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Electron-Rich Metal Cations Enable Synthesis of High Molecular Weight, Linear Functional Polyethylenes

  • Wei Zhang
    Wei Zhang
    Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
    More by Wei Zhang
  • Peter M. Waddell
    Peter M. Waddell
    Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
  • Margaret A. Tiedemann
    Margaret A. Tiedemann
    Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
  • Christian E. Padilla
    Christian E. Padilla
    Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
  • Jiajun Mei
    Jiajun Mei
    Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
    More by Jiajun Mei
  • Liye Chen
    Liye Chen
    Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
    More by Liye Chen
  • , and 
  • Brad P. Carrow*
    Brad P. Carrow
    Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
    *[email protected]
Cite this: J. Am. Chem. Soc. 2018, 140, 28, 8841–8850
Publication Date (Web):June 26, 2018
https://doi.org/10.1021/jacs.8b04712
Copyright © 2018 American Chemical Society
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Abstract

Group 10 metal catalysts have shown much promise for the copolymerization of nonpolar with polar alkenes to directly generate functional materials, but access to high copolymer molecular weights nevertheless remains a key challenge toward practical applications in this field. In the context of identifying new strategies for molecular weight control, we report a series of highly polarized P(V)–P(III) chelating ligands that manifest unique space filling and electrostatic effects within the coordination sphere of single component Pd polymerization catalysts and exert important influences on (co)polymer molecular weights. Single component, cationic phosphonic diamide-phosphine (PDAP) Pd catalysts are competent to generate linear, functional polyethylenes with Mw up to ca. 2 × 105 g mol–1, significantly higher than prototypical catalysts in this field, and with polar content up to ca. 9 mol %. Functional groups are positioned by these catalysts almost exclusively along the main chain, not at chain ends or ends of branches, which mimics the microstructures of commercial linear low-density polyethylenes. Spectroscopic, X-ray crystallographic, and computational data indicate PDAP coordination to Pd manifests cationic yet electron-rich active species, which may correlate to their complementary catalytic properties versus privileged catalysts such as electrophilic α-diimine (Brookhart-type) or neutral phosphine-sulfonato (Drent-type) complexes. Though steric blocking within the catalyst coordination sphere has long been a reliable strategy for catalyst molecular weight control, data from this study suggest electronic control should be considered as a complementary concept less prone to suppression of comonomer enchainment that can occur with highly sterically congested catalysts.

Introduction

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Tailored applications of polymeric materials demand access to a continuum of material properties, which in turn requires flexible polymerization methods with precise control over macromolecular structure. Polyolefin materials, polyethylene (PE), and polypropylene (PP), being the most common examples, are thus attractive targets for new methods development because they comprise the large majority of all polymer production by weight.(1) Though the semicrystalline nature of PE and isotactic PP engender strength and toughness, the associated hydrophobicity of these polymers also restricts their potential applications by limiting surface properties, compatibility and rheology. The incorporation of polar functional groups into the parent polyolefin structure, even in relatively minor amounts, can significantly alter the material properties of the resulting “functional” polyolefins and has long been the focus of synthetic efforts.
High molecular weight, linear functional polyethylenes (f-PE) represent a particularly attractive target for insertion polymerization because of their structural analogy to linear low-density polyethylene (LLDPE). Even though the $40 billion global market volume (2013) for LLDPE is forecast to continue expanding,(2) established polyolefin markets such as this are still insufficient to fully capture the massive volume of light olefins that have resulted from shale gas production.(3) The potential applications of polar analogues of LLDPE are manifold because of enhanced surface properties, compatibility in polymer blends, printability, dye retention, and degradability. New polyolefin classes therefore have added value both in their fundamental material properties but also for their potential to capture carbon that would otherwise be burned as fuel. A fundamental challenge to this end, however, is that few methods for functional polyolefin synthesis selectively generate linear polymer microstructures with high molecular weight.
Common synthetic approaches to incorporate polar functional groups into the hydrocarbon backbone of polyolefins are summarized in Scheme 1. High energy intensity methods, such as free radical copolymerizations of ethylene with polar alkenes, have been commercialized (Scheme 1a).(4) Copolymers formed by such methods are mechanistically biased toward microstructures with long chain branching and low crystallinity. On the other hand, the direct metal-catalyzed insertion copolymerization of olefins and industrial polar monomers represents a potentially ideal and tunable route to functional polyolefin materials. Significant progress has been made in this area over the last two decades using group 10 metal catalysts.(5) Brookhart’s cationic Pd and Ni complexes coordinated by an α-diimine ligand have been very successful in this regard, which tolerate acrylates or several other polar vinyl monomers to generate branched copolymers (Scheme 1b).(6) Mechanistic studies on these systems have established that branch formation occurs through a characteristic “chain walking” mechanism due to facile β-hydrogen elimination and reinsertion allowing monomer enchainment to occur at internal positions along the polymer chain. As a consequence of this mechanistic behavior, polar functional groups are positioned at the ends of branches.

Scheme 1

Scheme 1. Common Functional Polyolefin Synthesis Methods
Strictly linear functional polyolefins can be generated by ring opening metathesis polymerization (ROMP) or acyclic diene metathesis polymerization (ADMET).(7) Both of these methods provide highly controlled and tunable routes to well-defined copolymers, but the requisite postpolymerization hydrogenation and specialty monomers nevertheless impose major barriers to large-scale applications. Alternatively, Drent and Pugh discovered that phosphine-sulfonato Pd catalysts generate linear (co)polymers by insertion polymerization (Scheme 1c).(8) The contrasting catalyst behavior of Brookhart-type versus Drent-type polymerization catalysts has been established through theoretical studies to originate from the electronic (a)symmetry of the respective chelating ligands.(9) A variety of group 10 metal catalysts with other (P^O)-type ligands have since been shown to conserve the linear polymer selectivity of Drent-type catalysts and can make high molecular weight PE in some cases,(10) but very few can access high f-PE molecular weights (i.e., ≥105 g mol–1) needed for many applications.(11) Even Drent-type complexes, arguably the most successful catalyst class,(12) rarely achieve (co)polymer Mw exceeding 104 g mol–1.(13) The direct synthesis of f-PE with high molecular weight therefore remains a significant ongoing challenge that could benefit from new catalysts and rational design strategies.
A typical design strategy to increase polymer molecular weight using Drent-type and related catalysts is based on a steric blocking model (Scheme 2a) to inhibit the rate of chain transfer at these square planar metals that occurs through associative ligand substitution by monomer.(9a) Increased steric bulk at the ancillary ligand, which must emanate from phosphorus(III) substituents in Drent-type catalysts, therefore tends to increase polymer molecular weight by blocking monomer coordination at the apical sites. The liability of this common approach during insertion copolymerizations is that bulkier P(III) substituents exacerbate the relative energy barrier of enchaining any comonomer relative to ethylene, the smallest alkene (Scheme 2b). Drent-type catalysts therefore tend to exhibit an antagonistic relationship between polymer molecular weight and comonomer enchainment, and it is not clear a priori how catalysts could be designed to avoid this mechanistic paradox by variation of the P(III) substituents alone.

Scheme 2

Scheme 2. General Considerations of Catalyst Steric Effects on Insertion Copolymerizations
Computational studies on Drent-type phosphine-sulfonato Pd catalysts have also suggested that isomerization of the reacting ligands occurs prior to each monomer enchainment and that the most reactive intermediate has mutually cis π-alkene and tertiary phosphine ligands.(9) On the basis of this characteristic feature of (P^O)-coordinated polymerization catalysts, we hypothesized that steric bulk emanating from the O ligand rather than from the P ligand should still enhance polymer molecular weight but also lessen the impact of ligand size on ΔΔG for competing monomer enchainment because coordinated monomer is positioned trans to the O ligand (Scheme 2c). The substituents of P(V) oxides seem well-suited as a platform to test this hypothesis.(10,14) In particular, a phosphonic diamide group was attractive to us because it is derived from abundant secondary amines that could facilitate a high degree of structural tunability. An initial challenge to this end was the lack of any precedent for metal catalysis involving a phosphonic diamide in a chelating (P^O)-type ligand scaffold. We report here the first examples of single component Pd catalysts featuring a chelating P(V)–P(III) ligand based on a phosphonic diamide-phosphine (PDAP) motif. We have found these types of ligands do indeed provide a tunable platform for generating highly active Pd catalysts for synthesis of linear f-PE from industrial polar monomers, including high molecular weight examples that few other catalysts can match.

Results and Discussion

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PDAP-Pd Catalyst Synthesis and Steric Benchmarking

The highly polarized P–O bond in phenylphosphonic diamides functions as a good directing group for ortho-metalation, which provided a straightforward synthesis of PDAP ligands 1a1i by a one-pot sequence of directed ortho-lithiation followed by nucleophilic substitution at a chlorophosphine reagent (Table 1).(15) The PDAP ligands possessing a diarylphosphino moiety are air-stable and were purified by simple flash chromatography.(16) Air sensitive alkylphosphines were carried forward without purification. Ligand exchange of the PDAP for cyclooctadiene in (COD)Pd(CH3)Cl occurred within minutes at rt and gave the neutral complexes 2a2i. Exchange of the chloride ligand for 2,6-lutidine generated the final air and moisture-stable single component precatalysts (3a3i) in 15%–76% isolated yield over three steps. We next pursued a quantitative assessment of the space filling properties of PDAP ligands to test our initial hypothesis that the phosphonic diamide provides suitable steric properties.
Table 1. Synthesis of Phosphine–Phosphonic Diamide (PDAP) Ligands and Single Component Precatalystsa
complexR1R2R3yieldb (%)
3aMeHi-Pr51
3bi-PrHi-Pr43
3ci-PrHi-C5H1155
3di-PrHPh76
3eMeH2-(MeO)-C6H454
3fEtH2-(MeO)-C6H448
3gi-PrH2-(MeO)-C6H443
3hi-PrH2,6-F2C6H328
3iMeNMe22,4-(MeO)2C6H315
a

Reagents and conditions: (a) tert-butyl lithium (1.2 equiv), THF at 0 °C for 1 h the ClP(R3)2 (1.2 equiv), THF at 0 °C then warming to rt for 0.5 h; (b) (COD)PdCl(Me) (1.0 equiv), CH2Cl2 at rt for 0.5 h; (c) NaBArF4 (1.0 equiv), 2,6-lutidine (1.1 equiv), CH2Cl2 at −78 °C warming to rt for 0.5 h.

b

Yield over three steps. ArF = 3,5-(CF3)2C6H3.

To quantitate how the coordination chemistry of the aforementioned phosphonic diamide group differs from other common types of neutral and anionic oxygen ligands, we conducted a systematic steric analysis of a PDAP versus other common (P^O)-type ligands. To do so, we prepared and crystallographically characterized six (P^O)Pd(CH3)Cl coordination compounds relevant to olefin polymerization and oligomerization. The structures of these complexes differ only by the identity of the O group and should thus provide a clear indication of how the new PDAP ancillary ligand fills the primary coordination sphere relative to existing catalysts and, by corollary, could affect chain transfer during alkene polymerization.
Solid-state data for complexes 2g and 48 were analyzed by Cavallo’s SambVca 2.0 program to generate topographical steric maps (Figure 1).(17) Complexes possessing small ester (4), sulfonate (7), or carboxylate (8) groups expectedly display significant open space in the western hemisphere (O side) of their coordination sphere. On the other hand, the complex with a diisopropyl phosphonic diamide group (2g) fills significantly more space. Percent buried volume data for these complexes also indicate the diispropyl phosphonic diamide group in 2g fills more space than do other O ligands based on P(V) moieties, such as diethyl phosphonate in 5 or di-tert-butyl phosphine oxide in 6. Variation of the amine substituents in the PDAP ligand subsequently provided a straightforward means for catalyst tuning (vide infra).

Figure 1

Figure 1. Topographical steric maps of (P^O)Pd(CH3)Cl complexes 2g and 48 in which the Pd atom defines the center of the xyz coordinate system, the metal square plane defines the xz-plane and the z-axis bisects the O–Pd–P angle. Pd(CH3)Cl fragment omitted from each plot. Colors indicate occupied space in the +z direction toward the covalent ligands (red) or −z direction toward the dative ligands (blue).

Ligand-Dependent Catalyst Activity and Stability

The single component PDAP-Pd complexes 3a3i were next benchmarked as catalysts for ethylene polymerization (Table 2) to identify promising candidates for f-PE synthesis. Catalysts 3a3c possessing only alkyl substituents on both the P(V) and P(III) groups formed PE with decreasing activity as the size of the alkyl substituents increased (entries 1–3). In contrast, PDAP-Pd complexes were much more reactive with more hindered arene P(III) substituents. Complex 3d that contains a simple diphenylphosphino group formed only trace PE after 30 min (entry 4), but complexes possessing ortho-arene substituents were highly reactive (entries 5–11). The highest average turnover frequency (TOF) of 3.6 × 105 h–1 (100 s–1) was observed using 3h (entry 12) with electron-poor 2,6-difluorophenyl P(III) substituents, which ranks among the highest activities reported for any Pd polymerization catalyst(13a,14c) and even compares favorably to values first reported for cationic α-diimine nickel complexes (TOF up to 3.9 × 105 h–1) or classic homogeneous metallocene complexes such as [Cp2ZrMe]+[MeB(C6F5)3] (TOF up to 1.6 × 105 h–1).(6a,18)
Table 2. Ligand-Dependent Catalyst Activity and Polymer Structure Effects During Ethylene Polymerizationa
entrycatalyst[C2H4] (bar)time (min)yield (g)activity (kg molPd–1 h–1)Mwb (×10–3 g/mol)bMe br.cTmd (°C)
13a30607.1429005.21.80.5130
23b30151.081700391.62.5131
3e3c23302.601000811.416f124
43d3030trace     
53e30153.6058001201.61.4137
6 15303.5628001301.62.9f134
7 7302.4820001001.52.7f131
8 3.5301.19950212.57.0f126
93f30151.5625001201.43.4f136
103g30151.3522002401.413127
11 3.51201.35270161.621105
123h30156.3010000801.83.0f130
13g3i30152.1268001201.43.0f133
a

Conditions: catalyst (2.5 μmol) and toluene (0.1 L) were added to a 450 mL stainless steel autoclave at rt under N2, equilibrated to 90 °C, then charged with ethylene.

b

Determined by GPC with multidetection.

c

Methyl branches per 1000 C as determined by quantitative 13C NMR.

d

Peak Tm determined by DSC, second heating cycle.

e

5 μmol catalyst.

f

Determined by 1H NMR.

g

[Pd] = 12.5 μM.

Importantly, PE molecular weight also increased significantly upon variation of the amino substituents in 3e3g from Me or Et to i-Pr (e.g., entries 5, 9, and 10). Complex 3g formed PE with the highest molecular weight (Mw = 2.4 × 105 g mol–1), and this molecular weight is substantially higher than the PE formed by any other (P^O)Pd catalyst reported to date that possesses the same P(III) moiety. For instance, a phosphine-sulfonato palladium catalyst with identical ligand backbone and bis-o-anisylphosphino group generated PE with 6× lower Mw under similar reaction conditions.(19) Additionally, the dependence of PE molecular weight on pC2H4 saturates above ca. 7 bar using catalyst 3e (entries 5–8), but the catalyst activity increased proportionally with pC2H4 from 3.5 to 30 bar.
Methyl branching increases with increasing amino substituent size in 3e3g, from 1 to 13 per 1000 methylene carbons, leading to a corresponding depression of peak PE melting temperature (Tm) by 10 °C, respectively. Monomer pressure also affects branch density, which is most pronounced for catalyst 3g for which depression of Tm by 22 °C was observed upon decreasing pC2H4 from 30 to 3.5 bar (entries 10 and 11). This modulation of short chain branching contrasts the uniformly linear PE produced by Drent-type catalysts and highlights an advantage of the modular oxygen ligand in the PDAP scaffold.(20)
Another important catalyst property for practical applications in olefin polymerization is high thermal stability to allow for high turnover numbers and frequencies at reaction temperatures required for industrial reactors (ca. 80–140 °C).(21) Though group 10 α-diimine complexes developed by Brookhart and co-workers (e.g., 9, eq 1) are very reactive at mild temperatures, they can suffer from facile decomposition at reaction temperatures above ca. 60 °C.(6a,b,21,22) Many subsequent modifications to the classic α-diimine ligands have since been made with the goal of improved thermal stability,(23,13a) but the absolute activities reported for these are diminished compared to Brookhart’s original room temperature reactions. Salicylaldiminato nickel complexes developed by Grubbs (e.g., 10, eq 1) are similarly thermally sensitive, displaying excellent low temperature reactivity but deactivation occurs upon heating.(24) It is therefore instructive to benchmark the lifetime of PDAP-Pd catalysts during polymerizations at high temperature.
Thermal stability was assessed from the relationship between production of PE versus time during batch polymerizations. It was necessary to decrease the catalyst concentration (4.2 μM) and ethylene pressure (3.5 bar) to avoid fast saturation of the autoclave with PE during polymerizations at 90 °C. Under these conditions, steady production of PE was observed using catalyst 3i over a period of 24 h before a deviation in linearity was apparent (Figure 2). In contrast, a representative Brookhart (9) catalyst exhibited an initial burst of PE formation followed by rapid deactivation, consistent with literature observations. The prototypical Drent-type catalyst (e.g., 11, eq 1) displayed a slightly lower activity than did the PDAP-Pd catalyst 3i. The PDAP catalyst 3g was also tested, which gave a more measured accumulation of PE over time. Using this catalyst, linear PE productivity (R2 > 0.99) was observed over an even longer period (48 h) at 90 °C, which is indicative of excellent thermal stability at this temperature. The yield of PE (13.7 g) formed by 3g corresponds to 1.2 × 106 catalyst turnovers. A deviation from linearity was evident between 48 and 72 h, but the similar mass yields at 48 h using 3g, 3i, or 11 suggest to us that the autoclave may be saturated with polymer at this point. In any case, these data are consistent with the PDAP ligand engendering excellent catalyst thermal stability.

Figure 2

Figure 2. Batch ethylene (3.5 bar) polymerization catalyzed by 3g, 3i, or 911 (1.25 μmol) in toluene (300 mL) at 90 °C. Methylaluminoxane (1.25 mmol) added to the reaction with 9; Ni(cod)2 (1.25 mmol) added to reaction with 10.

Copolymerizations with Polar Alkenes

A preliminary series of small-scale reactions (i.e., 15 mL total volume) using PDAP-Pd complexes for copolymerizations of ethylene with a polar alkene (Table S6) was conducted to identify promising candidates for further study. During this initial screening, the most active catalyst for ethylene homopolymerization (3h) was found to generate poly(ethylene-co-methyl acrylate) with quite low molecular weight (Mw 9 × 103 g mol–1, Table S6, entry 2), similar to the copolymers formed using prototypical Drent catalysts (e.g., 11).(8c) On the other hand, copolymerization of ethylene with methyl acrylate (MA) catalyzed by the more electron-rich 3e generated the highest Mw (2.1 × 104 g mol–1) in the initial survey (Table 3, entry 1). The analogue 3i, which is more electron-rich yet isosteric to 3e, also generated promising polymer molecular weight (Table S6, entry 4). Catalysts 3e and 3i were selected for subsequent reactions.
Table 3. Copolymerization of Ethylene and Polar Alkenes Catalyzed by Cationic PDAP-Pd Complexesa
entrycatalystcomonomer (M)yield (g)productivity (kg molPd–1)Mwb (×10–3 g/mol)bMe brcinc. ratioc (mol %)FG distr. (main/chain ends)c,dTme (°C)ΔHe (J g–1)
1f3eMA (2.2)0.2626211.4102.394:6107
2 BA (1.1)2.8570471.43.44.6>98:2116116
33iMA (1.0)11.1740851.31.71.7n.d.i113107
4 MA (2.0)4.5300491.12.94.1n.d.i10695
5 MA (3.0)2.3150281.22.75.4>97:3100100
6 BA (1.0)7.0470841.21.32.7n.d.i113108
7g BA (1.0)15.410002201.60.50.7n.d.i125140
8h BA (1.0)15.210001901.81.60.3n.d.i135160
9g BA (0.5)26.518001902.02.00.3n.d.i135169
10 BA (2.0)3.9260531.51.38.2n.d.i10482
11 BA (3.0)2.9190621.50.89.6n.d.i9772
12 iBorA (1.0)5.9390921.2n.d.1.5n.d.i116108
13 iBorA (2.0)3.0200371.2n.d.2.4n.d.i10788
14 iBorA (3.0)2.7180181.5n.d.4.3n.d.i10065
15 AA (1.0)1.7110311.63.11.5n.d.i121125
16 AA (2.0)0.9161121.46.13.1n.d.i116120
17 AA (3.0)0.63423.01.36.64.085:1511183
18 BVE (1.0)0.7449211.71.30.5j90:10128186
19 BVE (2.0)0.7047391.61.30.2jn.d.i130165
20 BVE (3.0)0.5033192.41.50.4j87:13128165
a

Conditions: catalyst (15 μmol), comonomer, and toluene (300 mL) were added to a 450 mL autoclave at rt under N2, charged with ethylene (30 bar), then heated to 90 °C.

b

Determined by GPC with multidetection.

c

Methyl branches per 1000 C, comonomer incorporation, and functional group distributions determined by quantitative 13C NMR or 1H NMR.

d

Ratio of functional groups in the polymer main chain versus the chain ends.

e

Peak melting temperature determined by DSC, second heating cycle.

f

10 μmol [Pd], 15 mL total volume, 95 °C for 12 h.

g

pC2H4 = 40 bar, 70 °C.

h

pC2H4 = 40 bar, 60 °C.

i

Chain-ends were not detectible by 1H NMR; see SI for limit of detection estimation for each sample. n.d. = not detected.

j

Determination of BVE incorporation was complicated by overlapping 1H NMR resonances.

A series of copolymerizations (300 mL total volume) was next conducted using 3e or 3i (Table 3). The typical conditions for these reactions involved 50 μM [Pd], 30 bar of ethylene, and 1–3 M concentrations of comonomer in toluene at 90 °C.(25) Under these conditions, complex 3i generated poly(ethylene-co-butyl acrylate) with nearly double the molecular weight (Mw 8.4 vs 4.7 × 104 g mol–1, respectively) than did 3e (entries 2 and 6), which contrasts the weak poly(ethylene-co-methyl acrylate) molecular weight dependence on remote P(III) substituent electronic effects reported for Drent-type catalysts.(26) The magnitude of this difference was surprising because it is unlikely to originate from a classic steric blocking effect given that 3e and 3i are approximately isosteric. A rationalization for these polymer molecular weight differences based on ligand electronic effects is therefore favored (vide infra). A considerable difference in f-PE Mw is also observed between a prototypical Drent catalyst (11) and PDAP-Pd catalyst 3i with P(III) substituents of comparable size, the latter 10× larger under identical reaction conditions (see Table S8 versus Table 3, entry 3). Increasing pC2H4 to 40 bar and/or lowering the reaction temperature from 90 °C to 60–70 °C (Table 3, entries 6–8) generated poly(ethylene-co-butyl acrylate) with even higher molecular weights (Mw ∼ 2 × 105 g mol–1), which is highly desirable yet rare among linear functional polyolefins formed by insertion polymerization.(11b,13) The incorporation ratio (e.g., 2.7–9.6 mol %; Table 3, entries 6, 10, and 11) increased concomitantly with butyl acrylate (BA) concentration (1–3 M). This trend also occurred using methyl acrylate or isobornyl acrylate (iBorA) that possess smaller and larger ester substituents, respectively. All poly(ethylene-co-acrylate) copolymers generated by 3i are highly linear with few methyl branches (≤6 CH3 per 1000 C).
Catalyst 3i is also compatible with other more challenging polar monomers. The copolymerization of ethylene with acrylic acid (AA) or butyl vinyl ether (BVE) to generate statistical copolymers demonstrates the tolerance of PDAP-Pd catalysts to both acidic and nucleophilic reagents, albeit with reduction in catalyst activity and copolymer molecular weights (Table 3, entries 15–20). Contact angle (θ) measurements for the poly(ethylene-co-acrylic acid) with 1.5 mol % AA incorporation (78(3)°) or 3.1 mol % AA incorporation (65(3)°) reflect a significantly more hydrophilic polymer surface than commercial low density PE (102°).(27) This reinforces the substantial effects polar functional groups can exert on polyolefin material properties even at relatively low incorporation ratios (i.e., <10 mol %).
Structure analysis of the poly(ethylene-co-methyl acrylate) formed in Table 3, entry 1 by quantitative 13C NMR spectroscopy revealed a statistical distribution of methyl ester groups almost exclusively along the main chain (94 mol %) (Figure 3). Only 6 mol % of incorporated acrylate was positioned at the saturated chain-end, which corresponds to initiation of a new polymer chain via acrylate insertion into a Pd–H species. More importantly, no alkenyl ester groups were detectable at the unsaturated chain ends. This observation suggests chain transfer does not occur from the presumed catalyst resting state, a Pd-ester enolate complex, formed by enchainment of methyl acrylate.(12o) Analyses of 13C and 1H NMR data for other poly(ethylene-co-acrylate) samples in entries 3–14 indicate the sequence distribution of functional groups, heavily concentrated in the main chain versus chain ends in the copolymer formed by 3e, is conserved when using catalyst 3i. This microstructure formed by a PDAP-Pd catalyst compliments the f-PE typically formed by Drent-type catalysts, which generally have a significant fraction of unsaturated chain ends with functional groups. A potential connection between Pd-ester enolate stability and polymer molecular weight control is therefore conceivable but has received little attention to date. The frequent correlation between decreasing copolymer molecular weight and increasing polar monomer content in f-PE synthesis may suggest this effect deserves further consideration.

Figure 3

Figure 3. Assignment of characteristic 13C NMR resonances in poly(ethylene-co-methyl acrylate) in Table 3, entry 1. See SI for full spectrum (* = galvinoxyl).

The range of accessible molecular weights and comonomer content in the f-PE formed by 3i, which in the best cases are comparable to single site LLDPE samples, could provide a useful platform for comparative material property studies. An initial indication that unanticipated properties could be manifested in these functional analogues is apparent from their melting behavior, as determined by differential scanning calorimetry (DSC). The relative depression in peak melting temperature (Tm) of poly(ethylene-co-acrylate) samples with increasing comonomer content (Figure 4) track closely with literature values for LLDPE reported by Kim at lower comonomer content (ca. 0–5 mol %).(28a) At higher comonomer content (ca. 5–10 mol %), however, Tm decreases to a larger extent for the nonpolar poly(ethylene-co-octene) samples than for the functional analogues. The inclusion of methyl branching in this analysis maintains the same general trend (see Figure S1). The origin of these differences requires further study, but one potential cause could be statistical differences in the spacing between functional groups (i.e., ester versus hexyl) in the nominally random copolymer structures that arise from the catalytic behaviors of the respective metallocene or PDAP-Pd catalysts.

Figure 4

Figure 4. Relationship between melt temperature and comonomer content in f-PE generated by 3i (Table 3, entries 3–8 and 10–17). Literature values for hydrophobic analogues shown in open circles.(28)

Differentiating O Ligand Electronic Properties

In addition to the beneficial space filling characteristics of this new ligand class, our copolymerization data (e.g., Table 3, entries 1 vs 4 or 2 vs 6) suggested there could be an electronic component to the polymer molecular weight control because 3e and 3i are nearly isosteric and a steric blocking model (Scheme 2a) does not adequately account for the near doubling of Mw using the latter catalyst with either MA or BA. On the other hand, assessment of any significant ligand electronic differences is hindered by the general lack of systematic and quantitative data benchmarking the electronic properties of oxygen-based ligands to late transition metals,(29) particularly moieties commonly used in (P^O)-type ligands for alkene polymerization catalysts. We thus conducted additional experiments to build some understanding of how the coordination chemistry of the aforementioned phosphonic diamide group differs from other common neutral and anionic oxygen ligands. To do so, we returned to the structures of (P^O)Pd(CH3)Cl compounds 2g, 48, which differ only by the identity of the O group.
A systematic change in trans influence is evident in complexes 2g and 48 as measured by the change in Pd–C bond length (Figure 5) for the methyl ligand situated trans to the O ligand. Elongation of the Pd–C bond reflects a more strongly σ-donating oxygen atom in these complexes. The L-type oxygen ligands in 4 (ester) and 5 (phosphonate) are poor donors and exhibit expectedly weak trans influence. On the other hand, the X-type anionic donors in 7 (sulfonate) and 8 (carboxylate) displayed higher trans influence. A surprising observation was that the formally L-type phosphonic diamide group in 2g actually falls within the strongly donating X-type region. This suggests the phosphonic diamide σ-donicity is unusually strong for a neutral ligand and beyond what other P(V) ligands (i.e., phosphonate and phosphine oxide) exhibit.

Figure 5

Figure 5. Systematic variation in O ligand trans influence in (P^O)Pd(CH3)Cl complexes 2g and 48 determined from solid state data, and atomic charge in analogous (P^O)Rh(CO)Cl complexes 1217, determined by NBO analysis. Error bars indicate ±3σ.

The solid-state structure of 2g (Figure 6) provides some indication as to why the phosphonic diamide is such a strong donor among L-type oxygen ligands. The sum of the C–N–C and C–N–P bond angles was 355.5(3)° for N1 and 359.5(3)° for N2.(30) This trigonal planar geometry at each nitrogen atom is consistent with strong N–P multiple bond character through negative hyperconjugation. A more polarized P–O bond is then expected for the phosphonic diamide versus other P(V) oxides, such as phosphonates or phosphine oxides. A more polarized P–O bond has previously been correlated to stronger coordination of monodentate P(V) donors to lanthanide metals with the trend: phosphonate < phosphine oxide < phosphoramide.(31)

Figure 6

Figure 6. ORTEP diagram of 2g. Thermal ellipsoids are shown at 50% probability. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Pd–P2 2.2181(4), Pd–O1 2.163(1), Pd–C1 2.034(2), Pd–Cl1 2.3824(4), P1–O1 1.490(1), P2–Pd–O1 89.02(3), C23–N2–C26 114.8(1), C23–N2–P1 120.8(1), C26–N2–P1 123.9(1), C29–N1–C32 113.2(1), C29–N1–P1 113.3(1), C32–N1–P1 129.0(1).

A departure in the bonding trends between hard lanthanide and soft late transition metals is suggested from a natural bond order (NBO) analysis conducted on a related series of (P^O)Rh(CO)Cl complexes (1217) that possess the same ligands as 2g and 48. These complexes, which are geometrically similar to Pd(II), were initially targeted for their CO reporter ligand that reflects the net metal electron density (vide infra). The atomic charge (q) on the metal is plotted in Figure 5, which indicates that (i) higher trans influence generally correlates to decreasing metal charge in this series of complexes, and (ii) the PDAP ligand is a significant outlier in this trend.
The atomic charge (q) on oxygen in (1g)Rh(CO)Cl (16) was also the most negative versus any other complex in the series (Table 4), which suggests the PDAP exhibits the weakest ligand-to-metal charge transfer (LMCT) even though the phosphonic diamide is the strongest σ-donating L-type ligand. This contrasts the bonding trend of P(V) oxides to lanthanides for which phosphoramides show the highest LMCT.(31)
Table 4. Systematic Effects of O Ligand Identity on the Electronic Properties of 2g, 48, and 1217
     atomic chargec
entryXδC Pd–CH3 (ppm)ν COexptl (cm–1)aν COtheor (cm–1)bq(Rh)q(O)
1–CO2Me–1.219902083–0.3502–0.5950
2–P(O)(OEt)2–1.3d19822079–0.3475–1.029
3–P(O)tBu2–2.319752075–0.3704–1.048
4–SO3–2.1e,f1980f2077–0.3719–0.9528
–3.5g1971g
5–P(O)(NiPr2)2–2.519732076–0.3251–1.055
6–CO2–4.2f1971f2060–0.3764–0.7041
–5.3g1960g
a

Determined by 13C NMR in CD2Cl2 for 2g and 48.

b

Calculated for 1217 at ωB97XD/6-31G(d)/LANL2DZ (Rh) level; CPCM solvent model (CH2Cl2).

c

Determined by NBO analysis for for 1217.

d

From ref (14b).

e

From ref (12c).

f

(Et3NH)+ cation.

g

[Ph3P=N=PPh3]+ cation.

Additional spectroscopic data for complexes 2g, 48, and 1217 further support the general trend in O ligand σ-donicity and also the disconnection between donicity and metal electron density for PDAP-coordinated complexes (Table 4). A consistent increase in shielding of the Pd–CH3 resonances in 2g and 48 is observed by 13C NMR spectroscopy as the oxygen ligand fluctuates from the weakest L-type ester group (δC −1.2 ppm) to the strongest X-type carboxylate anion (δC −4.2 to −5.3 ppm),(32) corroborating the general trend in σ-donicity suggested by the trans influence data. Note that complex 2g exhibited a Pd–CH3 chemical shift of δC −2.5 ppm that is similar to the complex with a sulfonate (δC −2.1 to −3.5), which again suggests the donicity of this neutral phosphonic diamide group rivals oxygen anions.
The (P^O)Rh(CO)Cl complexes (1217) used for NBO analysis were also synthesized to determine the experimental carbonyl stretching frequency as a reporter of net metal electron density (Table 4). A general red shift in these stretching frequencies, measured in CH2Cl2 solution, was observed for (P^O) ligands that exhibit higher trans influence. The complex (1g)Rh(CO)Cl (16) with a phosphonic diamide group exhibited a CO stretch (1973 cm–1) that was blue-shifted compared to either the phosphine-sulfonato (1971 cm–1) or phosphine-carboxylate rhodium complexes (1960 cm–1) that possess a noncoordinating cation (PPN+).(27) The theoretical stretching frequencies determined for 1217 using DFT calculations generally agree with the experimental trend. The PDAP ligand thus appears to again be an outlier because the electron density at Rh is less than would be expected based on the high σ-donicity of the phosphonic diamide donor alone, which suggests that localization of positive charge at the metal by the PDAP exerts a significant competing electrostatic effect.
In total, these experimental and computational data establish a spectrum of electronic properties for common oxygen ligands used for the design of alkene polymerization and oligomerization catalysts. These data are also consistent with the conclusion that complexes with a phosphonic diamide group are less electron-rich than would be expected based purely on this O ligand’s high σ-donicity. Considering also that PDAP ligand 1g is still substantially more electron-releasing than is a prototypical Brookhart-type α-diimine ligand, such as Dipp-BIAN (ΔνCO = 25 cm–1),(6a,33,34) we propose that the PDAP-Pd polymerization catalysts 3a3i are best described as electron-rich metal cations. Consequently, these catalysts are electronically differentiated from both the very electrophilic, cationic Brookhart-type catalysts and the electron-rich, neutral SHOP-type(35) or Drent-type catalysts.
Finally, a potential electronic contribution to the polymer molecular weight control by PDAP-Pd catalysts can be inferred from the above data. Mechanisms of chain transfer (Scheme 3, middle) and chain-walking (Scheme 3, bottom) during group 10 metal-catalyzed insertion polymerization both generally proceed through initial formation of a metal hydride by β-H elimination from the propagating organometallic intermediate. The most stable isomer of this metal hydride in a P^O-coordinated Pd complex is the one with hydride and oxygen ligands mutually trans. Higher trans influence in the oxygen ligand, such as for the phosphonic diamide group, should then destabilize this intermediate and thereby disfavor chain transfer. This effect could contribute to the higher polymer molecular weight of 3i versus 3e because the dimethylamino group in the former should make the O ligand a stronger donor. Such an electronic effect may work synergistically with the higher steric shielding by the phosphonic diamide as compared to small sulfonate or carboxylate groups, thereby slowing chain transfer during ethylene propagation and also potentially from the catalyst resting state (Scheme 3, top) generated immediately after polar alkene enchainment. Future studies will attempt to scrutinize this hypothesis.

Scheme 3

Scheme 3. Mechanistic Pathways Controlling Polar Group Distribution in Functional Polyolefins

Conclusion

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A series of cationic palladium complexes ligated by a chelating phosphonic diamide-phosphine (PDAP) have been developed. Several of these PDAP-Pd complexes (e.g., 3e and 3i) are potent catalysts for insertion copolymerization with polar vinyl monomers. Importantly, poly(ethylene-co-acrylate) materials with Mw up to 2 × 105 g mol–1 were formed, which is a molecular weight threshold far higher than functional polyethylenes (f-PE) formed by the important Drent-type catalysts that possess analogous P(III) substituents (e.g., o-anisyl). This increase in molecular weight control highlights the utility of tunability in the O hemisphere of the coordination sphere of P^O-coordinated late transition metal catalysts, which is enabled by the modular amino groups in the PDAP structure.
Analysis of the f-PE structures formed by PDAP-Pd catalysts showed that the polar comonomer was incorporated almost exclusively into the polymer main chain, which complements copolymers formed by existing catalysts that tend to have significant functional group content at chain ends or at the ends of branches. Furthermore, the linear production of polymer over 24–48 h at 90 °C by PDAP-Pd catalysts 3g and 3i established the excellent thermal stability engendered by PDAP coordination, which is an important property needed for practical applications.
Quantitative analysis of the steric properties of a prototypical PDAP ligand (1g) versus other common chelating (P^O)-type ligands confirmed that the former fills more space than typical ester, sulfonate, phosphonate, phosphine oxide or carboxylate groups, which helps to inhibit coordinative chain transfer. Spectroscopic, solid-state structural and computational data are consistent with the phosphonic diamide being a more potent σ-donor to late metals, such as Pd or Rh, than any other neutral oxygen donor that was tested. We therefore conclude that PDAP-Pd catalysts can be described as electron-rich metal cations, which is distinct from existing catalyst types such as electrophilic, cationic Brookhart-type α-diimine Pd catalysts and electron-rich, neutral Drent-type catalysts. The potential of these ligands to access new and complementary catalytic behaviors in alkene polymerization when coordinated to Pd, and potentially other transition metals, should provide opportunities to explore material properties of high molecular weight polar-functionalized polyethylenes.

Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b04712.

  • Experimental procedures, crystallographic data, characterization data for ligands, metal compounds, and polymers (PDF)

  • Crystallographic structures (CIF)

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Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

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  • Corresponding Author
  • Authors
    • Wei Zhang - Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
    • Peter M. Waddell - Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
    • Margaret A. Tiedemann - Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
    • Christian E. Padilla - Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
    • Jiajun Mei - Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
    • Liye Chen - Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
  • Notes
    The authors declare the following competing financial interest(s): A patent was filed by Princeton University: Carrow, B. P.; Zhang, W. WO 2015/200849 A2, December 30, 2015.

Acknowledgments

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We thank Richard Register for helpful suggestions and access to analytical instrumentation, Adam Burns for assistance with DSC measurements, and Jason Brandt for assistance with manuscript revisions. C.E.P. thanks the NSF for a graduate fellowship. Bruker is thanked for sample analysis on a 10 mm cryoprobe 500 MHz NMR spectrometer. We thank the NSF (CHE-1654664), the Princeton Center for Complex Materials, a MRSEC supported by NSF Grant DMR 1420541, and Princeton University for financial support.

References

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  61. Guifu Si, Yinna Na, Changle Chen. Ethylene (co)Oligomerization by Phosphine‐Pyridine Based Palladium and Nickel Catalysts. ChemCatChem 2018, 10 (22) , 5135-5140. https://doi.org/10.1002/cctc.201800957OpenURL UNIV OF HOUSTON MAIN
  • Abstract

    Scheme 1

    Scheme 1. Common Functional Polyolefin Synthesis Methods

    Scheme 2

    Scheme 2. General Considerations of Catalyst Steric Effects on Insertion Copolymerizations

    Figure 1

    Figure 1. Topographical steric maps of (P^O)Pd(CH3)Cl complexes 2g and 48 in which the Pd atom defines the center of the xyz coordinate system, the metal square plane defines the xz-plane and the z-axis bisects the O–Pd–P angle. Pd(CH3)Cl fragment omitted from each plot. Colors indicate occupied space in the +z direction toward the covalent ligands (red) or −z direction toward the dative ligands (blue).

    Figure 2

    Figure 2. Batch ethylene (3.5 bar) polymerization catalyzed by 3g, 3i, or 911 (1.25 μmol) in toluene (300 mL) at 90 °C. Methylaluminoxane (1.25 mmol) added to the reaction with 9; Ni(cod)2 (1.25 mmol) added to reaction with 10.

    Figure 3

    Figure 3. Assignment of characteristic 13C NMR resonances in poly(ethylene-co-methyl acrylate) in Table 3, entry 1. See SI for full spectrum (* = galvinoxyl).

    Figure 4

    Figure 4. Relationship between melt temperature and comonomer content in f-PE generated by 3i (Table 3, entries 3–8 and 10–17). Literature values for hydrophobic analogues shown in open circles.(28)

    Figure 5

    Figure 5. Systematic variation in O ligand trans influence in (P^O)Pd(CH3)Cl complexes 2g and 48 determined from solid state data, and atomic charge in analogous (P^O)Rh(CO)Cl complexes 1217, determined by NBO analysis. Error bars indicate ±3σ.

    Figure 6

    Figure 6. ORTEP diagram of 2g. Thermal ellipsoids are shown at 50% probability. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Pd–P2 2.2181(4), Pd–O1 2.163(1), Pd–C1 2.034(2), Pd–Cl1 2.3824(4), P1–O1 1.490(1), P2–Pd–O1 89.02(3), C23–N2–C26 114.8(1), C23–N2–P1 120.8(1), C26–N2–P1 123.9(1), C29–N1–C32 113.2(1), C29–N1–P1 113.3(1), C32–N1–P1 129.0(1).

    Scheme 3

    Scheme 3. Mechanistic Pathways Controlling Polar Group Distribution in Functional Polyolefins
  • References

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