Solvent and Polymer Stereochemistry Play Key Roles in Deuterium

The Dow Chemical Company, 230 Abner Jackson Parkway, Lake Jackson, Texas 77566, United States. Macromolecules , Article ASAP. DOI: 10.1021/acs.macromo...
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Solvent and Polymer Stereochemistry Play Key Roles in Deuterium Exchange and Partial Racemization of Polypropylenes Brian M. Habersberger* and Daniel W. Baugh, III The Dow Chemical Company, 230 Abner Jackson Parkway, Lake Jackson, Texas 77566, United States S Supporting Information *

ABSTRACT: Recent reports have described deuterium exchange reactions on polyolefins; such isotopic labeling enables powerful analytical techniques, but exchange on isotactic polypropylene was previously unsuccessful. Here, we report successful exchange reactions on isotactic and syndiotactic polypropylenes, revealing the key role that the polymer stereochemistry and reaction solvent play in the mechanism of adsorption and exchange. Alteration of the polymer stereochemistry accompanies exchange in some cases, though molecular weight is preserved in all reactions.

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INTRODUCTION

Deuterium labeling of polymers is used throughout macromolecular science to measure chain conformations, thermodynamic interactions, aggregation, and other structural and dynamic elements of polymer systems, usually via the contrast that such labeling provides to small-angle neutron scattering (SANS) and other neutron-based measurements.1−12 Selective labeling of a particular block or other substructural element enables more sophisticated and powerful experiments through the method of contrast matching to isolate scattering from the targeted structure. However, the very high expense of synthesizing deuterium-labeled polymers has limited such techniques to a small set of model polymer structures. Recently, a facile and inexpensive method for labeling commercial ethylene-rich polyolefins using a heterogeneous Pt−Re catalyst on a SiO2 support has been reported, and the effect of molecular weight and short-chain branching has been investigated.13−16 However, across these studies, exchange could not be accomplished on polypropylene, a commercially important material representing over 20% of total global plastic production.17 Herein, we report deuterium exchange on isotactic (iPP) and syndiotactic (sPP) polypropylenes. Intriguingly, iPP only undergoes exchange in pentane (which at reaction conditions is a liquid−liquid phase separated system), while sPP can be exchanged in previously reported solvents and reactor conditions. Furthermore, this exchange can be accompanied by partial racemization of the chiral centers in these polypropylenes, which occurs without alteration of molecular weight. Both of these observations point to the mechanisms of polymer adsorption and exchange and the role that local stereochemistry plays in such physical and chemical processes. © XXXX American Chemical Society

EXPERIMENTAL SECTION

Polymers. Because of the previously reported molecular weight dependence of deuterium exchange on polyethylenes,14−16 iPP and sPP resins with similar molecular weight distributions were chosen for this work. A description of the resins and summary of the reaction results are provided in Table 1. In this table and throughout this

Table 1. Summary of Resins and Reaction Results sPP sPP-D-i8 sPP-H-n5 iPP iPP-D-n5 iPP-H-n5

Mwa (Da)

Đa

× × × × × ×

3.8 3.5 3.6 2.5 2.6 2.6

1.8 1.8 1.8 2.3 2.2 2.0

105 105 105 105 105 105

yDb 0.26

0.39

f mmc

f mrc

f rrc

0.00 0.01 0.02 0.91 d 0.71

0.11 0.10 0.20 0.08 d 0.20

0.88 0.90 0.79 0.01 d 0.09

Mw and Đ measured via refractive index detection SEC. bExtent of exchange, ±0.02, measured via 2H NMR. cMethyl triad stereosequence fractions, measured via 13C NMR. dTacticity not measured for iPP-Dn5. a

article, the reaction products are referred to as iPP/sPP-X-Y, where X is the isotope used in the reaction (H or D) and Y indicates the solvent (n5 for n-pentane or i8 for isooctane (2,2,4-trimethylpentane)). The iPP resin contains a small fraction (about 0.5%) of ethylene comonomer; while not reported in detail here, similar deuterium exchange and partial racemization were confirmed when exchange reactions were conducted on an iPP homopolymer. Reactions. Except for the variation in solvent, all reactions were conducted at previously reported concentrations, pressure, and temperature. For all reactions, 3 g of polymer was combined with 1.2 g of Pt−Re/SiO2 catalyst18 in 270 mL of solvent under 600 psi of D2 or H2 gas for 16 h in a 500 mL stirred stainless steel reactor. Received: December 13, 2017 Revised: January 30, 2018

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Macromolecules Following reaction, the reactor was quenched in ice water, and the gas was vented. Reaction solvent was evaporated, and the catalystcontaining polymer was recovered and redissolved in 1,2,4trichlorobenzene (TCB) at 150 °C. The solution was filtered through pearlite at 150 °C to remove the catalyst and then poured into cold methanol to precipitate the polymer. The polymer was then recovered via vacuum filtration and drying in a vacuum oven. Differential Scanning Calorimetry (DSC). DSC was performed using a TA Instruments Q2000 differential scanning calorimeter. Polymer samples were heated to 180 °C and then cooled at 10 °C/ min to −40 °C. The presented thermograms were collected upon a second heating cycle, also conducted at 10 °C/min. Nuclear Magnetic Resonance (NMR) Spectroscopy. Samples for 1H and 2H NMR were prepared in a Norell 1001-7 10 mm NMR tube by adding 100−130 mg of sample to 3.25 g of p-dichlorobenzened4/o-dichlorobenzene (50/50 w/w). One drop of 1,1,2,2-tetrachloroethylene-d2 was added as an internal 2H standard for quantitation. Samples for 13C NMR were prepared by adding approximately 2.7 g of a 50/50 mixture of tetrachloroethane-d2/o-dichlorobenzene containing 0.025 M chromium(III) acetylacetonate to 0.10−0.20 g of sample in a Norell 1001-7 10 mm NMR tube. All sample types were dissolved and homogenized by heating the tubes and their contents to 150 °C followed by visual inspection to ensure homogeneity. Data were collected using a Bruker 400 MHz spectrometer equipped with a Bruker Dual DUL high-temperature CryoProbe. For sPP, the 13C NMR chemical shifts were internally referenced to the rrrr propylene methyls (CH3 with no D substitution) at 20.35 ppm. For iPP, chemical shifts were referenced to the mmmm methyls at 21.90 ppm. Size Exclusion Chromatography (SEC). For SEC with infrared detection (SEC-IR), a Polymer Char GPC-IR high-temperature liquid chromatograph was used. TCB containing 200 ppm 2,6-di-tert-butyl-4methylphenol was used as the solvent at 150 °C. For separation media, four 20 μm mixed-porosity analytical columns from Agilent (Mixed A) were used. Molecular weight was calibrated against a series of 21 narrow polystyrene standards, referenced against linear homopolymer PE NIST NBS 1475, and then converted to polypropylene-equivalent molecular weight using Mark−Houwink parameters. 200 μL of a 2.0 mg/mL polymer solution was injected using a flow rate of 1.0 mL/ min. An infrared detector from Polymer Char (IR5) was used to detect the 2700−3000 cm−1 region, corresponding to saturated C−H stretching vibrations. Data processing was performed using GPC One Software from Polymer Char. Because C−H stretching vibrations are reduced in polymers that have undergone deuterium exchange, SEC with refractometer detection (SEC-RI) was used to confirm that the molecular architecture was unaltered by the exchange reaction. SEC-RI was conducted similarly to SEC-IR as described above, but using a Polymer Laboratories Model 220 instrument with four 10 μm mixed-porosity analytical columns from Agilent (Mixed B).

Figure 1. SEC-IR chromatograms of the unmodified and exchanged iPP (a) and sPP (b).

determine if reduced exchange is observed at high molecular weight as seen with polyethylene. Surprisingly, for iPP, changes in tacticity and, resultantly, the melting behavior accompanied isotope exchange; this partial racemization occurs without any significant chain scission or other alteration of the polymer backbone structure (see refractive index based SEC chromatograms in Figure S-1). After observing this change to the polymer microstructure, additional reactions were performed with hydrogen rather than deuterium to assess whether the observed partial racemization can be accomplished without isotope exchange. Reactions of both polymers with H2 in pentane confirm that partial racemization proceeds in the absence of deuterium. Heterogeneous catalytic epimerization of molten iPP to atactic polypropylene has been previously reported, though this solvent-free reaction at 270 °C under H2 for multiple weeks resulted in significant loss of molecular weight.19 DSC thermograms are presented in Figure 2. The sPP, as received, exhibits two melting peaks; this behavior is common

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RESULTS A combination of differential scanning calorimetry (DSC), nuclear magnetic resonance (NMR) spectroscopy, and size exclusion chromatography with infrared detection (SEC-IR) was used to assess the results of the exchange and partial racemization reactions. Table 1 summarizes the reactions performed on the polymers. As previously reported, deuterium exchange reactions performed on iPP in isooctane have been unsuccessful.13,16 However, such reactions performed in pentane resulted in an extent of exchange of 39% (determined via 2H NMR). Deuterium exchange was successfully performed on sPP in isooctane, yielding 26% exchange. The distribution of deuterium exchange with respect to molecular weight was assessed using SEC-IR and is shown in Figure 1. In contrast with polyethylene,14−16 exchange on polypropylene does not vary significantly across the measured molecular weight distributions considered here; further work is required to

Figure 2. DSC heating thermograms of the unmodified and exchanged/racemized sPP (a) and iPP (b); curves shifted vertically for clarity. A/A0 is the integrated area of the peaks normalized by the area of the corresponding unmodified polymer.

for syndiotactic polypropylenes and is caused by recrystallization on heating.20−22 Following all reactions, a decrease of the melting points was observed. Reactions performed in pentane result in the most significant decrease and broadening of the melting peak, consistent with reduction in the stereoregularity of the polymers. A small melting peak at 123 °C remains in B

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Macromolecules iPP-D-n5, suggesting that a fraction of the polymer did not experience significant change in tacticity. Because this was not observed for the reaction in H2, we interpret this as due to heterogeneity in the reactor due to phase separation (see discussion below). Additionally, there is a greater reduction in the melting point of iPP-D-n5 than iPP-H-n5; this is most likely due to the combined effects of deuterium exchange and tacticity change, as discussed subsequently. For sPP, exchange in isooctane reduces the melting point and relative height of the two melting peaks, but little broadening is observed. While both tacticity changes and deuterium substitution can cause changes in thermal behavior, this suggests that, relative to reaction in pentane, stereoregularity may be largely preserved in sPP-D-i8. The distribution of exchange between methyl, methylene, and methine protons was measured via 1H NMR, shown in Figure 3. For both polypropylenes, exchange heavily favors

Figure 4. 13C NMR spectra of the methyl carbon peak region for unmodified and partially racemized iPP (a) and sPP (b). Significant pentad peaks are labeled. Inset tables summarize triad fractions.

spectra of sPP and sPP-D-i8 are presented in Figure 5. Only rrrr peak assignments are shown in Figure 5; full peak

Figure 3. 1H NMR spectra of unmodified and exchanged sPP (a) and iPP (b). Dashed curves plot the peak integrals.

methyl protons. For sPP-D-i8, the relative amounts of exchange can be easily quantified; 40% of methyl, 12% of methylene, and 12% of methine protons are exchanged. For iPP, such quantitation is more difficult; the methylene protons in iPP are chemically distinct, and the 1H peak corresponding to the proton that is syn to the methyl group overlaps with the methyl proton peak. Nevertheless, the 1H spectra suggest that exchange on methyl protons is favored, consistent with the findings of Zeng et al. for exchange of highly branched model ethylene-co-butene polymers. Detailed information about the stereoregularity of the polypropylenes is available from 13C NMR. For polypropylenes that have been partially racemized with hydrogen, interpretation of the NMR spectra is straightforward, since the peak positions corresponding to different 5-carbon stereosequences are well-known.23 Figure 4 compares the 13C NMR spectra for unmodified and partially racemized iPP and sPP. Interpretation of the 13C NMR spectra of deuterium-labeled polymers is complicated by the peak shifting and splitting caused by deuterium substitution. The methyl-region 13C NMR

Figure 5. 13C NMR spectra of the methyl carbon peak region for unmodified and exchanged sPP. Inset compares the exchanged rrrr methyl peak area distribution to a statistically random distribution.

assignments and peak fitting/deconvolution are presented in the Supporting Information. The identified chemical shift, per deuteron, is 0.31 ppm upfield, and the coupling constant is 18.8 Hz. Both of these values are consistent with earlier findings for deuterium substitution on the methyl groups of polypropylene.24 In spite of the significant number of peaks seen in Figure 5, peak fitting analysis reveals that the tacticity of sPP-D-i8 is within measurement error, approximately the same as the neat sPP (see values in Table 1). The inset of Figure 5 compares the distribution of CH3, CH2D, CHD2, and CD3 measured in sPP-D-i8 to that expected from statistically random exchange with equivalent extent. C

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polymer backbone resides in a second plane. Either of these planes allows for the chain to sequentially adsorb to the planar catalyst surface, though evidently methyl adsorption is preferred. We hypothesize that in isooctane transition between methyl-plane and backbone-plane configurations requires complete desorption and resorption of the local chain segment. Similar behavior is observed for exchange of small-molecule cyclic alkanes in some cases, where for example the primary initial product of exchange on cyclopentane is C5D5H5.35 These adsorption configurations are sterically unfavorable for iPP, where neighboring methyl groups must be linearly arranged in order to reside in the same plane; these conformations are illustrated and compared in Figure 6. However, in pentane (a

Given that the exchange reaction is heterogeneous, it may be expected that once a CH3 group adsorbs to the catalyst, exchange of multiple protons for deuterons might occur rapidly, while other sites that adsorb subsequently (or not at all) would undergo little to no exchange due to the consumption of D2. Instead, the observed distribution is quite close to random, suggesting that in addition to the initial exchange from D2 subsequent exchange likely occurs among and between chains.

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DISCUSSION The preservation of tacticity in sPP-D-i8 indicates that the reduction in crystallinity and melting temperature observed via DSC is due solely to the effect of deuterium substitution. The effect of deuterium substitution on the crystallization and melting behavior of polyolefins has been widely studied, both theoretically and, in the case of perdeuteropolyethylene and isotactic polypropylene, experimentally.25−29 These prior studies report a difference in melting point between ordinary and perdeutero isotactic polypropylene of 3−5 °C for thermal treatment comparable to that used here.26−29 To our knowledge, there are no prior reports on the melting behavior of syndiotactic polypropylenes with any amount of deuterium substitution. Based on the higher temperature sPP melting peak in Figure 2, deuterium exchange results in a 12.5 °C reduction in melting point. This suggests that partial deuterium substitution is more disruptive to the crystal structure than complete substitution, though the crystallization behavior of polypropylenes is complex, and a more detailed thermal and crystallographic study, beyond the scope of the present report, is required to understand why partial labeling has such a large effect on the melting point. These results facilitate the preparation of matched pairs of deuterium-labeled and stereochemically modified polypropylenes, which are useful in polymer physical chemistry studies. Of equal interest and importance is the insight that these reactions provide into the mechanism of polymer adsorption and interaction. Reactions performed on iPP in isooctane and n-decane yield an insignificant amount of exchange and no alteration of the polymer structure,13 yet with all other conditions controlled, using n-pentane as the reaction solvent yields exchange and significant racemization. At these reaction conditions, iPP is fully miscible in isooctane and n-decane, but the LCST for iPP in n-pentane is 149 °C, indicating that polymer-rich and solvent-rich liquid phases are present during the reaction at 170 °C.30,31 In a recent study, Zeng et al. observed that very high molecular weight polyethylenes fail to undergo exchange in isooctane and suggested that operating above the LCST was responsible for the lack of exchange;16 our results suggest that a different mechanism is responsible for this observation. While further research is required to understand how and why the choice of solvent plays such a critical role in these reactions, our initial hypothesis is that two types of chemisorption are available to the polymer chains: σ-bonding, which preserves stereochemistry, and π-bonding, which, through the formation of planar adsorption intermediates centered on the methine carbons, enables racemization.32−34 Polymer adsorption represents a balance between the enthalpic interaction with the catalyst and the solvating force of the solvent. For sPP in isooctane, where the driving force for adsorption is relatively weak, the chain has access to a conformation in which sequences of neighboring methyl groups reside in a plane; simultaneously, the corresponding

Figure 6. Illustration of hypothesized planar adsorption conformation of sPP (green) and sterically prohibited planar conformation of iPP (red), with (a) methyl plane near (space-filling) and (b) backbone plane near (ball-and-stick). For clarity, non-methyl protons are not shown.

worse solvent than isooctane), the thermodynamics more strongly favor adsorption, enabling new adsorption configurations that allow for simultaneous adsorption of backbone and methyl carbons in both sPP and iPP. Kinetic studies, investigating exchange and racemization as a function of time, temperature, and solvent quality (which can be tuned continuously via blending alkane solvents), are required to further probe the mechanism, potentially enabling racemization-free exchange of iPP at lower temperatures or intermediate solvent quality, which is desirable for many studies requiring a labeled, but otherwise unaltered, isotactic polypropylene. Across several studies of deuterium exchange on polyolefins in isooctane, linear polyethylene undergoes the greatest amount of exchange, followed by short-chain branched polyethylenes, then sPP, and finally iPP fails to exchange.13,15,16 This sequence of reaction susceptibility mirrors the behavior of these polymers in interaction chromatography, a group of polymer separation techniques based on adsorption to graphite that is widely used in the plastics industry.36,37 For example, in solvent gradient interaction chromatography (SGIC), polymers are exposed to the graphite surface while dissolved in a poor solvent. The solvent quality is then gradually raised (via the introduction of a second, better solvent) at constant temperature, and elution of polymer is monitored during this gradient. In this process, iPP D

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(6) Fischer, E. W. Neutron-Scattering Studies on the Crystallization of Polymers. Polym. J. 1985, 17 (1), 307−320. (7) Bates, F. S.; Wignall, G. D. Isotope-induced quantum-phase transitions in the liquid state. Phys. Rev. Lett. 1986, 57 (12), 1429−32. (8) Balsara, N. P.; Fetters, L. J.; Hadjichristidis, N.; Lohse, D. J.; Han, C. C.; Graessley, W. W.; Krishnamoorti, R. Thermodynamic interactions in model polyolefin blends obtained by small-angle neutron-scattering. Macromolecules 1992, 25 (23), 6137−6147. (9) Gehlsen, M. D.; Rosedale, J. H.; Bates, F. S.; Wignall, G. D.; Hansen, L.; Almdal, K. Molecular weight scaling in critical polymer mixtures. Phys. Rev. Lett. 1992, 68 (16), 2452−5. (10) Balsara, N. P.; Lohse, D. J.; Graessley, W. W.; Krishnamoorti, R. Small-angle neutron-scattering by partially deuterated polymers and their blends. J. Chem. Phys. 1994, 100 (5), 3905−3910. (11) Higgins, J. S.; Benoît, H. Polymers and Neutron Scattering; Clarendon Press: Oxford, 1994. (12) Choi, S. H.; Bates, F. S.; Lodge, T. P. Molecular Exchange in Ordered Diblock Copolymer Micelles. Macromolecules 2011, 44 (9), 3594−3604. (13) Habersberger, B. M.; Lodge, T. P.; Bates, F. S. Solvent Selective Hydrogen-Deuterium Exchange on Saturated Polyolefins. Macromolecules 2012, 45 (19), 7778−7782. (14) Habersberger, B. M.; Hart, K. E.; Gillespie, D.; Huang, T. Molecular Weight Dependence of Deuterium Exchange on Polyethylene: Direct Measurement and SANS Model. Macromolecules 2015, 48 (16), 5951−5958. (15) Kang, S.; Zeng, Y.; Lodge, T. P.; Bates, F. S.; Brant, P.; LópezBarrón, C. R. Impact of molecular weight and comonomer content on catalytic hydrogen-deuterium exchange in polyolefins. Polymer 2016, 102, 99−105. (16) Zeng, Y.; López-Barrón, C. R.; Kang, S.; Eberle, A. P. R.; Lodge, T. P.; Bates, F. S. Effect of Branching and Molecular Weight on Heterogeneous Catalytic Deuterium Exchange in Polyolefins. Macromolecules 2017, 50 (17), 6849−6860. (17) Townsend Polypropylene Report, 2012. (18) Hucul, D. A.; Hahn, S. F. Catalytic hydrogenation of polystyrene. Adv. Mater. 2000, 12 (23), 1855−1858. (19) Suter, U. W.; Neuenschwander, P. Epimerization of vinyl polymers to stereochemical equilibrium. 2. Macromolecules 1981, 14 (3), 528−532. (20) Paukkeri, R.; Lehtinen, A. Thermal behaviour of polypropylene fractions: 2. The multiple melting peaks. Polymer 1993, 34 (19), 4083−4088. (21) Rodriguez-Arnold, J.; Zhang, A.; Cheng, S. Z.; Lovinger, A. J.; Hsieh, E. T.; Chu, P.; Johnson, T. W.; Honnell, K. G.; Geerts, R. G.; Palackal, S. J.; Hawley, G. R.; Welch, M. B. Crystallization, melting and morphology of syndiotactic polypropylene fractions: 1. Thermodynamic properties, overall crystallization and melting. Polymer 1994, 35 (9), 1884−1895. (22) Supaphol, P. Crystallization and melting behavior in syndiotactic polypropylene: origin of multiple melting phenomenon. J. Appl. Polym. Sci. 2001, 82 (5), 1083−1097. (23) Hansen, E. W.; Redford, K. Nuclear magnetic resonance spectroscopy of polypropylene homopolymers. In Polypropylene; Springer: 1999; pp 540−544. (24) Busico, V.; Caporaso, L.; Cipullo, R.; Landriani, L.; Angelini, G.; Margonelli, A.; Segre, A. Propene Polymerization Promoted by C 2Symmetric Metallocene Catalysts: From Atactic to Isotactic Polypropene in Consequence of an Isotope Effect. J. Am. Chem. Soc. 1996, 118 (8), 2105−2106. (25) Bates, F.; Keith, H.; McWhan, D. Isotope effect on the melting temperature of nonpolar polymers. Macromolecules 1987, 20 (12), 3065−3070. (26) Mezghani, K.; Phillips, P. J. Equilibrium melting point of deuterated polypropylene. Macromolecules 1994, 27 (21), 6145−6146. (27) Reddy, K. R.; Tashiro, K.; Sakurai, T.; Yamaguchi, N. Cocrystallization phenomenon between the H and D Species of isotactic polypropylene blends as revealed by thermal and infrared

never adsorbs, even at the lowest solvent quality, so it elutes immediately. Then, sequentially, sPP, branched polyethylenes, and finally linear polyethylenes elute from the graphite as solvent quality improves, indicating their relative strength of adsorption. This striking similarity points to the importance of stereochemistry in the physics of adsorption of both systems, in spite of the difference in adsorption mechanisms (chemisorption vs physisorption).

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CONCLUSIONS While previous efforts to exchange iPP in isooctane solvent were unsuccessful, deuterium exchange can be accomplished for sPP in these conditions. iPP can be exchanged in pentane (in spite of the fact that the reactor conditions are above the LCST for this system), but this exchange is accompanied by partial racemization, which is also observed for sPP in these conditions. We hypothesize that the differing ability of iPP and sPP to form a planar adsorption conformation is responsible for the observed difference in reaction and that these phenomena may be generalizable to other polymer adsorption systems. In addition to the utility provided by facile preparation of labeled polymers, these findings indicate that polymer stereochemistry and reaction solvent play a surprising and significant role in the adsorption and exchange process.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02633. 13 C NMR peak fitting and deconvolution; size exclusion chromatography results (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (B.M.H.). ORCID

Brian M. Habersberger: 0000-0002-9455-1342 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Philip Tyler and David Gillespie for assistance with SEC data and Kyle Hart for composing Figure 6. REFERENCES

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