Article pubs.acs.org/Macromolecules
Effect of Branching and Molecular Weight on Heterogeneous Catalytic Deuterium Exchange in Polyolefins Yiming Zeng,† Carlos R. López-Barrón,§ Shuhui Kang,§ Aaron P. R. Eberle,∥ 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 § ExxonMobil Chemical Company, Baytown, Texas 77520, United States ∥ ExxonMobil Research and Engineering Company, Annandale, New Jersey 08801, United States S Supporting Information *
ABSTRACT: Small-angle neutron scattering (SANS) is a powerful method for probing the structural properties of polymeric materials. Contrast between polymer chains can be obtained by labeling with deuterium, which provides an opportunity for analyzing individual chain behavior in bulk. A transition metal (Pt/Re)-catalyzed reaction in isooctane was used to exchange deuterium for hydrogen in various saturated hydrocarbon polymers, including a commercial polyethylene. We have investigated the role of two forms of molecular heterogeneity on the labeling reaction using narrow dispersity hydrogenated polybutadiene (hPBD) samples with controlled molecular weight and ethyl branch content (short chain branching). These materials were prepared by anionic polymerization, followed by catalytic hydrogenation. A monotonic increase of deuterium labeling from 65% to 84% was observed when molecular weight was increased from 4000 to 216 000. Increasing the molecular weight to 635 000, however, resulted in almost no exchange, which is possibly due to the existence of a lower critical solution temperature (LCST) in isooctane. A similar trend with molecular weight was found for an isotope-labeled commercial linear low-density polyethylene material with 2.5% butyl branches and molecular weight ranging between 1000 and 1 000 000. Variation of ethyl branches from 2 to 50 ethyl branches per 100 backbone carbons in hPBDs reduced the level of exchange from 78% to 34%, with deuterons preferentially entering the pendant methyl groups at higher levels of branching. The materials generated from this isotope exchange reaction proved to be viable materials for SANS, providing consistent single chain statistics through proper analysis strategies, which take into account the inhomogeneous distribution of deuterium along and among individual chains caused by partial labeling and the molecular weight dependence of exchange. These results suggest that for a given chain, isotope exchange occurs on the metal catalyst surface during relatively few adsorption steps.
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INTRODUCTION
exchanging deuterium contained in a solvent for hydrogen in PE in the presence of a heterogeneous transition metal catalyst.12,13 Subsequently, Habersberger and co-workers discovered that commercial PE, dissolved in certain saturated hydrocarbon solutions, could be labeled with substantial amounts of deuterium (up to 68% substitution for hydrogen) without any evidence of chain degradation by exposure to deuterium gas at elevated pressure and temperature over a heterogeneous Pt/Re catalyst supported on a wide-pore silicate substrate.14 Relaxing the need to employ deuterated monomers opens significant opportunities to use in situ small-angle neutron scattering (SANS) to unravel the molecular origins of various nonlinear mechanical properties associated with different grades of commercial PE,15 including the effects of the
The ability to explore the configuration of polymer chains in the bulk state using neutron scattering, first reported in 1972,1,2 represents one of the most important characterization tools in the field of polymer science and engineering.3 For homopolymers such as polyethylene (PE) this technique requires the use of deuterium labeling in order to provide scattering contrast in blends of normal (hydrogenous) and isotope-labeled, but otherwise chemically identical, chains. Mixing labeled and unlabeled polymers exposes segment−segment correlations, which define the local and overall chain conformations that characterize polymer coils through the coherent scattering of neutrons. Until recently, such isotope labeling was primarily accomplished by polymerizing deuterated monomers4−7 or saturating polydienes with deuterium (D2);8−11 these are restrictive and/or expensive methods that cannot be applied to most commercially produced plastics and elastomers. Crist and co-workers demonstrated the feasibility of swapping hydrogen isotopes on polyolefins in the 1980s, including © XXXX American Chemical Society
Received: June 14, 2017 Revised: July 29, 2017
A
DOI: 10.1021/acs.macromol.7b01268 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
calibrated against polystyrene standards with Mark−Houwink parameters α = 0.670 and K = 1.75 × 10−4 dL/g using universal calibration. The Mark−Houwink parameters for homopolymer polyethylene were obtained from literature values (α = 0.695 and K = 5.79 × 10−4 dL/g) while for LLDPE, the ethylene−hexene copolymers, are calculated from empirical equations according to comonomer or hexene content.20 From this characterization, Mw ≈ 113 kDa and Đ ≈ 2.46; this polymer contains 2.1% C4 branches according to the supplier, with a rather homogeneous branch distribution.25 Anionic Polymerization. Model polybutadienes (PBDs) were synthesized from 1,3-butadiene using anionic polymerization with nbutyllithium as the initiator following procedures described elsewhere.21 Cyclohexane was used as the reaction medium, except for one sample with 41 branches per 100 backbone carbons, where tetrahydrofuran was employed as the solvent. For each reaction, a 1 L Schlenk flask with at least three material adding ports was used as the reactor. The flask was evacuated and purged with argon three times prior to addition of 500 mL of purified solvent. The monomer was distilled twice from n-butyllithium cooled by a salt/ice bath and was kept in a glass buret cooled with salt/ice bath before addition into the reactor. Initiator was injected into the solvent by a gastight syringe. The mixture was stirred for 20 min at the reaction temperature, controlled by either a water bath (20−40 °C) or a dry ice/isopropanol bath (−77 °C). Purified monomer was then added to the reactor and allowed to react for 12 h, followed by termination by addition of degassed methanol. Branches were introduced into the polymers by 1,2-addition of butadiene monomers. The extent of branching was tuned by injecting polar modifier prior to addition of monomer as well as by varying reaction temperature. The modifiers used were tetrahydrofuran (THF) or bispiperidinoethane (DIPIP), providing different 1,2-addition contents at different [modifier]:[initiator] ratios.22 Molecular weight was adjusted by varying the ratio of monomer to initiator. The reaction conditions are summarized in Table 1.
distribution of molecular weight and types and concentration of short branches. In hindsight, the deuterium exchange mechanism reported by Habersbereger et al. should have been apparent years ago. Small hydrocarbon molecules were shown to exchange isotopes in the 1950s.16,17 Unsaturated polydienes and polystyrene have been saturated using deuterium to obtain neutron contrast since the 1980s, and degrees of isotope substitution in excess of stoichiometric addition were well documented. For example, catalytic deuteration of 1,4-polyisoprene over Pd/BaSO4 led to as many as six deuterons per C5H10 repeat unit, implying facile isotope exchange on the polymers while adsorbed on the metal surface.10,11 Balsara and co-workers demonstrated that pure polydienes deuterated over Pd/CaCO3 produced coherent neutron scattering, implying that the heavy isotope was not distributed uniformly across the chains, consistent with the notion that deuterium exchange occurred beyond exclusive addition to the double bonds.9 Nevertheless, the mechanisms responsible for such isotope exchange on fully saturated hydrocarbon polymers have not been delineated. Certain polymers such as isotactic polypropylene seem impervious to deuterium labeling, and the extent of deuterium substitution on PE depends on the specific solvent employed.14 This report examines the effects of molecular weight and short chain branching content on the extent of deuterium substitution for a series of model saturated polydienes and a commercial linear low-density PE (LLDPE) material. The isotope exchange reactions were performed using a Pt/Re/SiO2 catalyst in isooctane at 170 °C and 500 psi D2 pressure. Increasing molecular weight at low ethyl branch content in lowdispersity hydrogenated polybutadiene from Mw = 4 to 210 kDa leads to a gradual increase in deuterium content from 65% to 84% substitution. However, a 635 kDa sample failed to swap any deuterium for hydrogen, which we speculatively interpret on the basis of lack of solubility due to lower critical solution temperature behavior. Increasing the ethyl branch content beyond about 30% of the backbone carbon atoms reduces the extent of isotope exchange, which occurs preferentially at the pendant methyl groups as revealed by 1H NMR spectroscopy. Application of FTIR detection in conjunction with size exclusion chromatography revealed a similar molecular weight dependence of deuterium exchange in broad dispersity LLDPE. SANS experiments performed on selected pure deuterated polymers and blends with the corresponding hydrogenous compounds demonstrate that the catalytic exchange reaction provides viable materials for the investigation of polymer chain conformations by SANS, notwithstanding inhomogeneous labeling.
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Table 1. Polybutadiene (PBD) Polymerization Conditions and Products polymer PBD22-3 PBD25-9 PBD23-10 PBD22-12 PBD25-15 PBD27-19 PBD26-24 PBD34-33 PBD22-41a PBD31-50 PBD4-3 PBD7-3 PBD100-3 PBD210-3 PBD610-3
EXPERIMENTAL METHODS
Materials. Cyclohexane (CHX) and tetrahydrofuran (THF) were purified by purging with argon and passing through alumina and/or copper redox catalyst columns as described elsewhere.18 n-Butyllithium (Sigma-Aldrich) and bispiperidinoethane (Sigma-Aldrich) were used as received. 1,3-Butadiene (Sigma-Aldrich) was purified by distillation from n-butyllithium twice before use. Ultrawide-pore Pt/Re/SiO2 catalyst was provided by the Dow Chemical Company.19 Hydrogen gas, deuterium gas, methanol, ethylene glycol, isopropanol, and isooctane were used as received. A linear low-density polyethylene (LLDPE1) polymer was provided by ExxonMobil Chemical Company and used without modification. The molecular weight distribution was determined by size exclusion chromatography (SEC) at 145 °C with 1,2,4-trichlorobenzene (TCB) (Sigma-Aldrich) as the mobile phase. The molecular weight value was
modifier
[modifier]: [initiator]
THF THF THF THF THF THF THF
5 5 12 40 90 90 90
DIPIP
10
temp (°C)
Mwb (kDa)
Đ
40 30 20 25 40 40 30 20 −77 25 40 40 40 40 40
22 25 23 23 25 27 26 34 22 31 4 7 100 210 610
1.04 1.03 1.06 1.04 1.05 1.04 1.05 1.06 1.04 1.09 1.06 1.09 1.08 1.08 1.06
b
vinyl fractionc (%) 10 30 34 39 45 56 65 79 90 100 11 11 10 10 10
a
Tetrahydrofuran as the solvent; all others cyclohexane. bDetermined by SEC with THF as the mobile phase and universal calibration with PS standards. cDetermined with 1H NMR spectroscopy.
All polymers have vinyl (1,2-addition) fractions between 10% and 100%. The amount of branching was determined by 1H NMR spectroscopy (Varian UNITY 300, Varian INOVA 500) for the synthesized polybutadienes. The integrated peak area of protons in vinyl (1,2) groups and backbone (1,4) units between 4.8 and 5.8 ppm was used to calculate the fraction of vinyl side groups. Molecular weight averages and dispersities were verified using a SEC instrument B
DOI: 10.1021/acs.macromol.7b01268 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules operated at 30 °C with THF as the mobile phase and calibrated with polystyrene standards. Mark−Houwink parameters for branched polybutadienes were calculated by extrapolating published values according to the vinyl fraction of polymers; α = 0.670 and K ranges from 4.600 × 10−4 dL/g (10% vinyl) to 3.595 × 10−4 dL/g (100% vinyl).23 For polystyrene α = 0.717 and K = 1.251 × 10−4 dL/g. Several polybutadiene samples were also calibrated using a multiangle light scattering detector (Wyatt DAWN), with dn/dc = 0.130 mL/g, operated with SEC separation in THF.24 A comparison of these results can be found in the Supporting Information. Heterogeneous Catalytic Hydrogenation. Hydrogenated polybutadiene (hPBD) samples were prepared by addition of hydrogen to the unsaturated polymer samples in a 1 L stainless steel vessel. Polymers were dissolved in cyclohexane at a loading between 5 and 20 g per 500 mL of solvent. Pt/Re/SiO2 catalyst was used at a loading of 1 g of catalyst per 5 g of polymer. Polymer solution and catalyst were mixed in the vessel, which was then sealed and purged with argon for 15 min. The vessel was then pressurized with 500 psi H2, followed by heating to 170 °C for 17 h under magnetic stirring, during which the pressure in the reactor typically dropped by 50−150 psi. Hydrogenated polybutadiene with less than 3% branching is insoluble at room temperature in cyclohexane due to crystallinity. For these samples with Mw < 100 kDa, solvent was removed by filtration at room temperature followed by dissolution in 1,2,4-trichlorobenzene at 150 °C, filtration through a 0.22 μm filter (Millipore) at 130 °C (thus removing residual catalyst), and precipitation in methanol. Filtration of the high molecular weight hPBD and d-hPBD samples with this procedure, however, resulted in clogging of the filter membrane. Separation of catalyst from these samples was therefore performed by dissolving the polymers in TCB at 150 °C without stirring and recovering the polymers in the clear solution layer above the precipitated catalyst (removing most, but not all, of the catalyst). Hydrogenated polybutadienes with more than 3% branching were soluble at room temperature in cyclohexane and could be filtered through a 0.22 μm filter (Millipore) followed by polymer precipitation in cold methanol. Recovered polymers were dried under dynamic vacuum for at least 24 h prior to use. Integrity of chain architecture was verified by checking dispersities using SEC with 1,2,4-trichlorobenzene as the mobile phase at 135 °C (3% branching). All samples after hydrogenation retained dispersities Đ < 1.10; SEC traces are included in the Supporting Information. Saturation was confirmed by the disappearance of proton NMR signals between 4.8 and 5.8 ppm, which are associated with protons attached to unsaturated carbon atoms. These saturated polymers are referred to as hPBDxx-yy, where xx stands for weight-average molecular weight (kDa) of the polymer (calculated based on the parent PBD molecular weight) and yy is the number of branches per 100 backbone carbon atoms (calculated from the vinyl fraction in the PBD samples). Characteristics of the polymers are listed in Table 2. Isotope Exchange Reactions. Isotope exchange reactions were performed in a 1 L pressurized stainless steel vessel. For each reaction cycle, weighed amounts of polymer and Pt/Re/SiO2 catalyst were mixed with 500 mL of isooctane at loading ratios of 5 g of polymer/1 g of catalyst for LLDPE1 and 1 g of polymer/1 g of catalyst or 0.2 g of polymer/0.2 g of catalyst for hPBDs. The sealed vessel was purged with argon for 15 min then pressurized to 500 psi with deuterium gas. The reactions were conducted at 170 °C for 17 h, after which the solutions were cooled to room temperature and filtered through a 0.22 μm filter. Filtrate from samples with more than 3% branching was poured into cold methanol for precipitation. For samples with less than 3% branching, the solid polymer was dissolved in 1,2,4trichlorobenzene at 150 °C prior to filtration at 130 °C. The filtered solutions were subsequently poured into 2 L of cold methanol for precipitation. The precipitates were recovered and dried at 150 °C under vacuum for 12 h prior to any further characterization. SEC in 1,2,4-trichlorobenzene at 135 °C or in tetrahydrofuran at 30 °C was used to confirm that the molecular weight distribution was unaltered during the exchange reaction. SEC traces can be found in the Supporting Information.
Table 2. Polymers Used for H−D Exchange DL for deuterated specimens (%) polymer
Mwa (kDa)
Đb
branch fraction (%)c
FTIR
density
hPBD23-3f hPBD26-9 hPBD24-10 hPBD23-12 hPBD26-15 hPBD28-19 hPBD27-24 hPBD35-33 hPBD23-41 hPBD32-50 hPBD4-3g hPBD7-3 hPBD23-3 hPBD103-3 hPBD216-3 hPBD635-3 LLDPE1ad LLDPE1bd
23 26 24 23 26 28 27 35 23 32 4 7 23 103 216 635 113e 113e
1.06 1.03 1.05 1.05 1.04 1.05 1.06 1.06 1.04 1.09 1.08 1.09 1.06 1.09 1.08 1.09 2.46 2.46
2.6 8.8 10.1 12.1 14.5 19.4 24.1 32.9 41.0 50.0 2.9 2.8 2.6 2.6 2.7 2.6 2.1 2.1
78 65 60 59 76 62 67 69 55 34 65 66 69 72 84
