped with four 5 μm PL gel columns (Polymer Laboratories) and calibrated using 10 poly(methyl methacrylate) (PMMA) standards. Chromatograms were processed with Waters Empower software (version 2002); number-average molecular weight and polydispersity of polymers are given relative to PMMA standards.

a AP, acryloylpyrolidine; DEAAA, N,N-diethylacrylamide; DMAA, N,N-dimethylacrylamide; Mn, number average molecular weight; PDI, polydispersity.

polymer length increases.14,33 Thus, for polymers of the same tacticity but with molecular weights of 5000 and 10 000, one would expect a slightly higher performance from the polymer with a molecular weight of 5000. However, we believe the difference is small enough that we can compare the effect of tacticity for PAP1 (Mn = 5000) to PAP2 (Mn = 10 000). For the PDMAAs synthesized in this work, we have polymers of all three tacticities and molecular weights in the narrow range of 21 000-25 000 (GMM6, GMM8, and GMM9), making it possible for a good comparison. Further, Mw/Mn = PDI (Mn=number average molecular weight). For PAP1, because the polydispersity index PDI = 1.65, the Mw value is actually 8800, whereas for PAP2, the Mw value is 12 370. Thus, for Mw values, the polymers are not so different in size. It should also be pointed out that poly(N-monoalkyl(meth)acrylamide)s are also known to perform well as KHIs,

Experimental Section: High-Pressure Autoclave KHI Performance Tests KHI performance tests were carried out in high-pressure sapphire and stainless-steel autoclave equipment shown in Figure 3. We have two identical equipments for KHI tests, which we call cells 1 and 2. In this work, cell 2 was used throughout. In all of the experiments, we used a synthetic natural gas (SNG) mixture that gives structure II hydrates (Table 2). The aqueous phase was distilled water in all experiments. No liquid hydrocarbon phases were used. Two test methods were used, the ramping method and the constant cooling method. The ramping test method is discussed first below.

(33) Kelland, M. A.; Svartaas, T. M.; Ovsthus, J. RF-Rogaland Research, Research Report RF-97/001, 1997. (34) Gjertsen, L. H.; Fadnes, F. H. Measurements and predictions of hydrate equilibrium conditions, gas hydrates: Challenges for the future. Ann. N. Y. Acad. Sci. 2000, 912, 722–734. (35) Tohidi, B.; Burgass, R. W.; Danesh, A.; Ostergaard, K. K.; Todd, A. C. Improving the accuracy of gas hydrate dissociation point measurements. Ann. N. Y. Acad. Sci. 2000, 912, 924–931.

(36) Yamago, S.; Ray, B.; Iida, K.; Yoshida, J.; Tada, T.; Yoshizawa, K.; Kwak, Y.; Goto, A.; Fukuda, A. J. Am. Chem. Soc. 2004, 126, 13908–13909. (37) Mariott, W.; Chen, E. Y.-X. Macromolecules 2004, 37, 4741– 4743. (38) Ning, Y.; Chen, E. Y.-X. J. Am. Chem. Soc. 2008, 130, 2463– 2465.

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Figure 4. Ramping experiment with isotactic PAP, PAP1. Hydrate is first observed after 52 260 s (871 min), but no plug forms in 80 000 s (1333 min).

Figure 5. Ramping experiment with atactic PAP, PAP2.

of the cooling bath was adjusted to about 2 °C above the hydrate equilibrium temperature at the pressure conditions to be used in the experiment. (5) After the cell was purged twice with the SNG, the cell was loaded with SNG to the desired pressure while stirring at 600 rpm. (6) The cell temperature was held at ca. 20 °C for approximately 50 min before being dropped rapidly (in about 40 min) to 12 °C and held at this temperature for 4 h. (7) The cell temperature was then dropped rapidly (in ca. 10 min) to 11 °C and held there for 4 h. (8) The temperature ramping was repeated at 10, 9, 8, and 7 °C, at which point the experiment was terminated. The induction time, ti, for the first sign of hydrate formation was determined as the time from first stirring at 20 °C to when the first pressure drop occurred that was not due to the ramping down of the temperature. The total hold time was taken as the time when the pressure drop was rapid and fast hydrate formation occurred, resulting in a plug. Examples of typical plots are given in Figures 4 and 5 for AP1 and AP2, respectively. The second test method was the constant cooling method, in which the fluids were cooled with stirring from approximately 97 bar (for PAPs) to 63 bar (for PDMAAs) from 20 to 5 °C in 6 h. Examples of results are shown in the graphs in Figures 9 and 10.

The pressure was usually 92 bar at the first ramp temperature of 12 °C. The equilibrium temperature for distilled water at this pressure was calculated using Calsep’s PVTSim software to be 20.9 °C. Laboratory experiment to determine the equilibrium temperature by standard slow hydrate dissociation gave equilibrium temperatures of approximately 21.0 °C at 90 bar.34,35 The cooling rate near the equilibrium temperature was 0.14 °C/h. From our experience, the equilibrium temperature could have been a little lower if we had used a cooling rate of 0.05 °C/h, which would have given very good agreement with the predicted equilibrium temperature, but this makes the experiments considerably more time-consuming. For the rest of this paper, we will assume that the equilibrium temperature for our SNG-distilled water system at 92 bar is 20.9 °C. This means that for the first ramp, the temperature is 12 °C and the subcooling (ΔT) is 8.9 °C. The same initial procedure for the preparation of a KHI ramping experiment and filling of the cell was followed in all high-pressure experiments in both sapphire cells. (1) The chemical to be tested was dissolved in distilled water to the desired concentration, usually 5000 ppm (0.5 wt %). (2) The magnet housing of the cell was filled with the aqueous solution containing the chemical to be tested. The magnet housing was then mounted in the bottom end piece of the cell, which thereafter was attached to the sapphire tube and the cell holder. (3) The desired amount of the aqueous solution was filled in the cell (above the cell bottom) using a pipet; the top end piece was mounted; and the cell was placed into the cooling bath (plastic cylinder). (4) The temperature

Results and Discussion Before conducting KHI performance experiments, we had to first check the solubility of the poly(N,N-dialkylacrylamide)s. Isotactic PAP, PAP1, was soluble in distilled water 2557

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Table 3. Sapphire Cell Ramping KHI Experiments with Luvicap 55W, Isotactic PAP (PAP1), and Atactic PAP (PAP2) at 90 bar chemical

induction time (min)

Tonset (°C)

total delay time (min)

temperature for rapid growth (°C)

slow crystal growth time (min)

no inhibitor Luvicap 55 W Luvicap 55 W PAP1 PAP1 PAP1 PAP1 PAP1 PAP2 PAP2 PAP2 PAP2 PAP2

1333a 1353 935 957 1082 1137 969

430 >462 679 50 0 0 0 42

a

Experiments terminated at these times without rapid formation of hydrate as a plug.

at room temperature (20-22 °C) but gave a slightly cloudy solution. The catalyst is detached from the polymer, so that this does not affect the solubility. The atactic PAP, PAP2, is however completely soluble at room temperature. For the GMM series of PAPs, atactic GMM1 was water-soluble, GMM2 gave cloudy solutions, and GMM3 was was not soluble in water, even after sonication or after first dissolving it in a polar organic solvent, such as acetone. For the poly(N,N-diethylacrylamide)s, atactic GMM3 was completely water-soluble but isotactic GMM4 was not very soluble. Therefore, we could not compare the performance of different tactic poly(N,N-diethylacrylamide)s. For the PDMAAs, we found that atactic GMM6, isotactic GMM8, and syndiotactic GMM9 were fully soluble in water, with only the latter showing slight cloudiness. Good solubility for all three tacticities is not surprising because the N,Ndimethylacrylamide monomer is more hydrophilic than the other two acrylamide monomers in this study. Moreover, these three PDMAAs are of similar molecular weight, making it possible for a good comparison of the dependence of tacticity on KHI performances, independent of any molecular-weight differences. Experiments with PAPs. The first ramping tests were conducted with isotactic PAP1 and atactic PAP2 and compared to Luvicap 55W, a vinyl caprolactam/vinyl pyrrolidone polymer, supplied by BASF, Germany. We carried out five parallel experiments on each polymer because there is scattering in the data as a result of the stochastic nature of the hydrate formation process. The results are summarized in Table 3. Examples of the graphical results for the pressure drop during the ramping are shown in Figures 4 and 5 for experiments with PAP1 and PAP2, respectively. The pressure drops rapidly after about 50 min because of the large drop in the temperature from 20 to 12 °C. The pressure stays stable for 240 min at each temperature that the cell is ramped. Although tests with Luvicap 55W were not of interest with regard to studying the effect of tacticity, we carried out a couple of tests to ascertain the performance of the tactic polymers relative to this commercial KHI product. Luvicap 55W performed fairly well, giving induction times in the two tests of 505 and 380 min, compared to blank tests with no inhibitor. Hydrates were first observed at 11 °C (ΔT = 9.9 °C) in both tests with Luvicap 55W because of the start of a pressure drop and not the ramping down of the temperature. There was a period of slow hydrate growth in both tests, but hydrate plugs ultimately formed in both tests, also at 11 °C. Both the PAPs, PAP1 and PAP2, atactic and isotactic, respectively, performed better than Luvicap 55W.

Independent sample t tests have been used to test for statistical differences in induction times between the two inhibitors PAP1 and PAP2. p values smaller than 0.05 are considered to indicate statistical significant differences.39 An independent sample t test for differences in induction times for PAP1 and PAP2 gave a p value of 0.014; i.e., there is a clearly significant difference in the induction time results for the two polymers. Thus, from the data, we see that PAP2 gives longer induction times. If tacticity did not matter, PAP1 with the lower and more optimal molecular weight would be expected to perform somewhat better than PAP2. Instead, the results suggest that PAP2 is a better nucleation inhibitor than PAP1, which means that polymer tacticity does affect structure II gas nucleation inhibition with this class of polymer. One can rationalize these results, in that the side groups in isotactic PAP1 are more crowded together than in atactic PAP2, which results in less water perturbation for the isotactic PAP1 and, therefore, less effect on disrupting hydrate nucleation. This assumes that this is the primary nucleation inhibition mechanism operating for this polymer class. For atactic PAP2, we observed fairly fast hydrate formation immediately after the first sign of nucleation. This suggests that, of the two tactic polymers, PAP1 is a better inhibitor of hydrate crystal growth than PAP2. This can possibly be rationalized by considering that the isotactic PAP1 has a higher concentration of side groups along one side of the polyvinyl backbone than atactic PAP2, which may give it better adsorption onto the growing hydrate crystal surfaces. Interestingly, we found that isotactic PAP1 gave long periods of slow crystal growth periods after initiation of hydrate formation, to such a degree that we did not observe a rapid growth leading to a plug in two of the five tests, at which time the experiments were stopped. For example, in the second experiment listed in Table 2, the total delay time is 1033 min, which is no better or worse than the total delay time with atactic PAP2, but the crystal growth delay is long (422 min) because nucleation was first observed after 611 min in this experiment. In the other four experiments with isotactic PAP1, the crystal growth delay times (total delay time minus induction time) were 679 and 617 min and over 430 and 462 min in the last two experiments, at which point they were terminated. (39) Myers, R. H.; Myers, S. L.; Walpole, R. E.; Ye, K. Probability and Statistics for Engineers and Scientists; Pearson Education: Upper Saddle River, NJ, 2007.

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We also carried out several constant cooling tests in the same sapphire cell over 6 h with 5000 ppm PAP1 or PAP2, cooling from 20 to 5 °C. The pressure at 20 °C was ca. 97 bar, and we used a 1:1 decane/distilled water mixture by volume. The equilibrium temperature at 97 bar is ca. 18.8 °C. Isotactic PAP1 gave first hydrate formation after 350, 331, and 329 min in three tests detected as a deviation from the pressure drop as a result of cooling the fluids in a closed system. The onset temperatures at these times were 5.20, 5.85, and 5.86 °C, respectively, fairly close to the minimum experimental temperature of 5 °C. Fast hydrate formation, detected by a faster pressure drop and an exothermic increase in the cell temperature, did not occur in any of these experiments, indicating good inhibition of hydrate crystal growth. This concurs with our observations in the ramping tests discussed earlier. In contrast, no sign of hydrate formation was detected in four tests with atactic PAP2 even after cooling to the minimum temperature of 5 °C, at which point the cell was warmed. These results also confirm the results from the ramping experiments that PAP2 is the best nucleation inhibitor.

Experiments with PDMAAs. The second set of experiments that we carried out, using the same ramping test procedure from 12 to 7 °C, were with the three different tacticities of PDMAA, with molecular weights in the range of 21 000-25 000. The results are summarized in Table 4. Examples of the graphical results for the pressure drop during the ramping are shown in Figures 6-8. Atactic PDMAA is known to be a poorer KHI than atactic poly(N,Ndiethylacrylamide) or PAP;12 therefore, it was expected that induction times would be lower with all of the PDMAAs compared to PAP1 and PAP2. This is also what was observed; all three tacticities of PDMAA gave much shorter induction times than PAP1 or PAP2. Although there was a fair amount of scattering, it appears that the syndiotactic GMM9 gives longer induction times (ti) than the atactic or isotactic PDMAAs. Thus, the p values for the induction time results for GMM9 compared to GMM8 and GMM6 are 0.023 and 0.029, respectively, which are lower values than 0.05, suggesting a significant difference in performance of GMM9 compared to the other two polymers. This can be rationalized by the fact that the syndiotactic polymer GMM9 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 assuming that is the dominant nucleation inhibition mechanism. With regard to the average of the total delay times (or total hold times, ta), the isotactic GMM8 polymer gave clearly shorter times than the syndiotactic polymer, while the atactic polymer gave the longest times ( p value is 0.025 for GMM6 versus GMM9 total delay 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. To further confirm the results with the PDMAAs, we also ran some constant cooling experiments in the same sapphire cell as the ramping tests. In these tests, the temperature is lowered constantly from 20 to 5 °C in a 6 h period. The cell is then cleaned, and the test is repeated. Three tests were carried out on each PDMAA polymer. We chose to conduct the tests at a lower pressure (ca. 63 bar at 20 °C) to give a lower driving force for hydrate formation because it was clear that the PDMAAs had fairly low KHI performance compared to the PAPs discussed earlier. Tests were conducted in distilled

Table 4. Sapphire Cell Ramping KHI Experiments with PDMAAs Using DI Water, 600 rpm, 5000 ppm Additive. The Pressure is 94 bar at 12 °C PDMAA GMM9 (syndiotactic)

GMM6 (atactic)

GMM8 (isotactic)

a

ti (min)

Tonset (°C)

ta (min)

temperature at rapid growth (°C)

241 327 503 147 300 67 82 108 79 112 115 61 55 175 105 135 152 158

12 11 11 12 11 12 12 12 12 12 12 12 12 12 12 12 12 12

439 698 918 613 504 703 >416a 1019 943 917 234 124 143 258 157 208 168 170

11 10 9 10 11 10 e11 9 9 9 12 12 12 12 12 12 12 12

Unplanned experiment termination.

Figure 6. Ramping experiment with syndiotactic PDMAA, GMM9.

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Figure 7. Ramping experiment with atactic PDMAA, GMM6.

Figure 8. Ramping experiment with isotactic PDMAA, GMM8.

Figure 9. Example of a 6 h constant cooling test, using 5000 ppm isotactic PDMMA (GMM8). The upper line is the pressure.

water using 5000 ppm polymer. The equilibrium temperature at 63 bar is ca. 18.8 °C. All results are given in Table 5, and graphical examples of test results are shown in Figures 9

and 10. The pressure dropped throughout each test as a result of the temperature drop and the cell tests being carried out in a closed system. It was possible to see a deviation from 2560

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Figure 10. Example of a 6 h constant cooling test, using 5000 ppm syndiotactic PDMMA (GMM9). The upper line is the pressure.

The question then remains as to why isotactic GMM8 shows little ability to slow the growth of hydrate crystals in the ramping tests but does show this property in the constant cooling tests. A possible partial explanation is that the onset temperature for hydrate formation is very high (13.3814.08 °C) in the constant cooling tests with isotactic GMM8, such that the driving force for further growth of these crystals is comparatively low compared to the tests with atactic GMM6 or syndiotactic GMM9. This would lead to long hydrate slow growth periods for GMM8, as observed. However, this does not explain why the total delay time for isotactic GMM8 is significantly lower in the ramping tests than the atactic polymer, yet both polymers gave similar temperatures for rapid hydrate formation in the constant cooling tests. Because of this apparent inconsistency in the ramping tests, we went back and conducted three further ramping tests on isotactic GMM8 a few months later. These results, entered last in Table 4 in italics, fitted well with the earlier results for GMM8 using this method and only served to underline the apparent inconsistency. At this time, we do not have a rationalization for this seemingly contradictory behavior regarding hydrate crystal growth inhibition by the isotactic PDMAA, GMM8, in ramping tests at 90 bar and constant cooling tests at 63 bar. As pure speculation, there may be a pressure affect on the polymer structure in solution. However, for field applications, it is the time to nucleation of the first hydrate crystals (the induction time) that is most critical to operators and all of our results on the time to nucleation are consistent.

Table 5. Constant Cooling Test Results (6 h) on 5000 ppm PDMAAs in Distilled Water, Cooling from 63 bar and 20 °C polymer GMM6 (atactic) GMM8 (isotactic) GMM9 (syndiotactic)

onset temperature (°C)

rapid hydrate formation temperature (°C)

11.00 10.22 12.31 14.08 13.38 13.81 5.36 6.20 5.85

9.28 8.41 8.28 8.54 7.74 8.29 5.36 6.20 5.85

the pressure drop when hydrate formation was first detected. When a fast pressure drop occurred, we observed a hydrate plug in the cell. Both the temperature at which onset of hydrate formation occurred and the temperature at which fast hydrate formation occurred are recorded in Table 5. From the results, it is clear that the best KHI, with the lowest onset temperatures, is syndiotactic GMM9 ( p values compared to GMM6 and GMM8 are 0.004 and 0.002, respectively). This fits well with the results obtained using the temperature ramping method at 90 bar. The poorest KHI is isotactic GMM8, which may be marginally worse than atactic GMM6 (the p value for GMM6 compared to GMM8 is 0.057, which is borderline to a statistically significant difference). Both GMM8 and GMM6 gave very short induction times using the temperature ramping method at 90 bar. However, the constant cooling tests at 63 bar are at less severe conditions and have allowed for a clearer ranking of these two polymers. We did not observe any slow hydrate growth in the experiments with syndiotactic GMM9. A graphical example of a test result is shown in Figure 10. This is probably because the driving force at the onset is so high that rapid hydrate formation ensues immediately. The temperature at which rapid hydrate formation occurred for the atactic and isotactic polymers was fairly similar. However, both polymers showed a phase of slow hydrate growth, with isotactic GMM8 giving the longest periods in all three experiments. This does not fit the results with GMM8 conducted using the ramping method, but it does fit the results that we obtained with the PAPs, in which isotactic PAP1 gave longer slow growth times than atactic PAP2.

Conclusions Because atactic polyvinyl-based polymer KHIs may not be optimal for KHI performance, we have synthesized various poly(N,N-dialkyacrylamide)s, a known class of KHI polymer, of all three tacticities and similar molecular weights using metallocene-mediated stereospecific coordination polymerization of polar vinyl monomers to optically active, stereoregular polymers. 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 KHI performance of this class of polymers as demonstrated for both the PAP and PDMAA series. This suggest that tacticity control in commercial KHI polymers, such as polyvinylcaprolactam 2561

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and poly(isopropylmethacrylamide), could significantly improve their performance. However, at present, there appear to be no easy polymerization methods available to give good tactic control in these classes of KHIs. We are currently investigating methods to control the tacticity in these and other classes of KHI polymers. In ramping and constant cooling tests, isotactic PAP (PAP1) gave shorter induction times than atactic PAP (PAP2). However, the isotactic PAP gave longer periods of slow hydrate crystal growth than the atactic PAP, as demonstrated in the ramping tests. Assuming that the primary nucleation inhibition mechanism is the perturbation of the water structure, these observations can be can rationalized, in that the side groups in isotactic PAP are more crowded together than in atactic PAP2, which results in less water perturbation for the isotactic PAP and therefore less effect on disruption of hydrate nucleation. However, isotactic PAP gave longer periods of slow hydrate crystal growth than atactic PAP. This can possibly be rationalized by considering that the isotactic PAP1 has a higher concentration of side groups along one side of the polyvinyl backbone than atactic PAP2, which may give it better adsorption onto the growing hydrate crystal surfaces. All three different tacticity PDMAAs were poorer nucleation inhibitors than the PAPs. In ramping and constant cooling experiments, 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 assuming that is the dominant hydrate nucleation inhibition mechanism. In the ramping tests at 90 bar, the isotactic PDMAA gave clearly shorter slow crystal growth periods than the syndiotactic polymer, while the atactic polymer generally 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. However, isotactic PDMAA did give long slow hydrate growth periods using the 6 h constant cooling method at 63 bar. At the moment, we do not have a rationalization for this seemingly contradictory behavior. Probing the structure of these polymers in water at various pressures using laboratory techniques or molecular modeling may help answer this. In addition, studies on tetrahydrofuran hydrate crystal growth inhibition by polymers of different tacticities may also shed light on these issues. These studies are in progress. Acknowledgment. The work carried out at Colorado State University was supported by the National Science Foundation (NSF-0718061).

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