and Low-Density Polyethylene (LDPE) - ACS Publications - American

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Direct Synthesis of Polar-Functionalized Linear Low-Density Polyethylene (LLDPE) and Low-Density Polyethylene (LDPE) Yinna Na, Shengyu Dai, and Changle Chen* CAS Key Laboratory of Soft Matter Chemistry, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei 230026, China S Supporting Information *

ABSTRACT: Late-transition-metal catalysts have great potentials to incorporate polar comonomers during olefin polymerization. The preparation of polar-functionalized polyolefins with different microstructures and topologies is a highly fascinating concept. In this contribution, we demonstrate this possibility through palladium-catalyzed ethylene copolymerizations with polar-functionalized α-olefins and their terpolymerizations with α-olefins. A phosphine−sulfonate−palladium catalyst (PO-Pd) afforded highly linear polyethylene during ethylene homopolymerization. Linear low-density polyethylene (LLDPE) was obtained from the PO-Pd-catalyzed copolymerizations of ethylene with α-olefins; in these copolymerizations, the partial or complete replacement of α-olefins with polar-functionalized α-olefins led to the formation of polar-functionalized LLDPE (P-LLDPE). A specially designed α-diimine palladium catalyst (NN-Pd) afforded polyethylenes with tunable branching densities (16−37 per 1000 carbon atoms), melting points (101−113 °C), and densities (0.89−0.92 g/cm3), which closely resemble those of low-density polyethylene (LDPE). The NN-Pd-catalyzed copolymerizations of ethylene with polar α-olefins generated analogues of polar-functionalized low-density polyethylene (P-LDPE). The mechanical and surface properties of these polar polyolefin materials were studied in detail, and their properties were further improved/ modified through cross-linking reactions.

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INTRODUCTION Polyolefins represent approximately half of the world’s plastics.1 Hundreds of different grades of polyolefins are commercially available, with an incredible variety of properties and applications. However, their chemical compositions are surprisingly limited to polyethylene, polypropylene, and some copolymers. This apparent contradiction originates from the unique and thorough molecular-level control of the polymerization processes and polymer microstructures facilitated by transition-metal-based catalysts. For example, the supposedly simple polyethylene can be categorized into many different types based on their densities and microstructures; these include high-density polyethylene (HDPE, ρ > 0.941 g/cm3; branching density 80/ 1000C)41−46 to a much higher extent than LDPE or P-LDPE generated by free-radical processes (20−40/1000C). Our

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RESULTS AND DISCUSSION

The syntheses of the P-LLDPE analogues were achieved through phosphine−sulfonate−palladium-catalyzed (PO-Pdcatalyzed) ethylene copolymerizations with some polar αolefins (Scheme 2a). While the PO-Pd complex had been reported previously,50 its X-ray structure (Scheme 2b) had not. Phosphine−sulfonate−palladium-type catalysts have been shown to afford highly linear polymers in ethylene polymerization and copolymerization reactions.51−59 PO-Pd is highly tolerant of polar functionalities (Table 1, entries 1−6) and can mediate the efficient copolymerizations of ethylene with 6chloro-1-hexene (m-Cl), methyl 10-undecenoate (mCOOMe), 10-undecenoic acid (m-COOH), and 10-undecenol (m-OH). It should be noted that the latter three polar comonomers have the added advantage of being biorenewable.60,61 This catalytic system realized high catalytic activities (105 g mol−1 h−1), high comonomer incorporations (1.9− 10.7%), high copolymer molecular weights (Mn: (3.8−19.4) × 104 g mol−1), and high polymer melting points (up to 120 °C). A comparison of the activities of the various polar comonomers may not be accurate due to the mass-transport effect, since large amounts of product precipitate from solution after 1 h of polymerization. The copolymer parameters (comonomer B

DOI: 10.1021/acs.macromol.8b00467 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 2. (a) PO-Pd-Catalyzed Ethylene Copolymerizations and Terpolymerizations with Polar α-Olefins and α-Olefin; (b) Molecular Structure of PO-Pda

a Selected bond lengths (Å) and angles (deg): Pd1−C29 2.121(3), Pd1−O1 2.149(2), Pd1−P1 2.2667(9), Pd1−S2 2.3655(9), C29−Pd1−O1 178.66(11), C29−Pd1−P1 88.03(8), O1−Pd1−P1 92.95(7), C29−Pd1−S2 91.21(8), O1−Pd1−S2 87.71(7), P1−Pd1−S2 173.23(3).

Scheme 3. (a) NN-Pd-Catalyzed Ethylene Copolymerization with a Polar α-Olefin; (b) Molecular Structure of NN-Pda

a Selected bond lengths (Å) and angles (deg): Pd1−C63 2.22(2), Pd1−Cl1 2.359(7), Pd1−N1 2.153(14), Pd1−N2 2.166(17). N1−Pd1−N2 79.2(5), N2−Pd1−C63 100.5(8), C63−Pd1−Cl1 82.4(7), N1−Pd1−Cl1 100.5(4).

Table 2. NN-Pd-Catalyzed Copolymerizationsa entry 1 2 3 4 5 6i 7i 8i 9i 10i 11i 12i 13i 14i 15i 16i

comonomer (mmol)

T (°C)

yield (g)

act.b

m-COOMe(60) m-COOMe(120) m-COOMe(240) m-COOMe(25)j m-COOMe(50)j m-COOMe(100)j m-COOMe(120) m-COOH(120) m-OH(120) m-Cl(120)

20 40 60 80 100 30 30 30 30 30 30 30 60 30 30 30

4.6 6.0 5.1 3.3 1.3 25.4 12.7 8.5 8.3 0.16 0.07 0.04 1.3 13.5 0.66 0.42

182.8 240.0 202.4 132.4 50.4 21.1 10.54 7.06 6.94 0.13 0.05 0.02 1.05 11.25 0.55 0.35

incorpc (%)

Mnd (×10−4)

PDI

Be

Tmf (°C)

0.67 0.86 1.67 1.85 3.58 6.81 1.45 0.90 0.89 0.19

15.4 25.9 2.4 1.8 0.4 76.9 124 39.8 36.2 4.8 3.5 0.6 4.7 42.5 1.7 9.3

4.0 2.7 2.7 6.8 3.1 2.3 3.1 3.1 2.4 5.1 4.4 2.1 3.5 4.0 4.4 2.8

22 27 29 30 37 21 17 15 18 26 28 46 34 15 27 14

113 110 108 104 101 112 112 112 110 115 111 68 104 111 114 118

σMg (MPa)

εBh (%)

ρ

24

950

27

930

0.92 0.91 0.90 0.89 0.89 0.92 0.92 0.92

26 brittle 14

800 brittle 870

0.91 0.90 0.94 0.94

Conditions: 10 μmol of precatalyst, 1.2 equiv of NaBAF (sodium tetrakis(3,5-bis(trifluoromethyl)phenyl)borate), 5 mL of CH2Cl2, total volume of toluene and comonomer: 95 mL, ethylene pressure = 8 atm, 15 min polymerization time was used for entries 1−5. bActivities are the averages of at least two runs and are in units of 104 g mol−1 h−1. cComonomer incorporation ratio was determined by 1H or 13C NMR spectroscopy in C2D2Cl4 at 120 °C. dMolecular weight and PDI were determined by GPC in trichlorobenzene at 150 °C using polystyrene standards. eB = branches per 1000 carbons; branching numbers were determined by 1H NMR spectroscopy, and branches terminated with functional groups were added to the total branches. fDetermined by DSC. gTensile strength (averages of at least two specimens). hElongation at break (averages of at least two specimens). i 12 h polymerization time was used for entries 6−16. jEthylene pressure = 1 atm. a

C


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Figure 1. Stress vs strain curves for the polymer products generated using PO-Pd and NN-Pd. Samples from (a) Table 1, entries 1−6; (b) Table 1, entries 7−10; (c) Table 1, entries 11−14; and (d) Table 2, entries 6, 8, 14, and 16. Results on multiple specimens (Figure S106) are shown for reproducibility.

incorporation, molecular weight, and melting point) are comparable with those of LLDPE. Interestingly, the PO-Pdcatalyzed copolymerization of ethylene and 1-octene (1-Oct) generated a copolymer (Table 1, entry 2) with a similar molecular weight and comonomer incorporation as m-COOMe (Table 1, entry 3). At higher comonomer concentration (Table 1, entries 7−10), higher comonomer incorporation ratios were obtained. However, the catalytic activities and the copolymer molecular weights were reduced. Similar catalytic activities, molecular weights, and melting points were observed for the terpolymerizations of ethylene, 1Oct, and these polar comonomers, as were observed for the corresponding copolymerizations (Table 1, entries 11−13). Similar levels of incorporation were observed for the polar comonomers and 1-Oct, which indicate that the palladium catalyst possesses similar reactivity toward 1-Oct and the polar comonomers under the current polymerization conditions. The P-LDPE analogues were synthesized through α-diimine palladium-catalyzed ethylene copolymerizations with polar αolefins (Scheme 3a). As mentioned above, we recently developed a series of diarylhydryl-based α-diimine palladium catalysts that produce polyethylenes with a very wide range of branching densities.47−49 During our catalyst-screening studies, we developed the thienyl-phenyl substituted α-diimine palladium catalyst NN-Pd that generated polyethylenes with branching densities between 16 and 37 per 1000 carbon atoms, melting points between 101 and 113 °C, and densities between 0.89 and 0.92 g/cm3, which are in the ranges of typical LDPE materials (Table 2, entries 1−6). The branching densities can be tuned by adjusting the polymerization conditions, such as temperature and ethylene pressure. Only methyl and long-chain branches were formed in these polymers, with no branch-onbranch structures observed (Figures S38−S40). Despite the differences in their microstructures, their similar branching densities, melting points, and densities make them close analogues.

NN-Pd mediated the efficient copolymerizations of ethylene with m-COOMe, m-COOH, m-OH, and m-Cl. In the presence of m-COOMe and under 8 atm of ethylene, high activities ((7.0−10.5) × 104 g mol−1 h−1), very high copolymer molecular weights (Mn up to 1.24 × 106), and moderate comonomer incorporations (0.7−1.7%) were achieved (Table 2, entries 7−9). Lower activities and copolymer molecular weights were observed at lower ethylene pressures (Table 2, entries 10−12); however, the comonomer incorporation ratios were greatly enhanced (1.8−6.8%). In addition, the polymer branching densities were greatly enhanced at lower ethylene pressures. The comonomer incorporation ratio and branching density were observed to increase at a higher polymerization temperature (60 °C vs 30 °C; Table 2, entry 13 vs 8), along with decreased activity and copolymer molecular weight. The m-COOH comonomer exhibited a higher activity, similar incorporation, and similar copolymer molecular weight compared with those of m-COOMe (Table 2, entry 14 vs 8), while greatly reduced activities and copolymer molecular weights were observed for m-OH and m-Cl, indicating great poisoning effect of these two polar comonomers (Table 2, entries 15 and 16). The densities, mechanical properties, and surface properties of these polar-functionalized polyolefins were analyzed in detail. The presence of alkyl branches in LLDPE can lower its density compared to that of HDPE. PO-Pd catalyzed the formation of homopolyethylene with a density of 0.95 g/cm3 (Table 1, entry 1), which is a typical value for HDPE. Branching lowers the densities of P-LLDPE generated through PO-Pd catalysis, while the presence of heavy heteroatoms (oxygen and chlorine) results in a density increase. In this system, the copolymer and terpolymer products exhibit densities in the 0.89−0.94 g/cm3 range (Table 1), which are lower than that of homopolyethylene and comparable to those of LLDPE. NN-Pd generated homopolyethylenes with densities of 0.89−0.92 g/cm3 (Table 2, entries 1−6), which are comparable with those of D


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Figure 2. Water contact angles of the polyethylene and copolymer products generated using (a) PO-Pd and (b) NN-Pd.

commercially available LDPE. The densities of the P-LDPE materials (0.92−0.94 g/cm3) generated by NN-Pd are generally higher than their nonpolar counterparts (Table 2, entries 7− 16), which suggest that the heavy-heteroatom factor plays an important role in these cases. For the PO-Pd case, the polymer structures changed from highly linear (HDPE) to branched (PLLDPE). For the NN-Pd case, the polymer structures remained branched for both LDPE and P-LDPE. This is probably the main factor that leads to the different trends in density change during the introduction of polar functional groups for these two systems. There are many reports on the physical properties of polyolefins generated by late-transition-metal catalysts;62−66 however, there are very few studies on the physical properties of polar-functionalized polyolefins. Tensile strengths were measured for selected polymer and copolymer samples (Figure 1a). Dog-bone-shaped tensile-test specimens were obtained by melt-pressing the polymeric products at 30−35 °C above their melting temperatures. The test specimens had widths of 2 mm, gauge lengths of 25 mm, and thicknesses of 0.4 mm. The POPd-generated polyethylene sample (Table 1, entry 1) exhibited typical thermoplastic behavior, with stress-at-break and strainat-break values of 36 MPa and 790%, respectively. This sample also exhibited yielding and strain-hardening phenomena based on the increased slope of the curve at break point.67 The E/1Oct copolymer sample (Table 1, entry 2) exhibited elastomeric properties,68 with a lower and broader yield maximum, a lower stress-at-break value (11 MPa), and similar strain-at-break value (770%) compared with those of the polyethylene sample. The E/m-COOMe copolymer (Table 1, entry 3) contains lower levels of comonomer, similar melting point, and similar density as the E/1-Oct copolymer. Interestingly, E/m-COOMe showed much higher stress-at-break (41 MPa) and strain-atbreak (1050%) values than the E/1-Oct copolymer. The E/mCOOH copolymer (Table 1, entry 4) had lower stress-at-break (11 MPa) and strain-at-break (380%) values than the E/mCOOMe copolymer. This observation is currently not fully understood but may be due to interactions involving the COOH group within the copolymer chain. These intramolecular interactions may reduce intermolecular chain entanglement and correspondingly damage the mechanical properties of the material. It should be noted that the excellent mechanical properties of the E/m-COOMe copolymer may partially be due to its high molecular weight. To probe this issue, esterification reaction of the E/m-COOH copolymer was carried out. After esterification, both the stress-at-break value

and strain-at-break value were increased, supporting the hypothesis of intramolecular COOH interactions (Figure 1a and Figure S5). It is possible that this intramolecular interaction is sensitive to the levels of comonomer incorporated. The E/m-OH copolymer sample (Table 1, entry 5) stands out as having the highest yield strength, while the E/m-Cl copolymer (Table 1, entry 6) exhibits similar tensile properties as the E/1-Oct copolymer. It should be noted that the E/m-Cl copolymer is different from the rest of polar functionalized copolymers, in the regard that a short chain comonomer m-Cl (six carbon units versus the 11 carbon units in the rest of polar comonomers) was used because of its commercial availability. This may contribute to influence the mechanical properties, since long side chains may be able to induce better intermolecular chain entanglement. The copolymers formed at higher comonomer concentrations and correspondingly bearing higher comonomer incorporations (Table 1, entries 7− 10) exhibited lower stress-at-break and strain-at-break values (Figure 1b). Tensile properties were also modulated through the terpolymerization of ethylene with 1-Oct and polarfunctionalized α-olefins (Figure 1c and Table 1, entries 11− 14). The NN-Pd-generated polyethylene and copolymer samples (Figure 1d and Table 2, entries 8, 14−16) have typical thermoplastic properties; the incorporation of various polar groups can also modulate the stress-at-break and the strain-atbreak values of these materials. In addition to modulating mechanical properties, the introduction of polar functional groups has the potential to improve many other properties of polyolefin materials, such as adhesion, wettability, barrier properties, surface properties, and miscibility with additives and other polymers.69−72 The surface properties (related to printability, paintability, etc.) of these polar-functionalized polymeric products were studied by measuring their water contact angles (WCAs). WCA measurement samples were prepared by the evaporation of ∼5 wt % polymer solutions in toluene on glass slides. The solvent was evaporated on a hot plate for 30 min, and a second or a third layer of the polymer solution was applied to produce a thicker film. WCAs of 109° and 105° were determined for pure polyethylene when generated by PO-Pd and NN-Pd, respectively (Figures 2a and 2b). In both cases, the incorporation of polar functional groups led to a significant decrease (of up to 20°) in the WCA values compared to pure polyethylene; clearly, this is an efficient strategy for modulating/improving the surface properties of polyolefin materials. Generally, the polymer samples from NN-Pd tend to E


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Figure 3. (a) E/m-COOH-copolymer cross-linking (sample from Table 1, entry 8) using toluene diisocyanate. (b) E/m-OH-copolymer crosslinking (sample from Table 1, entry 9) using toluene diisocyanate. (c) E/m-COOMe-copolymer cross-linking (sample from Table 2, entry 13) using ethylene glycol. IR spectra before and after cross-linking for (d) the E/m-COOH copolymer and (e) the E/m-OH copolymer. Stress−strain curves before and after cross-linking for (f) the E/m-OH and E/m-COOH copolymers and (g) the E/m-COOMe copolymer; results on multiple specimens (Figure S106) are shown for reproducibility.

form smoother polymer films than those from PO-Pd, which are probably due to their highly branched microstructures as well as their higher solubility in toluene solution. The polymer microstructures, the type of polar groups, and the smoothness of the polymer films may all influence the WCA measurement. The properties of these polar-functionalized polyolefins can be further improved/modified through postpolymerization functionalization. For example, the incorporated COOMe, COOH, OH, or Cl groups can be converted into other functional groups through known organic transformations. Furthermore, these functional groups provide reactive sites for cross-linking. It is well established that cross-linking reactions improve the mechanical, thermal, and physicochemical properties of a variety of polymeric materials.73 In this study, we examined the cross-linking of the E/m-COOH and E/m-OH copolymers using toluene diisocyanate (Figure 3a,b). Following cross-linking, both copolymers were insoluble in common organic solvents, even at elevated temperatures. In addition, the appearance of N−H, C−N, CO, and C−O related peaks in their IR spectra supports the formation of cross-links (Figure 3d,e). Furthermore, the stress−strain curves reveal that both the stress-at-break and strain-at-break values were improved following cross-linking (Figure 3f); in both cases, the melting points were slightly lowered after cross-linking (Figures S56−

S58). Utilizing another type of cross-linking reaction, the E/mCOOMe copolymer was cross-linked with ethylene glycol (Figure 3c) in the presence of 1,5,7-triazabicyclo[4.4.0]undec5-ene (TBD). The cross-linked product from the PO-Pdgenerated E/m-COOMe copolymer was very stiff and could not be molded to prepare a specimen for tensile testing. However, the cross-linked product from the NN-Pd-generated E/m-COOMe copolymer could be molded. Clearly, crosslinking can dramatically improve the physical properties of these copolymers (Figure 3g).

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CONCLUSIONS In summary, polar-functionalized polyolefins with microstructures that closely resemble those of LLDPE and LDPE were synthesized using a phosphine−sulfonate−palladium (PO-Pd) catalyst and a specially designed α-diimine palladium (NN-Pd) catalyst. PO-Pd efficiently mediates the copolymerizations of ethylene with 6-chloro-1-hexene, methyl 10undecenoate, 10-undecenoic acid, and 10-undecenol. High catalytic activities (105 g mol−1 h−1) were achieved in this system, leading to copolymers (P-LLDPE) with high comonomer incorporations (1.9−10.7%), high copolymer molecular weights (Mn: (3.8−19.4) × 104 g mol−1), high polymer melting points (up to 120 °C), and tunable densities F


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Macromolecules (0.89−0.94 g/cm3). Terpolymerizations of ethylene with these polar comonomers and 1-octene were also realized. NN-Pd also efficiently mediated copolymerizations of ethylene with these polar comonomers. High catalytic activities (105 g mol−1 h−1) were achieved in this system, leading to copolymers (P-LDPE) with high comonomer incorporation (0.7−6.8%), high copolymer molecular weights (Mn up to 1.24 × 106), high polymer melting points (104−118 °C), tunable branching densities (14−46/1000C), and tunable densities (0.92−0.94 g/ cm3). The incorporation of polar groups significantly influenced the mechanical and surface properties of the resulting polymeric materials. Moreover, these polar groups provide reactive sites for further functionalization, leading to the further improvement of their properties.

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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.8b00467. Experimental procedures, NMR spectra for polymer products (PDF) X-ray crystal structure of PO-Pd (CIF) X-ray crystal structure of NN-Pd (CIF) checkCIF/PLATON report for PO-Pd (PDF) checkCIF/PLATON for NN-Pd (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.C.). ORCID

Shengyu Dai: 0000-0003-4110-7691 Changle Chen: 0000-0002-4497-4398 Author Contributions

Y.N. and S.D. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (NSFC, 21690071, 51522306, and 51703215).

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REFERENCES

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