Solvent Selective Hydrogen–Deuterium Exchange on Saturated

Article pubs.acs.org/Macromolecules

Solvent Selective Hydrogen−Deuterium Exchange on Saturated Polyolefins Brian M. Habersberger,† Timothy P. Lodge,†,‡ and Frank S. Bates*,† †

Department of Chemical Engineering and Materials Science and ‡Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, United States S Supporting Information *

ABSTRACT: A heterogeneous Pt−Re/SiO2 catalyst was used to perform H−D exchange reactions on the linear saturated polyolefins poly(ethylene), poly(ethylene-alt-propylene), and isotactic poly(propylene) in the saturated hydrocarbon solvents decalin, decane, heptane, and isooctane. Exchange reactions using deuterium gas were selective toward the polymer in the case of linear and branched alkane solvents for poly(ethylene) and poly(ethylenealt-propylene), whereas reactions conducted in decalin result in virtually no isotope exchange with the polymer. Essentially, no exchange occurred with poly(propylene) in either linear (decane) or branched (isooctane) solvents. In all cases, polymer backbone bonds were unperturbed, preserving the molecular weight distribution. These results suggest that competitive adsorption between the polymer and solvent plays a significant role in heterogeneous catalysis of polymers. Furthermore, this strategy provides a simple and efficient method for postsynthesis labeling selected polyolefins with deuterium, for example, for use in small-angle neutron scattering.

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INTRODUCTION Small-angle neutron scattering (SANS) is a powerful technique1 that can be used to probe single chain conformational statistics,2 blend phase behavior,3−5 block copolymer and surfactant self-assembly,6,7 micellization,8−10 and micelle equilibration.11,12 By virtue of selective deuterium labeling, single chains or even parts of chains can be examined even in multicomponent phases. The conventional way to accomplish selective labeling is to synthesize the polymer of interest using isotopically labeled monomers. This versatile approach has the drawbacks of being expensive and of producing labeled and unlabeled polymers with slightly different molecular weight averages and distributions; exactly matched protonated and deuterated polymer pairs are preferred for many experiments. Another option is available if the polymer of interest can be prepared from an unsaturated precursor, such as poly(ethylenealt-propylene) from poly(isoprene) or poly(cyclohexylethylene) from poly(styrene); saturation with deuterium leads to a partially labeled product, which, when blended with the matched hydrogenous version, permits interrogation of single chain configurations in the melt, glassy or crystalline states by SANS. A third option, direct catalytic H−D exchange, is in principle optimal in many cases, especially for polyolefins. Given that polyolefins such as poly(ethylene) and poly(propylene) account for more than 90% (by volume) of the synthetic polymer market, and that these materials exist in many different structural variants due to advances in catalyst technology, it is particularly important to enable SANS analysis of such products after polymerization. An ideal process would exchange a controlled fraction of protons with deuterons randomly on each © 2012 American Chemical Society

chain without altering the molecular weight average or distribution. While there has recently been renewed interest in H−D exchange on small molecules and some alkane oligomers,13−15 catalytic H−D exchange on polymers has received relatively little attention in the literature.16−18 In pioneering work, Nicholson and Crist demonstrated exchange of up to 60% on poly(ethylene) using a rhenium catalyst.19 However, their method has drawbacks: deuterium was exchanged from excess cyclohexane-d6 (which is very expensive) and the catalyst was reported to be sensitive to polar impurities, requiring thorough drying of all reaction components. Here, we demonstrate a related method for producing partially deuterated polyolefins, a technique that is both facile and free of the above drawbacks. Furthermore, our results demonstrate that the extent of H−D exchange is strongly influenced by the molecular structures of the saturated hydrocarbon polymer and solvent.

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EXPERIMENTAL METHODS

Materials. Table 1 summarizes the characterization of the polymers used in exchange reations. High-density poly(ethylene) (HDPE) and isotactic poly(propylene) (iPP) were used in reactions as received, while poly(ethylene-alt-propylene) (PEP) was prepared by hydrogenating a poly(isoprene) precursor (94% 1−4 and 6% 3−4 addition) using the heterogeneous Pt−Re/SiO2 catalyst. PEP was recovered by precipitating into methanol then drying under vacuum. Exchange Reactions. In each experiment, 5.0 g of HDPE, iPP, or PEP was combined with 2.0 g of a Pt−Re/SiO2 catalyst (provided by Received: August 30, 2012 Revised: September 10, 2012 Published: September 24, 2012 7778

dx.doi.org/10.1021/ma301814n | Macromolecules 2012, 45, 7778−7782

Macromolecules

Article

Table 1. Characteristics of Polymers polymer HDPE iPP PEP

origin Dow Aldrich anionic synthesis

Table 2. Summary of Deuterium Exchange Experiments

Mn (kg/mol) a

21 97b 98c

Mw (kg/mol) a

52 340b 110c

a

Measured via SANS. bAs described by the vendor. cMeasured via SEC of the polyisoprene precursor using universal calibration against polystyrene standards.

the Dow Chemical Co.) in 450 mL of solvent (decalin, decane, isooctane, or heptane from Sigma-Aldrich) in a sealed reactor and pressurized with 600 psi of D2 at 170 °C for 16 h. Following the reactions, various procedures were used to recover the polymers. Semicrystalline polymers in decane or decalin were heated to a temperature at which they dissolved (>130 °C), filtered to remove the catalyst, then allowed to cool and precipitate, after which they were filtered again to remove solvent. Semicrystalline polymers in heptane or isooctane were first filtered at room temperature to remove most of the solvent, then the polymer/catalyst mixture was redissolved in xylenes and the above procedure was repeated. PEP in heptane was filtered at room temperature then precipitated into methanol. Each polymer was dried thoroughly under vacuum before further characterization was performed. In the case of semicrystalline polymers, approximately 80% of the reaction solvent, containing 2.1−3.5% deuterium, was recovered. NMR. Nuclear magnetic resonance (NMR) spectroscopy was used to assess the deuterium content of PEP and the various solvents. 1H and 2H experiments were performed on a Bruker Avance III spectrometer at 500 and 76 MHz, respectively. Samples for 1H NMR were prepared in benzene-d6 with pyridine as an internal standard, while those for 2H NMR were prepared in heptane with benzene-d6 as an internal standard. Density Measurements. The deuterium content of semicrystalline polymers was assessed using density measurements made with a density gradient column. The column was prepared with a mixture of ethylene glycol and 2-propanol and calibrated using glass beads of known density. Measurements were made a minimum of two days after the insertion of at least four small pieces of the sample of interest into the column. For all samples, the pieces all came to rest within 1 mm of each other, consistent with a margin of error of less than 1%. SANS. For SANS experiments, blends of unmodified HDPE and HDPE reacted in isooctane were prepared by dissolving an appropriate amount in boiling xylenes, then precipitating the solution into cold methanol. After drying under vacuum, blends were pressed into discs and sealed between two quartz windows with a 1.5 mm spacer. SANS experiments were performed at 150 °C using the High Flux Isotope Reactor at Oak Ridge National Laboratory. Measured scattering patterns were azimuthally isotropic and were averaged to produce onedimensional plots of intensity vs the scattering wave vector q = 4πλ−1 sin(θ/2). Scattering data were corrected for background and empty cell scattering, sample transmission, sample thickness, and detector sensitivity. FTIR. Fourier transform infrared spectroscopy was performed on HDPE samples pressed to a uniform thickness of 50 μm. Transmission measurements were made directly on these films.

polymer

solvent

ypolymer

ysolvent

f polymera

fsolventa

HDPE HDPE HDPE HDPE iPP iPP PEP

decalin decane heptane isooctane decane isooctane heptane

4 at 525 cm−1. Unlabeled peaks represent intermediate chain sequences to those described. The split peak at 725 cm−1 for the unmodified HDPE is due to in-phase and out-of-phase rocking motions which are polarized along different crystallographic axes; this effect vanishes when the crystal structure is disrupted by solvation, melting, or introduction of irregularities.38,39 The inset of Figure 4 compares the ratio of the integrated area of the (CH2)n>4 peak for each exchanged polymer (A) to the area of the unmodified HDPE (AH). This area is roughly proportional to the concentration of sequences of five CH2 groups. A statistical model was used to generate the curve, which predicts the concentration of (CH2)5 as a function of y if exchange happens in an entirely random fashion. The measured data points fall significantly above the curve, suggesting that at a very local scale, the exchange may be nonuniform. However, as shown by the SANS experiments, this local “blockiness” does not extend to length scales probed by small angle scattering.

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CONCLUSIONS These results identify a simple and efficient method for preparing isotopically labeling polyolefins, with the exception of isotactic poly(propylene). This new tool for characterizing the conformational properties and phase behavior is particularly applicable to commercially relevant polyolefins (e.g., metallocene based), where the use of deuterated monomers is not feasible; specimens produced in industrial scale polymerization facilities can now be evaluated directly. The sensitivity of the exchange reaction to both solvent and polymer structure offers insight into the mechanism of competitive adsorption and exchange. At length scales probed by SANS, the polymer chains are uniformly labeled, although FTIR reveals some degree of local blockiness.

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ASSOCIATED CONTENT

S Supporting Information *

1

H NMR spectra and cumulative integral of spectra of PEP and dPEP. This material is available free of charge via the Internet at http://pubs.acs.org. 7781

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(30) Suter, U. W.; Neuenschwander, P. Macromolecules 1981, 14 (3), 528−532. (31) deGennes, P.-G. Scaling Concepts in Polymer Physics; Cornell University Press: Ithaca, NY, 1979. (32) Fetters, L.; Lohse, D.; Richter, D.; Witten, T.; Zirkel, A. Macromolecules 1994, 27, 4639. (33) Bates, F. S.; Wignall, G. D.; Koehler, W. C. Phys. Rev. Lett. 1985, 55, 2425. (34) Bates, F. S.; Wignall, G. D. Phys. Rev. Lett. 1986, 57, 1429. (35) Balsara, N. P.; Lohse, D. J.; Graessley, W. W.; Krishnamoorti, R. J. Chem. Phys. 1994, 100, 3905. (36) Dubner, W. S.; Schultz, J. M.; Wignall, G. D. J. Appl. Crystallogr. 1990, 23, 469. (37) Tasumi, M.; Shimanouchi, T.; Kenjo, H.; Ikeda, S. J. Polym. Sci., Part A 1966, 4, 1011. (38) Krimm, S. J. Chem. Phys. 1954, 22, 567. (39) Krimm, S.; Liang, C. Y.; Sutherland, G. B. B. M. J. Chem. Phys. 1956, 25, 549.

AUTHOR INFORMATION

Corresponding Author

*E-mail: (F.S.B.) [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported with funding from the Abu DhabiMinnesota Institute for Research Excellence, a partnership between the Petroleum Institute of Abu Dhabi and the Department of Chemical Engineering and Materials Science of the University of Minnesota, and partially by the MRSEC Program of the National Science Foundation under Award Number DMR-0819885. Neutron scattering experiments were performed using the High Flux Isotope Reactor at Oak Ridge National Laboratory.

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