Precision Long-Chain Branched Polyethylene via Acyclic Diene

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Precision Long-Chain Branched Polyethylene via Acyclic Diene Metathesis Polymerization Hong Li,† Giovanni Rojas,†,‡ and Kenneth B. Wagener*,† †

The George and Josephine Butler Polymer Research Laboratory, Department of Chemistry, University of Florida, Gainesville, Florida 326011-7200, United States ‡ Departamento de Ciencias Químicas, Universidad ICESI, Cali, Colombia S Supporting Information *

ABSTRACT: A series of polyethylenes containing 21-carbon alkyl branches have been synthesized by acyclic diene metathesis (ADMET) polymerization. These 21-carbon alkyl branches are precisely placed on every 15th, 19th, 21st, 23rd, and 39th carbon along the polymer backbone. Precision of primary structures of all polymers is verified by 1H and 13C NMR spectroscopy. All polymers present well-defined melting profiles, even at a high branch incorporation (13.3% mol). The melting temperature increases as the branch frequency decreases, similar to what we observed for shortchain branched polyethylenes. These observations together with a good linear relationship derived from Flory’s theory suggest the exclusion of 21carbon side chains from polyethylene crystal units.

polyethylene crystal unit into the amorphous region, resulting in the same melting behavior that has never been observed before for all the polymers with branches longer than two carbons.11,12 When compared to commercial PE, ADMET PE shows sharper melting transitions and narrower lamella thickness distributions. Herein, we report the synthesis and characterization of polyethylenes with 21-carbon alkyl branches precisely placed on every 15th, 19th, 21st, 23rd, and 39th carbon along the polymer backbone. This paper extends the scope of the PE model study from short-chain branching (shorter than 15 carbons) to longchain branching (21 carbons), providing a basis for further research on the effects of long alkyl branches on polyethylene properties. Careful design of symmetrical long-chain branched monomers is the key to achieving precision ADMET polymers. In the past years, new methodologies for the preparation of ADMET monomers have been widely explored and developed by our research group, which now makes the synthesis of long-chain branched polyethylene possible. By taking advantage of our published methodologies for making lengthy spaced α,ωalkenyl alcohols13 and nitrile alkylation/decyanation chemistry,14,15 we are able to prepare symmetrical diene monomers with −C21H43 groups placed in the middle of the monomer 6, 8, 9, 10, and 18 methylene spacers from the terminal double bonds (Scheme 1).

High molecular weight polyethylene (PE) with narrow molecular weight distribution can be produced using singlesite metallocene-based catalysts.1,2 Although these polymers have superior thermal and mechanical properties, they are difficult to process. Therefore, long-chain branches, usually longer than 60 carbons, are introduced into the polymer backbone via copolymerizing ethylene with vinyl-terminated macromonomers to ease the processabilty while still maintaining the thermal and mechanical properties.3 As is well-known, copolymerization of ethylene with αolefins via chain-propagation chemistry leads to the structural defects in the final polymer. Inevitable chain-transfer and chainwalking processes produce alkyl branches with varying lengths randomly spaced on the polymer backbone.4 Acyclic diene metathesis (ADMET) polymerization has proven to be one of the most effective methods in producing precisely branched polyolefins.5−7 Through careful monomer design, it is possible to control the identity and frequency of alkyl groups on the polyethylene backbone via ADMET polymerization, which makes the study of the resulting primary structure feasible. Over decades, short-chain branched (from methyl to pentadecyl alkyl groups) precision ADMET polyethylenes have been systematically studied by our research group.8−10 The structure−property relationships of these model short-chain branched PEs have provided a better understanding of the morphology and crystallization behavior of low density and linear low density polyethylenes. For example, different sizes of alkyl groups have been precisely placed on every 21st and 39th carbon on the polyethylene backbone, and the results show that side-chain alkyl groups (except methyl and ethyl groups) are expelled from the © XXXX American Chemical Society

Received: September 2, 2015 Accepted: October 20, 2015

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DOI: 10.1021/acsmacrolett.5b00641 ACS Macro Lett. 2015, 4, 1225−1228

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ACS Macro Letters

at 5.4 ppm (Figure 1e) and the carbon signal at 130 ppm (Figure 1f) in the spectra of the hydrogenated polymer. The absence of unsaturation was corroborated by the disappearance of the out-of-plane C−H (969 cm−1) bend absorption in the alkene region of the infrared (IR) spectra (SI Figure 1). The precise nature arises from the symmetrical diene monomer and is carried to the final polymer, which can be verified by closely examining a portion of the 13C NMR of the final polymer from 10 to 45 ppm (Figure 2). Note that the

Scheme 1. Synthesis of Long-Chain Branched ADMET Polyethylenes

Solubility issues are inevitable in the syntheses. It is a problem we have successfully addressed. For example, although 3a−3d were soluble in THF at 0 °C, 3e (20-bromoicos-1-ene) was not. Therefore, after the addition of 3e, instead of keeping the solution at 0 °C, the reaction mixture was warmed to room temperature, and additional THF was added to maintain solubility. The same problem occurred in the decyanation of 2,2-di(icos-19-en-1-yl)tricosanenitrile (4e), and rather than conducting this reaction at 0 °C, it was carried out at room temperature to avoid precipitation. Via ADMET polycondensation chemistry, monomers 5a−5d were condensed to the corresponding unsaturated polymers under bulk conditions with high vacuum, but when polymerizing 5e, the viscosity increased dramatically after a few hours and inhibited the effective stirring and further polymerization. Therefore, 5e was polymerized in dichloromethane (DCM) under reflux. The reflux of DCM removes the ethylene, a side product generated during polycondensation, driving the equilibrium toward the formation of polymer. Exhaustive hydrogenation with high pressure of H2 and Pd/C or Wilkinson’s catalyst yielded saturated precision ADMET polymers 7a−7e. The evolution from monomer to final polymer was monitored by 1H and 13C NMR. As an example, Figure 1

Figure 2. 13C NMR spectrum assignments for saturated polymer 7a.

presence of alkyl groups precisely installed on the polymer backbone affects the chemical shifts of carbons located within 3 CH2 units from an individual branch.16 As an example, the spectrum of polymer 7a is dominated by a singlet at 29.96 ppm, corresponding to the methylenes on the polyethylene main chain and 21-carbon alkyl side chain. The chemical shift at 37.65 ppm is attributed to the branching point (methine). With 21 carbons in the side chain, there is sufficient distance from the pendant end methyl group to avoid effects on methylenes near the branching point. Thus, the methylene 1, 2, and 3 carbons from the branching point exhibit identical chemical shifts, 33.96, 30.43, and 26.97 ppm, respectively. The remaining four peaks (14.35, 22.93, 29.61, and 32.17 ppm) belong to the methyl and methylene groups on the end of the alkyl side chain. The assignments are in good agreement with previously reported experimental and theoretical data. All other saturated polymers (7b to 7e) show the same carbon resonances as well. Therefore, it can be concluded that via ADMET 21-carbon alkyl branches were precisely placed along the polymer backbone on every 15th, 19th, 21st, 23rd, and 39th carbon without any structural defects typically observed during chaingrowth chemistry. Model polymers generated by metathesis polymerization can be viewed as copolymers of ethylene and α-olefins. Because of the precise nature of ADMET polymers, it is easy to calculate the comonomer content using the branch frequency (m) according to the following relationship 2 mol % of comonomer = × 100 (1) m

Figure 1. 1H and 13C NMR spectra of monomer 5a (a,b), unsaturated polymer 6a (c,d), and saturated polymer 7a (e,f).

shows the 1H and 13C NMR spectra of polymer 7a and its precursors. The successful ADMET polymerization with Grubbs’ first generation catalyst is evidenced by the disappearance of the terminal olefin proton signals at 4.9 and 5.8 ppm (Figure 1a) and the appearance of the internal proton signals at 5.4 ppm (Figure 1c) in 1H NMR. 13C NMR (Figure 1b and 1d) shows that, after polymerization, signals attributed to terminal olefin at 114.29 and 139.49 ppm disappeared with the formation of new internal olefin signals at around 130 ppm (cis at 130.10 ppm and trans at 130.56 ppm). Subsequent exhaustive hydrogenation under high H2 pressure using Pd/C as a catalyst generated ADMET polyethylene with 21-carbon branches on every 15th carbon along the polymer backbone (7a). Saturation was verified by the absence of the proton signal

Molecular weights of all saturated polymers were measured by gel permeation chromatography (GPC) and diffusionordered NMR spectroscopy (DOSY) (SI Table 1). The melting behavior of all saturated long-chain branched polyethylenes (7a to 7e) was investigated by differential scanning calorimetry (DSC). It has been well studied that the melting behavior of shortchain branched PE is influenced by the amount and distribution of branches.17 The same is true for long-chain branched PE. However, due to the heterogeneity and unwanted structural 1226

DOI: 10.1021/acsmacrolett.5b00641 ACS Macro Lett. 2015, 4, 1225−1228

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ACS Macro Letters defects built in commercial PE via chain-propagation chemistry, the interpretation of thermal results has been limited. The situation is different here. The identity of the branch is predetermined, which is the 21-carbon alkyl branch. The distance between two branching points is held constant for each polymer and increases systematically from 15 carbons to 39 carbons. The precise nature of long-chain branched PE reported in this paper allows us to interpret melting behavior as a function of branch frequency (BF). The melting profiles of the five polymers are displayed in Figure 3. It is obvious that the melting points of all polymers

Figure 4. Flory equation treatment for long-chain and butyl branched ADMET PE (the dashed line represents butyl branched PE; the solid line represents 21-carbon branched PE).

predicts, the butyl branched ADMET polyethylenes exhibit a linear relationship, which is consistent with the fact that butyl branches are excluded from the crystal unit. The same holds true for the 21-carbon branched PE data (Figure 4). A linear relationship is also obtained; the long side chains are expelled as well. The two slopes are different, which can be explained by the fact that the Flory theory focuses only on the crystallizable units in the polyethylene backbone, regardless of the nature of the branch. More interesting results are observed when comparing our polymers with short-chain branched PE having the same branch frequency, for example PE with butyl branches on every 15th, 21st, and 39th carbon11,21 (Table 1).

Figure 3. DSC visual overlay of 21-carbon branched ADMET polyethylenes.

are lower than that of the pure ADMET PE (134 °C), indicating that the presence of long side chains alters the ease of crystallization and suppresses the melting temperature of the final polymer. As a function of the branch frequency, the decrease of BF (from 7a to 7e) results in the increase of the melting point of the final polymer. This comes as no surprise as the decrease of defects (branch concentration) in the polymer structure increases the intermolecular forces between polymer chains, resulting in higher melting temperatures. Compared to randomly long-chain branched PE synthesized by Ziegler− Natta or metallocene-based catalysts, our precision polymers display narrower melting profiles even with a high concentration of branches (7a, 13.3% mol).18,19 The melting behavior described above for long-chain branched PE is the same as we observed for short-chain branched PE, falling in line with our previous observations of branches being excluded from the crystal units. To further examine this situation, Flory’s theory20 was applied to both short-chain and long-chain branched polyethylenes. Flory’s theory of copolymer crystallization considers the counits as the defects that are completely expelled from the crystal region. In other words, the melting points of the polymer relate only to the portion of the polyethylene backbone which effectively participates in the crystallization. This relationship can be described by Flory’s equation20 (eq 2), where Tm0 is the melting point of the perfect crystal (418.7 K for polyethylene), R the gas constant, ΔHf the heat of fusion per repeating unit (8.284 kJ/mol), and P the sequence propagation probability of ethylene units which equals the mole fraction of crystallizable units for random copolymers. 1 1 R = 0 − ln P Tm ΔHf Tm (2)

Table 1. DSC Data for Saturated 21-Carbon Branched and Butyl Branched ADMET PE branch on every nth carbon, n 15 19 21 23 39

ADMET polyethylenes

Tm (°C)

ΔHf (J/g)

7a PE-butyl 7b 7c PE-butyl 7d 7e PE-butyl

31 −33 44 52 14 58 70 75

65 13 65 57 57 59 62 66

In the case of PE with butyl branches on every 15th carbon, the low melting temperature and heat of fusion result from the relatively large concentration of defects of butyl branches. Therefore, we initially thought that PE with 21-carbon branches, much larger than butyl branches, with the same branch frequency would generate a totally amorphous material. Instead of being amorphous, polymer 7a exhibits a sharp and well-defined melting profile (Figure 3). The melting temperature of 7a was more than 60 °C higher than the corresponding butyl branched PE, and the heat of fusion increased by a factor of 5. The same phenomenon is observed in the case of PE with branches on every 21st carbon. Considering the branches are expelled from the crystal region, the higher melting temperature and heat of fusion indicate that there exist different crystal morphologies with better packing in long-chain branched polymers 7a and 7c. A series of 21-carbon alkyl group branched polyethylenes have been synthesized via ADMET polymerization. 1H NMR, 13 C NMR, and IR spectroscopy provide unambiguous evidence for the precise primary structures of the monomers and final polymers. The thermal behaviors of all polymers were investigated by DSC. All 21-carbon branched PEs display

By using eq 2, the reciprocals of melting points are plotted versus the natural logarithms of the mole fraction of ethylene units of butyl branched PE (Figure 4). Just as the theory model 1227

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(17) Few, C.; Wagener, K.; Thompson, D. Macromol. Rapid Commun. 2014, 35 (2), 123−132. (18) Piel, C.; Starck, P.; Seppala, J.; Kaminsky, W. J. Polym. Sci., Part A: Polym. Chem. 2006, 44 (5), 1600−1612. (19) Russell, K.; McFaddin, D.; Hunter, B.; Heyding, R. J. Polym. Sci., Part B: Polym. Phys. 1996, 34 (14), 2447−2458. (20) FLORY, P. Trans. Faraday Soc. 1955, 51 (6), 848−857. (21) Inci, B.; Wagener, K. B. J. Am. Chem. Soc. 2011, 133 (31), 11872−11875.

well-defined melting profiles even with high comonomer incorporation. The melting temperatures for these polyethylenes increase as the branch frequency decreases. These observations together with the good linear relationship derived from Flory’s theory suggest the exclusion of 21-carbon branches from the polyethylene crystal units. Presently we are continuing this research by gathering X-ray scattering data for these long-chain branched ADMET polyethylenes to gain a full understanding of crystal morphologies of these polymers.

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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/acsmacrolett.5b00641. Materials, instrumentations, experimental procedures, and characterization data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]fl.edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Science Foundation (DMR1505778) for the financial support for this project. This material is based upon catalyst work supported by, or in part by, the Army Research Office under the grant W911NF1310362. We also thank Materia Inc. for their generous donation of the catalyst used in this project.

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REFERENCES

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DOI: 10.1021/acsmacrolett.5b00641 ACS Macro Lett. 2015, 4, 1225−1228