Long-Chain Branch Measurement in Substantially Linear Ethylene

Oct 5, 2017 - Long-Chain Branch Measurement in Substantially Linear Ethylene Polymers by 13C NMR with Halogenated Naphthalenes as Solvents. Zhe Zhou ,...
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Long-Chain Branch Measurement in Substantially Linear Ethylene Polymers by 13C NMR with Halogenated Naphthalenes as Solvents Zhe Zhou,* Dan Baugh, Philip P. Fontaine, Yiyong He, Zhi Shi, Sukrit Mukhopadhyay, Rongjuan Cong, Bill Winniford, and Matt Miller The Dow Chemical Company, 2301 N Brazosport Blvd., Freeport, Texas 77541, United States ABSTRACT: Substantially linear ethylene polymers (SLEPs) are important commercial products which are used in various applications such as packaging, electrical insulation, toys, pipes, footwear, roofing, automotive, fabrics, and much more. SLEPs can be produced using molecular catalysts which can lead to long chain branching (LCB). The amount of LCB has an influence on viscoelastic properties which affect film production and processing as well as mechanical and optical properties. Thus, it is important to accurately measure LCB content. 13C NMR is one of the methods that have been used to characterize LCB. It is quite challenging to measure LCB with 13C NMR in the presence of short chain branches (SCBs) longer than four carbons due to the overlap of LCB signals with SCB signals. In this paper, we describe the use of halogenated naphthalenes as suitable solvents to separate 13C signals related to LCB from SCB. The new method presented here allows for better quantification of LCB in polymer samples with a diverse array of branching types.

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Figure 1A, Cα),14,15,17−19 (ii) use signal of branch carbon (Br in Figure 1A, CBr),20−23 and (iii) use of a combination of CBr, Cα, and Cβ (β in Figure 1A).24 Measuring LCB in some SLEPs with short chain branching (SCB) has challenges due to overlapping signals from SCB. This challenge is exacerbated by the use of catalyst systems with a capacity for in situ comonomer generation, which generally leads to a variety of different branch lengths, and significantly more complex polymer microstructures.25 Baugh et al. demonstrated that selected solvents, including naphthalene/naphthalene-d8, enable separation of CBr and CBr′ signals to measure LCB content in the presence of C4 and C6 branching.26 Here we present halogenated naphthalenes as suitable solvents for measuring LCB content in SLEPs with 13C NMR. We use the combination of the signals from the branch (CBr), alpha (Cα), and beta (Cβ) carbons to measure LCB content. The low sensitivity issue related to CBr has been significantly mitigated with the high temperature 10 mm cryoprobe technology.27−30

INTRODUCTION Substantially linear ethylene polymers (SLEPs) comprise an important class of commercial products which are used in various applications such as packaging, electrical insulation, toys, pipes, footwear, roofing, automotive, fabrics, and much more. The global demand of polyethylene in 2014 was 167 billion pounds, of which 46% was high-density polyethylene (HDPE) and 31% was linear low-density polyethylene (LLDPE). Using molecular catalysts for the production of SLEPs has become increasingly important in industry.1−5 During olefin polymerization reactions with molecular catalysts, some of the polymer chains are terminated to produce a vinyl group, forming vinyl-terminated macromers.6,7 These vinylterminated macromers can then be reincorporated back into a growing polymer backbone to form long chain branching (LCB).8,9 LCB improves viscoelastic properties of SLEPs,10,11 which are important for cast and blown film, foaming, and blow molding processes. Therefore, it is important to quantify LCB content to enable production of improved resins for targeted applications. Several techniques including rheology,12 triple detector gel permeation chromatography (TDGPC),13 and 13C NMR14,15 have been used to characterize LCB in SLEPs. NMR, when performed correctly, is an intrinsically quantitative method.16 LCB as determined by 13C NMR has typically been defined as a chain longer than six carbons and has the structure shown in Figure 1, commonly referred to as Y-PE type LCB.14 To fully characterize LCB in SLEPs, a combination of rheology, TDGPC, and 13C NMR would be the best approach. Several 13C NMR methods have been used to measure the LCB content: (i) use signal of alpha carbons (α in © XXXX American Chemical Society

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EXPERIMENTAL SECTION

Polyolefins are provided by The Dow Chemical Company. Polymer no. 1 is a HDPE made with ethylene only. Polymer no. 2 is a HDPE with LCB and a small amount of ethyl branching. Polymer no. 3 is a polyethylene−1-octene copolymer (E/O) with low octene content and without LCB. Polyethylene−1-hexene (E/H) is a copolymer with low hexene content and without LCB. All solvents, 1-octene, and 1decene were obtained from Sigma-Aldrich. Received: August 7, 2017 Revised: September 14, 2017

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DOI: 10.1021/acs.macromol.7b01692 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. Molecular structures of polymers depicting long chain branching (A) and C6 short chain branching (B). P indicates polymer chain; alpha, beta, and branch carbon atoms are labeled in red.

Figure 2. 13C NMR of polymer no. 1 which is a HDPE made without feeding comonomer to the reactor. NMR was run with a 400 MHz spectrometer at 120 °C with TCE-d2 containing 0.025 M Cr(acac)3 as the solvent, the inverse gated single 90° pulse was used, and the recycle delay was 7.3 s. NMR. The samples were prepared by adding approximately 2.7 g of solvent(s) to the polymer sample (0.25−0.30 g) in a 10 mm NMR tube, which was then purged with N2 for 10 min. The samples were dissolved and homogenized by heating the tube and its contents at 140−150 °C. The NMR spectra were collected using Bruker 400 or 600 MHz spectrometers equipped with Bruker 10 mm hightemperature helium cryoprobes at 120 °C.27−30 NMR parameters are detailed in the following Results and Discussion section. GC-MS. 5.0 g of polymer no. 1 was weighed in a 20 mL glass bottle with a closed top silicone-lined cap. About 4.0 mL of methylene chloride were added to the bottle, and the resulting mixture was shaken for 24 h at room temperature. The methylene chloride extracts were transferred to a 2 mL autosampler vial and then analyzed by GCMS in full scan mode and selected ion monitoring (SIM) mode on a Thermo Scientific Trace GC Ultra connected with DSQ II mass spectrometry. GC conditions are split injection, split ratio 20:1, injection volume 2 μL; inlet temperature 280 °C, helium carrier gas at constant flow rate 1.0 mL/min; VF-5 ms capillary column (30 m length, 0.25 mm i.d., and 0.25 μm film) from Agilent Technologies (part number CP8944); GC oven temperature program: initial 40 °C (no hold), increase to 325 °C at 10.0 °C/min ramp rate, then held for 15 min; mass spectrometer transfer line temperature 220 °C; ion source temperature 200 °C; full scan range 29−600 Da; SIM ions 55 and 83 Da; detector gain 4.0 × 105.

from a branch (CBr), alpha (Cα), and beta (Cβ) carbons to measure LCB content here. LCB = 1000 × (IBr + Iα /3 + Iβ /3)/3Itotal

(1)

where IBr is the integral of CBr peak, Iα is the integral of Cα peak, Iβ is the integral of Cβ peak, and Itotal is the total integrals of carbon signals, excluding those from solvents. The contributions from SCBs to the corresponding integrals needed to be subtracted out to obtain IBr, Iα, and Iβ when the SCBs have signals in the corresponding chemical shift ranges. Polymer no. 1 is a HDPE made without feeding comonomer to the reactor. The 13C NMR spectrum shown in Figure 2 was obtained on a 400 MHz spectrometer at 120 °C with TCE-d2 containing 0.025 M Cr(acac)3 as the solvent. An inverse gated single 90° pulse was used, and the recycle delay was 7.3 s, which should provide quantitative NMR results.16,31 It is interesting to see from Figure 2 that polymer no. 1 contains C2 and C4 branches14 although no comonomers were fed to the reactor. The LCB result in polymer no. 1 is listed in Table 1. Equation 1 was used with IBr (contribution from one C4 branch carbon was subtracted), Iα (the contribution from two C4 branch alpha carbons was subtracted), and Iβ (the contribution from two C2 branch beta carbons and the contribution from two C 4 branch beta carbons were subtracted). As C2 and C4 branches were detected in polymer no. 1, this polymer was further analyzed with a solvent blend of 1chloronaphthalene/para-dichlorobenzene-d4 (Cl-NA/PDCB-

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RESULTS AND DISCUSSION LCB Measurement with 13 C NMR/Detection of Alkenes with GC-MS. As discussed in the Introduction, several NMR methods have been used to measure the LCB content with 13C NMR. We use the combination of the signals B

DOI: 10.1021/acs.macromol.7b01692 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 1. Branch Information of Polymer No. 1 Which Is a HDPE Made without Feeding Comonomers to the Reactora

Table 2. Branch Information of Polymer No. 1 Which Is a HDPE Made without Feeding Comonomers to the Reactora

polymer no. 1

branch/1000C

STD

polymer no. 1

branch/1000C

STD

C2 branch/1000C C4 branch/1000C LCB/1000C from eq 1

0.30 0.22 2.18

0.02 NA 0.02

C2 branch/1000C C4 branch/1000C C6 branch/1000C LCB/1000C from eq 1

0.30 0.19 0.41 1.73

0.03 0.04 0.03 0.20

NMR was run with 400 MHz NMR at 120 °C with TCE-d2 as solvent containing 0.025 M Cr(acac)3. STD stands for standard deviation. NA: STD is not available due to only one peak is available.

a NMR was run at 120 °C with Cl-NA as the solvent. STD stands for standard deviation.

d4, 90:10, w:w). PDCB-d4 was used for shimming and locking. Inverse gated single 90° pulse and 100 s recycle delays were used to ensure quantitative NMR results. A 100 s delay is far more than 5 times the spin−lattice relaxation time of all carbons in the polymer without relaxation agent of Cr(acac)3.31 The reason for not using Cr(acac)3 here is to have better resolution. Baugh et al. previously observed an issue when using naphthalene as a solvent.26 In general, 13C NMR experiments on polyolefins are performed at around 120 °C, but the upper part of NMR tube temperature is much cooler. As melting temperature of naphthalene is 80 °C and it has a relatively high vapor pressure, naphthalene vapor can solidify in the upper part of the NMR tube, and this process continues during the NMR experiment. This vaporization/condensation has a potential to degrade NMR resolution and result in a polymer concentration gradient during the long 13C NMR acquisitions used for measuring LCB. This led us to explore “liquid naphthalene” derivatives such as 1-chloronaphthalene (Cl-NA). The NMR spectrum of polymer no. 1 and spectra of control polymers E/H and E/O in Cl-NA from 400 MHz NMR are shown in Figure 3. It is evident from Figure 3 that polymer no. 1 also contains a C6 branch. Three additional peaks corresponding to C4 branch were also resolved with Cl-NA solvent. This information would not be extractable with conventional NMR solvents, such as TCE-d2 or a blend of o-dichlorobenzene and TCE-d2. The LCB result in polymer no. 1 with Cl-NA solvent is listed in Table 2. Again, eq 1 was used with IBr (contribution from one C6 branch carbon was subtracted), Iα (contributions from three C6 branch

carbons and two C4 branch carbons were subtracted), and Iβ (contributions from two C6 branch carbons and two C4 branch carbons were subtracted). The LCB content in Table 2 is lower than the value in Table 1 due to the contribution of C6 branch to the LCB value in Table 1. In general, NMR LCB is defined as chains longer than C6 branch. Thus, C6 branch needs to be removed from NMR LCB calculation. Further examination of the 13C NMR spectrum of polymer no. 1 in Cl-NA solvent (Figure 4), acquired on a 400 MHz NMR, revealed that there might be other short branches longer than C6 (medium chain branches, MCBs), such as C8, shown as a shoulder to the peak at 32.117 ppm. Polymer no. 1 was rerun with a 600 MHz spectrometer, and the result is shown in Figure 5 including MestReNova32 curve fitting (purple line) of the C6 branch resonance and an additional peak at 32.135 ppm. It is highly possible that the peak at 32.135 is from C8 branch, and its level is 0.28 ± 0.03/1000C. Since it appears there is more than one peak on the upfield side of the peak at 32.135 ppm in Figure 5, there might be other MCBs longer than C8 branch present in polymer no. 1 sample. To confirm the above NMR observation of MCBs, GC-MS analysis was conducted. Methylene chloride was added to polymer no. 1 pellets. Polymer no. 1 exhibited relatively low solubility in methylene chloride at room temperature, but residual oligomers readily migrated and desorbed from the pellets into the methylene chloride solvent. As shown in Figure 6, small amounts of 1-octene (C8H16) and 1-decene (C10H20) were detected and confirmed by both retention of authentic standards and mass spectra. A C9 alkane is coeluted with 1-

a

Figure 3. 13C NMR of polymer no. 1 which is a HDPE made without feeding comonomers to the reactor and E/H and E/O controls. NMR were run with 400 MHz NMR at 120 °C with Cl-NA/PDCB-d4 (9:1, w:w) as the solvent. An inverse gated single 90° pulse was used. Polymer no. 1 used 100 s pulse delay. C

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Figure 4. 13C NMR of polymer no. 1 which is a HDPE made without feeding comonomers to the reactor, E/H, and E/O. NMR were run with 400 MHz NMR at 120 °C with Cl-NA/PDCB-d4 (9:1, w:w) as the solvent. An inverse gated single 90° pulse was used. Polymer no. 1 used 100 s pulse delay.

blend of polymer no. 2 (a HDPE which has some LCB) and polymer no. 3 (an E/O with low octene content and without LCB) was used. Figure 8 shows 13C NMR of the branch, alpha, and beta regions of this polymer blend in halogenated-NA/ PDCB-d4 (9:1, w:w) with a 400 MHz spectrometer. Br and Br′ signals are separated but not baseline separated, and the separation of α and β signals is not optimal. I-NA solvent is the worst compared with other halogenated-NAs in this regard. The effects of a higher magnetic field on the separation of the peaks was also explored. 13C NMR of the branch, alpha, and beta regions of a blend of polymer no. 2 (a HDPE which has some LCB) and polymer no. 3 (an E/O with low octene content and without LCB) in halogenated-NA/PDCB-d4 (9:1, w:w) with a 600 MHz spectrometer is shown in Figure 9. The NMR peaks related to LCB (Br, α, and β in Figure 1) are well separated from signals related to branches from octene C6 branch (Br′, α′,α″, β′, and β″ in Figure 1) with 600 MHz and halogenated-NAs. LCB in typical LLDPE E/O could be measured with either the Br, or α, or β, or a combination of the Br, α, and β signals as shown in eq 1. It should be noted that it would be more challenging to use the branch, alpha, and beta signals to measure LCB of E/O with higher octene content for two reasons. First, accurate quantitation becomes more difficult as the SCB/LCB ratio increases. Second, the possibility of isolated LCB will be reduced, and the possibility of LCB near a C6 branch will increase. This C6 branch will change the Br, α, and β signal chemical shifts dramatically which affect LCB measurement. One possibility to characterize LCB in higher octene content E/O polymers is to start with the concept of long chain end (LCE), i.e., use the third carbon from the chain end at about 32.2 ppm to get LCE/1000C (C6 is well separated from LCE in Figure 5). For polymers terminated solely with an unsaturated group, the contribution of saturated polymer chain ends to LCE/1000C can be subtracted using the measured unsaturation content to obtain LCB/1000C. This concept of LCE/1000C can also be used to characterize LCB in highly branched LDPE polymers because the contributions from C6, C7, and even C8 can be removed for better LCE/LCB characterization of LDPE with the use of a 600 MHz spectrometer and halogenated

Figure 5. 13C NMR of polymer no. 1 which is a HDPE made without feeding comonomers to the reactor. NMR was run with 600 MHz NMR at 120 °C with Cl-NA/PDCB-d4 (9:1, w:w) as the solvent. An inverse gated single 90° pulse and 100 s pulse delay were used. The MestReNova32 curve fitting (purple line) is also shown in Figure 5.

octene; therefore, the peak at 3.76 min retention time is larger than the 1-decene peak (6.61 min retention time). The other unlabeled peaks shown in Figure 6 are branched alkanes that are most likely from residual Isopar E solvent used for making the polymer. Figure 7 shows a series of residual alkenes from dodecene (C12H24) to hexacosene (C26H52) which were also detected in the extract, and the most abundant alkene detected is octadecene (C18H36). Longer chain alkenes up to C44 were also detected at much lower concentrations. The catalyst used to make polymer no. 1 is a zirconium-based molecular catalyst; similar catalysts were reported to make vinyl-terminated dimers, trimers, etc., from ethylene and these in situ generated comonomers can be enchained to form SCB and MCBs.25 Therefore, the LCB value in Table 2 contains C8 and even longer MCBs. C8 and longer MCBs will not affect rheological properties of SLEPs,33 which further supports the notion that a combination of rheology, TDGPC, and 13C NMR would be the best approach to fully characterize LCB in SLEPs. Other halogenated naphthalenes were also explored for separation of signals from SCB and LCB. For this purpose, a D

DOI: 10.1021/acs.macromol.7b01692 Macromolecules XXXX, XXX, XXX−XXX

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Figure 6. GC-MS selected ion chromatograms showing residual 1-octene and 1-decene in extracts from polymer no. 1. Top: extracts in CH2Cl2. Bottom: 1-octene (131 ppm) and 1-decene (137 ppm) authentic standards. GC retention times are labeled below chemical names.

Figure 7. GC-MS total ion chromatogram of residual C12−C26 alkene oligomers in extracts from polymer no. 1. GC retention times are labeled below the chemical formula.

optimized conformation of TMS is used to compute the tumbling averaged NMR shielding tensor, using the same functional and 6-311+g** basis set.38 Consideration of the triple-ζ basis set and adding diffuse functions to the e basis is expected to improve the accuracy of predicted NMR shielding tensors. The effect of dielectric medium on the shielding tensors is also included using the PCM model. The magnitude of shielding is averaged over all the four C atoms (CTMS) to compute the reference for predicting chemical shifts of long and short chain branching points in E/O polymer with LCB. Figure 10 depicts molecular structures of model isomers of E/O polymers with NMR LCB (chain longer than six carbons). Initial geometries of the isomers are obtained following the above-mentioned optimization protocol (as done in the case of TMS). These serve as initial geometries for generating a library of conformations. For each isomer, a library of conformations is generated by a random sampling procedure, where all the dihedral angles are allowed to rotate between 0° and 180° with steps of 10°. Geometries of each of the generated

naphthalenes as shown in Figure 5. These results will be detailed in a separate paper. Computing Branch 13C Chemical Shifts of C6 and C10. It is evident from Figure 9 that the Br signal appears at lower field than the Br′ signal. We tried to explain this phenomenon with the following computation methodology. Density functional theory (DFT) based methods were used to compute 13C chemical shifts of long and short chain branching points (Br and Br′ in Figure 1). 34,35 At first, the geometry of tetramethylsilane (TMS) is optimized using long-range corrected hybrid functional with atom−atom dispersion correction (ωB97xD) and 6-31g* basis set.36 The effect of dielectric medium (benzene) is incorporated by considering polarizable continuum model (PCM).37 Benzene is chosen as a surrogate for naphthalene because of similarity in dielectric constant (dielectric constant of benzene is 2.3, whereas for naphthalene it is 2.5). Frequency calculations were performed, and the minimum ground state potential energy surface (PES) was confirmed by the lack of imaginary frequencies. The E

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Figure 8. 13C NMR a blend of polymer no. 2 (a HDPE which has some LCB) and polymer no. 3 (an E/O with low octene content and without LCB) in halogenated-NA/PDCB-d4 (9:1, w:w) with a 400 MHz spectrometer. *The peak is from ethyl branch in polymer no. 2.

Figure 9. 13C NMR a blend of polymer no. 2 (a HDPE which has some LCB) and polymer no. 3 (an E/O with low octene content and without LCB) in halogenated-NA/PDCB-d4 (9:1, w:w) with a 600 MHz spectrometer. *The peak is from ethyl branch in polymer no. 2.

Figure 10. Molecular structures of model isomers of E/O polymer with LCB; long chain (CBr) and short chain (CBr′) branch carbons are highlighted in red.

F

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conformations are optimized using molecular mechanics. Universal force field parameters are used for this approach. All the conformations whose energies are higher than the initial conformation by 10 kcal/mol are omitted. Among the remaining conformations, 200 low-energy conformations are chosen for each isomer, which are then subjected to geometry optimization using the DFT approach. These calculations were performed using Conformers module, as implemented in Materials Studio. The geometry optimization of 200 conformations of isomer1 and isomer-2 are performed using the same procedure as that for TMS. The tumbling averaged NMR shielding tensors for the branch carbon atoms (CBr and CBr′ in Figure 10) are computed at the ωB97xD/6-311+g** level. The effect of dielectric constant on the shielding coefficients is also included using the same procedure as that for TMS. The computed tumbling averaged shielding coefficients of CBr and CBr′ are subtracted from CTMS to obtain the chemical shifts of the branch points in each conformation. The Boltzmann-weighted chemical shifts are computed using the procedure similar in ref 25 (temperature is considered to be 298 K). The DFT-based geometry optimization and the calculations of NMR shielding tensors were performed using the G09 suite.39 Based on the computational methodology, the energies and chemical shifts of a variety of conformations of model polymers are computed. The maximum energy difference between these conformations is ∼0.5 kcal/mol. The computed Boltzmann average chemical shift for CBr is 44.3 ppm, and the same for CBr′ is 43.4 ppm; their standard deviations are 0.6 and 0.7 ppm. Although the deviation between absolute value of the computed chemical shift and the experimental results is more significant than those reported in the literature,35 the computed results indicate that the C atom at the long chain (modeled as C10-branch) branched point has higher chemical shift compared to that at the short chain (modeled as C6 chain) branched position. The deviation from the experimental results can be an artifact of the chosen density functional. Although the absolute computed values deviate from experimental results, the computational results indicate that the chemical shift of CBr should be higher than CBr′, which is validated by experimental observation. Furthermore, the standard deviation of computed chemical shifts indicates that the widths associated with CBr and CBr′ peaks are expected to be comparable for long chain and short chain branches, which is also validated by experimental results. In conclusion, LCB is one of the most important parameters to control SLEPs processing properties, for example, melt strength and melt viscosity. 13C NMR with a suitable solvent(s) and high magnetic field NMR as presented in this paper can provide a good determination of LCB content. The utility of the methodology described here was demonstrated by analyzing a polymer sample containing a complex mixture of LCB, MCB, and SCB. Hence, accurate quantification of LCB is possible even for samples with diverse microstructural features.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Mr. Ismael Salazar for helping to run some of the NMR experiments as well as Dr. David Redwine, Dr. Linh Le, and Dr. Kebe Beshah for very helpful discussions.

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REFERENCES

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Corresponding Author

*(Z.Z.) Tel 1-979-238-1387; Fax 1-979-238-0752; e-mail [email protected]. ORCID

Zhe Zhou: 0000-0003-0869-2112 G

DOI: 10.1021/acs.macromol.7b01692 Macromolecules XXXX, XXX, XXX−XXX

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Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian, Inc.: Wallingford, CT, 2009.

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DOI: 10.1021/acs.macromol.7b01692 Macromolecules XXXX, XXX, XXX−XXX