Copolymerization of Ethylene and Vinyl Fluoride by Self-Assembled Multinuclear Palladium Catalysts

The self-assembled multinuclear PdII complexes {(Li-OPOOMe2)PdMe(4-5-nonyl-pyridine)}4Li2Cl2 (C, Li-OPOOMe2 = PPh(2-SO3Li-4,5-(OMe)2-Ph)(2-SO3−-4,5-(OMe)2-Me-Ph)), {(Zn-OP-P-SO)PdMe(L)}4 (D, L = pyridine or 4-tBu-pyridine, [OP-P-SO]3− = P(4-tBu-Ph)(2-PO32−-5-Me-Ph)(2-SO3−-5-Me-Ph)), and {(Zn-OP-P-SO)PdMe(pyridine)}3 (E) copolymerize ethylene and vinyl fluoride (VF) to linear copolymers. VF is incorporated at levels of 0.1–2.5 mol% primarily as in-chain -CH2CHFCH2- units. The molecular weight distributions of the copolymers produced by D and E are generally narrower than for catalyst C, which suggests that the Zn-phosphonate cores of D and E are more stable than the Li-sulfonate-chloride core of C under copolymerization conditions. The ethylene/VF copolymerization activities of C–E are over 100 times lower and the copolymer molecular weights (MWs) are reduced compared to the results for ethylene homopolymerization by these catalysts.


Introduction
The coordination-insertion copolymerization of ethylene with polar vinyl monomers by Pd II catalysts has been extensively studied [1][2][3][4][5][6][7][8]. Vinyl halides are particularly challenging polar comonomers due to (i) their poor competition with ethylene for binding to Pd II catalysts [9,10], (ii) the formation of inactive L n Pd-X complexes by β-X elimination of L n PdCH 2 CXR species (generated by 1,2 CH 2 =CHX insertion or 2,1-CH 2 =CHX insertion followed by chain walking) [11][12][13][14][15][16][17][18][19][20], and (iii) the low insertion reactivity of L n PdCHXCH 2 R species formed by 2,1 CH 2 =CHX insertion [16,21]. We previously reported that (PO)PdMe(L) catalysts (A, Figure 1) that contain phosphine-arenesulfonate ligands (PO − ) copolymerize ethylene and vinyl fluoride (VF) to linear copolymers with up to 0.55 mol% VF incorporation [22][23][24][25]. The catalyst activities are significantly reduced and the polymer molecular weights (MWs) are also reduced in ethylene/VF copolymerization compared to ethylene homopolymerization under the same conditions. For example, {P(2-Et-Ph) 2 (2-SO 3 -Ph)}PdMe(py) exhibits an activity of 296 kg·mol −1 ·h −1 in ethylene homopolymerization and produces polyethylene (PE) with M n = 16,560 Da (300 psi ethylene, toluene, 80 • C), while in ethylene/VF copolymerization the activity and polymer M n are reduced to 5 kg·mol −1 ·h −1 and 6800 Da, respectively (300 psi total pressure, VF/ethylene = 4/1, toluene, 80 • C) [22]. The ethylene/VF copolymers produced by (PO)PdMe(L) catalysts contain internal -CH 2 CHFCH 2 -units formed by 1,2 and/or 2,1 VF insertion into growing (PO)PdR species. The copolymers also contain -CH 2 CHFCH 3 , -CH 2 CHF 2 , and -CH 2 CH 2 F chain ends. The -CH 2 CHFCH 3 units are formed by 2,1 VF insertion into (PO)PdH species followed by chain growth. It was proposed that the -CH 2 CHF 2 and -CH 2 CH 2 F groups are generated by VF or ethylene insertion of (PO)PdF species (formed by β-F elimination of (PO)Pd(CH 2 CHFR) species), followed by chain growth. Strong support for this proposal was provided by the demonstration that (PO BpOMe )PdF(2,6-lutidine)  Figure 1) [35][36][37]. "Pd4 cage" catalyst B produces linear PE with a high MW and linear ethylene/VF copolymers that contain up to 3.6 mol% VF (Table 1, entries 8,9). The ethylene/VF copolymers produced by B contain internal -CH2CHFCH2-units as well as -CH2CHFCH3 and -CH=CHF end groups, but no NMR-detectable -CH2CF2H or -CH2CH2F chain ends. The -CH=CHF chain ends are most likely formed by 2,1 VF insertion into growing (Li-OPO)Pd-R species, followed by β-H elimination. However, B undergoes partial disassembly to the monomeric (Li-OPO)PdMe(py') "Pd1" species under polymerization conditions, which strongly influences the MWs and molecular weight distributions (MWDs) of the polyethylene and ethylene/VF copolymers it produces. For example, B produces a high-MW ethylene/VF copolymer with a broad bimodal MWD (Mw = 494 kDa, PDI = 310; Table 1, entry 9) in hexanes suspension, due to competing copolymerization by intact B (which generates the high-MW fraction) and monomeric (Li-OPO)PdR species (which generates the low-MW fraction). In contrast, B produces a low-MW copolymer with a narrow MWD  Figure 1) [35][36][37]. "Pd 4 cage" catalyst B produces linear PE with a high MW and linear ethylene/VF copolymers that contain up to 3.6 mol% VF (Table 1, entries 8,9). The ethylene/VF copolymers produced by B contain internal -CH 2 CHFCH 2 -units as well as -CH 2 CHFCH 3 and -CH=CHF end groups, but no NMR-detectable -CH 2 CF 2 H or -CH 2 CH 2 F chain ends. The -CH=CHF chain ends are most likely formed by 2,1 VF insertion into growing (Li-OPO)Pd-R species, followed by β-H elimination. However, B undergoes partial disassembly to the monomeric (Li-OPO)PdMe(py') "Pd 1 " species under polymerization conditions, which strongly influences the MWs and molecular weight distributions (MWDs) of the polyethylene and ethylene/VF copolymers it produces. For example, B produces a high-MW ethylene/VF copolymer with a broad bimodal MWD (M w = 494 kDa, PDI = 310; Table 1, entry 9) in hexanes suspension, due to competing copolymerization by intact B (which generates the high-MW fraction) and monomeric (Li-OPO)PdR species (which generates the low-MW fraction). In contrast, B produces a low-MW copolymer with a narrow MWD (M w = 4200, PDI = 2.4; entry 8) in toluene solution, indicative of nearly complete dissociation to monomeric species. We recently reported several new multinuclear Pd "cage" catalysts (C-E) [39,40]. Figure 1), in which four (Li-OPO OMe2 )PdMe(py') units are arranged around the periphery of a Li 4 S 4 O 12 •Li 2 Cl 2 cage [39]. Compound C is much less susceptible to cage dissociation than B. For example, C undergoes only 6.5% dissociation to monomeric species in CDCl 2 CDCl 2 solution at 80 • C ([Pd 4 ] initial = 4.7 mM), whereas B undergoes 38% dissociation under these conditions. In toluene, C is only partially dissociated into monomeric species and therefore produces PE with broad MWD as expected for a multi-site catalyst. In contrast, in hexanes suspension at 80 • C, C is resistant to disassembly and exhibits nearly ideal single-site behavior in ethylene homopolymerization and produces high-MW PE (M w = 1473 kDa, PDI = 2.3, Figure 2). The solvent effects on the MWDs of the PEs produced by B and C may appear to be opposite but simply reflect the relative stabilities of B and C toward disassembly to monomeric species.
Polymers 2020, 12, x FOR PEER REVIEW 4 of 8 The availability of these new, more stable multinuclear Pd assemblies provides an opportunity to probe the reactivity of Pd cage catalysts while reducing the complications from thermal cage disassembly. In this paper we report the ethylene/VF copolymerization behavior of C-E.  [39] and ethylene/VF copolymer (Table  1, entry 4) generated by C determined by high temperature GPC. Polymerization conditions: hexanes solvent, 80 °C.

Results and Discussion
Ethylene/VF copolymerizations were carried out using C as the catalyst at 80 °C in toluene or hexanes (Scheme 1), and the results are shown in Table 1. Experimental details and copolymer The availability of these new, more stable multinuclear Pd assemblies provides an opportunity to probe the reactivity of Pd cage catalysts while reducing the complications from thermal cage disassembly. In this paper we report the ethylene/VF copolymerization behavior of C-E.

Results and Discussion
Ethylene/VF copolymerizations were carried out using C as the catalyst at 80 • C in toluene or hexanes (Scheme 1), and the results are shown in Table 1. Experimental details and copolymer characterization data are provided in the Supplementary Materials. In toluene, C produces low-MW copolymers with ≤0.25 mol% VF incorporation (entries 1,2). In contrast, in hexanes slurry, C generates a high-MW copolymer with 0.87 mol% VF incorporation at a VF/ethylene feed ratio of 0.36 (entry 3), which is increased to 2.5 mol% at a VF/ethylene feed ratio of 0.92 (entry 4). The microstructure of the copolymer produced by C is similar to that from B (Table 2, Figure 3), with major in-chain -CH 2 CHFCH 2 -units and minor VF-derived -CH 2 CFHCH 3 and cis-CH=CHF end groups [41][42][43][44][45][46][47].

Results and Discussion
Ethylene/VF copolymerizations were carried out using C as the catalyst at 80 °C in toluene or xanes (Scheme 1), and the results are shown in Table 1. Experimental details and copolymer aracterization data are provided in the Supplementary Materials. In toluene, C produces low-MW polymers with ≤0.25 mol% VF incorporation (entries 1,2). In contrast, in hexanes slurry, C generates high-MW copolymer with 0.87 mol% VF incorporation at a VF/ethylene feed ratio of 0.36 (entry 3) hich is increased to 2.5 mol% at a VF/ethylene feed ratio of 0.92 (entry 4). The microstructure of the polymer produced by C is similar to that from B (Table 2, Figure 3), with major in-chain 2CHFCH2-units and minor VF-derived -CH2CFHCH3 and cis-CH=CHF end groups [41][42][43][44][45][46][47].   The availability of these new, more stable multinuclear Pd assemblies provides an opportunity to probe the reactivity of Pd cage catalysts while reducing the complications from thermal cage disassembly. In this paper we report the ethylene/VF copolymerization behavior of C-E.

Results and Discussion
Ethylene/VF copolymerizations were carried out using C as the catalyst at 80 °C in toluene or hexanes (Scheme 1), and the results are shown in Table 1. Experimental details and copolymer characterization data are provided in the Supplementary Materials. In toluene, C produces low-MW copolymers with ≤0.25 mol% VF incorporation (entries 1,2). In contrast, in hexanes slurry, C generates a high-MW copolymer with 0.87 mol% VF incorporation at a VF/ethylene feed ratio of 0.36 (entry 3), which is increased to 2.5 mol% at a VF/ethylene feed ratio of 0.92 (entry 4). The microstructure of the copolymer produced by C is similar to that from B (Table 2, Figure 3), with major in-chain -CH2CHFCH2-units and minor VF-derived -CH2CFHCH3 and cis-CH=CHF end groups [41][42][43][44][45][46][47].  The ethylene/VF copolymers produced by C in hexanes exhibit broad MWDs (Table 1, entries 3,4; Figure 2). Given the nearly ideal single-site behavior observed in ethylene homopolymerization The availability of these new, more stable multinuclear Pd assemblies provides an opportunity to probe the reactivity of Pd cage catalysts while reducing the complications from thermal cage disassembly. In this paper we report the ethylene/VF copolymerization behavior of C-E.

Results and Discussion
Ethylene/VF copolymerizations were carried out using C as the catalyst at 80 °C in toluene or hexanes (Scheme 1), and the results are shown in Table 1. Experimental details and copolymer characterization data are provided in the Supplementary Materials. In toluene, C produces low-MW copolymers with ≤0.25 mol% VF incorporation (entries 1,2). In contrast, in hexanes slurry, C generates a high-MW copolymer with 0.87 mol% VF incorporation at a VF/ethylene feed ratio of 0.36 (entry 3), which is increased to 2.5 mol% at a VF/ethylene feed ratio of 0.92 (entry 4). The microstructure of the copolymer produced by C is similar to that from B (Table 2, Figure 3), with major in-chain -CH2CHFCH2-units and minor VF-derived -CH2CFHCH3 and cis-CH=CHF end groups [41][42][43][44][45][46][47].  The ethylene/VF copolymers produced by C in hexanes exhibit broad MWDs (Table 1, entries 3,4; Figure 2). Given the nearly ideal single-site behavior observed in ethylene homopolymerization The availability of these new, more stable multinuclear Pd assemblies provides an opportunity to probe the reactivity of Pd cage catalysts while reducing the complications from thermal cage disassembly. In this paper we report the ethylene/VF copolymerization behavior of C-E. Figure 2. Molecular weight distributions of polyethylene (PE) [39] and ethylene/VF copolymer (Table  1, entry 4) generated by C determined by high temperature GPC. Polymerization conditions: hexanes solvent, 80 °C.

Results and Discussion
Ethylene/VF copolymerizations were carried out using C as the catalyst at 80 °C in toluene or hexanes (Scheme 1), and the results are shown in Table 1. Experimental details and copolymer characterization data are provided in the Supplementary Materials. In toluene, C produces low-MW copolymers with ≤0.25 mol% VF incorporation (entries 1,2). In contrast, in hexanes slurry, C generates a high-MW copolymer with 0.87 mol% VF incorporation at a VF/ethylene feed ratio of 0.36 (entry 3), which is increased to 2.5 mol% at a VF/ethylene feed ratio of 0.92 (entry 4). The microstructure of the copolymer produced by C is similar to that from B (Table 2, Figure 3), with major in-chain -CH2CHFCH2-units and minor VF-derived -CH2CFHCH3 and cis-CH=CHF end groups [41][42][43][44][45][46][47].  The ethylene/VF copolymers produced by C in hexanes exhibit broad MWDs (Table 1, entries 3,4; Figure 2). Given the nearly ideal single-site behavior observed in ethylene homopolymerization tral cage, leading to the formation of new active Pd species [23,25,[48][49][50][51][52]. Consistent wit sal, -CH2CH2F and -CH2CHF2 end groups, which are formed by ethylene and VF inserti pecies in copolymerization by mononuclear catalysts A, are not observed in the copol ced by C (Figure 3) [23]. This process provides a potential catalyst deactivation pathway. igure 3. 19 F{ 1 H} NMR spectrum of ethylene/VF copolymer (o-dichlorobenzene-d4, 120 °C) produced y C (Table 1, entry 4). thylene/VF copolymerization by D (L = 4t Bu-py) was investigated in a m e/chlorobenzene (49/1) solvent at a VF/ethylene feed ratio of 0.92 (Table 1, entry poses in the presence of free 4t Bu-py (and other Lewis bases). Therefore, 1 equiv B(C6F5 py was added to a solution of D in chlorobenzene prior to dilution with toluene to sequ Bu-py that will be displaced by monomer, in order to minimize catalyst deactivation. U onditions, D produces a linear copolymer with 0.96 mol% VF incorporation. The MWD o mer formed by D is unimodal but somewhat broadened (PDI = 4.1), which can be attrib presence of several diastereomeric forms of the catalyst. The copolymer contains in-ch HCH2-and a small amount of VF-derived -CH2CFHCH3 chain ends. Catalyst E beh rly to D, incorporating 1.1 mol% VF in the form of both -CH2CFHCH2-(major) a HCH3 (minor) units (Table 1, entry 7). The MWDs of the copolymers formed by E are na 2.4), indicative of nearly ideal single-site catalysis. These results suggest that the multinu res of D and E are substantially retained during ethylene/VF copolymerization. he ethylene/VF copolymerization activities of C-E are over 100 times lower than the eth olymerization activities, and the copolymer MWs are reduced compared to the resul ne homopolymerization, as observed previously for A and B and mononuclear (PO)P sts.

clusions
Pd cage" catalysts C-E copolymerize ethylene with VF to linear copolymers with 0.1 t VF incorporation depending on the catalyst and reaction conditions. VF is incorpo The ethylene/VF copolymers produced by C in hexanes exhibit broad MWDs (Table 1, entries 3,4; Figure 2). Given the nearly ideal single-site behavior observed in ethylene homopolymerization by C in hexanes, it is unlikely that the broadening of the MWD of the ethylene/VF copolymers formed in this solvent is due to thermal disassembly of the cage structure ( Figure 2). One possibility is that nucleophilic Pd-F species generated by 1,2-VF insertion and β-F elimination react with the Li + ions in the central cage, leading to the formation of new active Pd species [23,25,[48][49][50][51][52]. Consistent with this proposal, -CH 2 CH 2 F and -CH 2 CHF 2 end groups, which are formed by ethylene and VF insertion of Pd-F species in copolymerization by mononuclear catalysts A, are not observed in the copolymer produced by C (Figure 3) [23]. This process provides a potential catalyst deactivation pathway.
Ethylene/VF copolymerization by D (L = 4-t Bu-py) was investigated in a mixed toluene/chlorobenzene (49/1) solvent at a VF/ethylene feed ratio of 0.92 (Table 1, entry 5). D decomposes in the presence of free 4-t Bu-py (and other Lewis bases). Therefore, 1 equiv B(C 6 F 5 ) 3 per 4-t Bu-py was added to a solution of D in chlorobenzene prior to dilution with toluene to sequester the 4-t Bu-py that will be displaced by monomer, in order to minimize catalyst deactivation. Under these conditions, D produces a linear copolymer with 0.96 mol% VF incorporation. The MWD of the copolymer formed by D is unimodal but somewhat broadened (PDI = 4.1), which can be attributed to the presence of several diastereomeric forms of the catalyst. The copolymer contains in-chain -CH 2 CFHCH 2 -and a small amount of VF-derived -CH 2 CFHCH 3 chain ends. Catalyst E behaves similarly to D, incorporating 1.1 mol% VF in the form of both -CH 2 CFHCH 2 -(major) and -CH 2 CFHCH 3 (minor) units (Table 1, entry 7). The MWDs of the copolymers formed by E are narrow (PDI ≤ 2.4), indicative of nearly ideal single-site catalysis. These results suggest that the multinuclear structures of D and E are substantially retained during ethylene/VF copolymerization.
The ethylene/VF copolymerization activities of C-E are over 100 times lower than the ethylene homopolymerization activities, and the copolymer MWs are reduced compared to the results for ethylene homopolymerization, as observed previously for A and B and mononuclear (PO)PdRL catalysts.