Reactivity of Difunctional Polar Monomers and Ethylene

Article pubs.acs.org/Macromolecules

Reactivity of Difunctional Polar Monomers and Ethylene Copolymerization: A Comprehensive Account Shahaji R. Gaikwad,† Satej S. Deshmukh,† Vijay S. Koshti,† Suparna Poddar,† Rajesh G. Gonnade,‡ Pattuparambil R. Rajamohanan,§ and Samir H. Chikkali*,†,∥ †

Polymer Science and Engineering Division, ‡Center for Material Characterization, and §Central NMR Facility, CSIR - National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411008, India ∥ Academy of Scientific and Innovative Research, Anusandhan Bhawan, 2 Rafi Marg, New Delhi 110001, India S Supporting Information *

ABSTRACT: A comprehensive picture of insertion of 1,1disubstituted difunctional olefins, their ability to double the functional group density at the same level of incorporation as that of monofunctional olefin, and copolymerization with ethylene has been demonstrated. Exposure of a palladium complex [{P∧O}PdMe(L)] (P∧O = κ2-P,O−Ar2PC6H4SO2O with Ar = 2-MeOC6H4; L = C2H6OS) to methyl 2acetamidoacrylate (MAAA) revealed slight preference for 1,2insertion over 2,1-insertion (1.0:0.7). In contrast, insertion of electron-deficient 2-(trifluoromethyl)acrylic acid (TFMAA) unveiled selective 2,1-insertion {via [(P∧O)PdC5H6F3O2] (11)}. The unstable intermediate 11 undergoes β-hydride and β-fluoride elimination to produce subsequent insertion and elimination products. The identity of elimination products (E/Z)-2-trifluoromethyl)but-2-enoic acid [17(E/Z)] and 2(difluoromethylene)butanoic acid (13) was fully established by 1−2D NMR spectroscopy. These insertion experiments, taken together with insertion rates, suggest that MAAA and TFMAA are amenable to insertion. Polymerization of ethylene with MAAA, TFMAA, acetamidoacrylic acid, 2-bromoacrylic acid, dimethyl allylmalonate, and allylmalonic acid was catalyzed by [{P∧O}PdMe(L)] (L = C2H3N) (5.ACN), and the highest incorporation of 11.8% was observed for dimethyl allylmalonate (DMAM). The changes in the surface properties of the copolymers after incorporation of difunctional olefins were evaluated by measuring the water contact angle. Copolymer with highest (11.8% of DMAM) incorporation revealed a reduced water contact angle of 76°. These findings demonstrate that 1,1-disubstituted difunctional olefins are amenable to polymerization, and incorporation of difunctional olefins in polyethylene backbone leads to the production of relatively hydrophilic polyethylene copolymers.

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INTRODUCTION Coordination−insertion polymerization of ethylene and propylene is one of the most well-established transformations, and roughly 145 million tons of polyolefins are produced annually using this process.1−4 Polyolefins are inherently nonpolar, which limits their application in adhesives, paints, binders, and printing inks, among others. Introducing even a few percent of functional groups on the nonpolar polyolefins is anticipated to significantly enhance material properties of these polymers.5−8 Despite the tremendous progress of olefin polymerization, insertion (co)polymerization of functional olefins is a highly sought-after area of research, in both academia and industry.5−8 The following three features associated with this transformation inhibit the insertion of polar monomers and make it a demanding target.9 (A) One of the major hurdles has been coordination of the functional group installed on the polar monomer to the metal to form σcomplex (Figure 1, A). The consequence of this is catalyst poisoning and loss of activity. (B) The electron-deficient nature of the functional olefin leads to the weak π-complex formation © XXXX American Chemical Society

(as compared to ethylene) and therefore lower or no polar monomer incorporation (Figure 1, B). (C) Formation of the stable chelate complex after insertion of the polar monomer or β-X elimination has been the finishing bottleneck (Figure 1, C and D). The first breakthrough in the functional olefin polymerization was reported by Brookhart in the mid-1990s.10 A cleaver choice of metal, accompanied by diimine ligand, enabled the insertion copolymerization of methyl acrylate (MA) with ethylene (Figure 1, complex 1) for the first time.11 A significant development was the discovery of o-phosphinebenzenesulfonate by Drent et al. in 2002.12 After that, the field of functional olefin polymerization has been energized by the presence of palladium complex of o-phosphinebenzenesulfonate (2). The palladium complex 2 catalyzes insertion copolymerization of a wide range [Figure 1, (iii)] of polar vinyl and allyl monomers with decent activities. Incorporations of as high as Received: June 24, 2017

A

DOI: 10.1021/acs.macromol.7b01356 Macromolecules XXXX, XXX, XXX−XXX

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Thus, the aim of several investigations was to improve the incorporation of functional groups on the polyethylene backbone to make functional polyethylene. We anticipated that insertion of difunctional monomer would double the functional group density in the resultant copolymer, at a similar percentage incorporation of monofunctional polar monomer. In a preliminary communication, Gaikwad et al. reported the insertion copolymerization of industrially relevant, 1,1-disubstituted olefins, ethyl cyanoacrylate (ECA: super glue), and trifluoromethylacrylic acid (TFMAA).38 A neutral, acetonitrile ligated palladium−phosphinesulfonate complex 5.ACN [{P∧O}PdMe(L)] (P∧O = κ2-P,O−Ar2PC6H4SO2O with Ar = 2-MeOC6H4; L = CH3CN) enabled 6.5% ECA and 3% TFMAA incorporation. Beyond this initial report, no comprehensive picture of insertion modes of 1,1-disubstituted polar olefins and their reactivity exists.39 We now present a full account of organometallic intermediates encountered in insertion and explain the mode of insertion of 1,1-disubstituted difunctional olefins. Remarkably, an electron-deficient 1,1-disubstituted functional olefin revealed preferentially 2,1-insertion, whereas relatively electronrich difunctional olefin follows 1,2-insertion, along with 2,1insertion. The scope of the method has been demonstrated by insertion copolymerization of a wide array of difunctional olefins and difunctional allylic monomers. Finally, the practical significance of the resultant copolymers has been evaluated by determining the water contact angle.

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RESULTS AND DISCUSSION Insertion of 1,1-Disubstituted Difunctional Polar Olefins. The mode of insertion of polar vinyl monomers in a metal−carbon bond is a balancing act between electronic effects and the steric restrictions. In the classical Cossee−Arlman type mechanism, electron-rich monosubstituted olefins display 1,2insertion,40 whereas electron-poor monosubstituted olefins (mostly functional olefins) follow the 2,1-insertion mode.41 This reactivity pattern can be rationalized by assuming that the electronic parameters govern the insertion regioselectivity. In a four-membered transition state, the electrophilic metal atom migrates to the electron-rich carbon atom of the double bond, while the metal-bound nucleophilic carbon atom migrates to the electron-poor carbon atom of the olefin.42 However, this reactivity pattern can be reversed by the appropriate choice of ligand substituents which destabilize the transition state.43,44 Thus, literature reports will convince that both ligand parameters and the monomer structural features influence the regioselectivity of insertion. On the other hand, the insertion chemistry of 1,1disubstituted difunctional monomers is relatively underexplored. The electron-rich 1,1,-disubstituted olefin, isobutene,45 or 2,4-dimethyl-1-pentene46 is known to follow 1,2-insertion. The preferred 1,2-insertion is most likely due to the steric congestion around the disubstituted carbon center. Notably, little information exists on the insertion regioselectivity of difunctional 1,1-disubstituted olefins.47 We choose trifluoromethylacrylic acid (TFMAA) and methyl 2-acetamidoacrylate (MAAA) as the representative of 1,1-disubstituted difunctional olefins. Insertion of Methyl 2-Acetamidoacrylate (MAAA). MAAA is a particularly challenging monomer as there are three donor (ester carbonyls, the N lone pair, and the amide carbonyl) sites that can potentially chelate to the metal. In our attempts to trap the insertion products, complex 5 was treated

Figure 1. State of the art in functional olefin polymerization: (i) major challenges (insrt = insertion; X = leaving group); (ii) most successful catalysts; and (iii) representative list of functional olefins investigated.

52% (of methyl acrylate) could be achieved, although at the cost of reduced polymer molecular weight.13 Detailed investigations by Jordan,14,15 Mecking,16−18 Noza19−23 ki, Sen,24 Claverie,25−27 Rieger,28 and Chen29−32 unveiled the fundamental elements responsible for the success of complex 2. The following structural features associated with complex 2 make it stand out as the most successful catalyst to date. (1) The net neutral charge, which reduces the acidity of the metal center. Thus, the affinity of the metal toward functional groups is reduced, making it functional group tolerant. In addition, palladium is a soft acid, and therefore it has inherently lower affection toward functional groups. (2) Repulsion of filled d-orbitals on palladium by the oxygen lone pairs (on SO3) and consequent back-donation (dπ−pπ) from palladium (dπ) to empty π*-orbital of the functional olefin. (3) Ethylene insertion takes place from a trans-π complex, which is in equilibrium with a nonproductive cis-σ complex. Thus, the ability of 2 to stabilize the trans-π-complex over cis-σ-complex enables incorporation of functional olefins. A judicious steric tuning of complex 2 afforded copolymers with significantly higher molecular weights (up to 177 000 g/mol).33 Inspired by the success of 2, a new bisphosphine−monoxide ligand class with one strong and one weak σ-donor, a structural feature that mimics phosphinesulfonate, was introduced.34 The resultant cationic complex 3 was found to catalyze insertion copolymerization of polar vinyl monomers.35 Along the same line, a palladium complex (4) bearing imidazo [1,5-a]quinolin-9-olate1-ylidene ligand was recently reported.36,37 Complex 4 not only catalyzes ethylene-functional olefin copolymerization, but even propylene−polar monomer copolymerization was achieved. B


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can be assigned to 2,1-insertion product 9/9′. Assignment of these products was corroborated by 1−2D NMR spectroscopy (Figure 2 and Figures S4−S18). Although insertion products were less abundant, the 2D NMR {H−H COSY, H−C HSQC, P−H HMBC} spectra of isolated compounds clearly established the formation of 8/8′ and 9/9′. Proton NMR of compound 8 revealed a singlet at 1.41 ppm that can be ascribed to methyl (−CH3) protons, while Pd-bound diastereotopic −CH2 protons appeared as a multiplet at 1.37 and 1.28 ppm (Figure S8). The 13C NMR of compound 8 revealed downfield shift of the amide carbonyl (173.6 ppm) as compared to ester carbonyl. This observation directly suggested chelation through amide carbonyl and preferred formation of six-membered chelate complex 8 over five-membered ester carbonyl chelate 8′. The spectroscopic findings were further corroborated by single crystal X-ray diffraction, and the existence of 8 could be unambiguously ascertained. Suitable crystals for single crystal X-ray studies were obtained by recrystallizing 8 from dichloromethane. The phosphine and the sulfonate oxygen (P−Pd−O = 94°) are mutually cis to each other, whereas the chelating amide carbonyl (P1−Pd−O4 = 173°) is situated trans to phosphine (Figure 3). The newly (after insertion) formed chelate displays

with excess (2 equiv) methyl 2-acetamidoacrylate in tetrachloroethane to obtain a residue (res-I). 31P NMR of the residue (res-I) in DMSO-d6 revealed three resonances at 26.1, 20.9, and 19.1 ppm in a ratio 1.0:8.5:0.73 (Supporting Information, Figure S3). Although the major peak is identified as multinuclear unreacted 7n48 at 20.9 ppm (Scheme 1), the Scheme 1. Possible Insertion Products of 1,1-Disubstituted Difunctional Monomer: Methyl 2-Acetamidoacrylate (Top) and Trifluoromethylacrylic Acid (Bottom)

remaining two peaks can be assigned to insertion products 8 and 9 (Figure 2 and Scheme 1). Work-up of above reaction mixture led to isolation of two compounds. 31P NMR of the first compound revealed a peak at 25.39 ppm which can be attributed to 1,2-insertion product 8/8′, while the second compound displayed a 31P NMR resonance at 19.04 ppm which

Figure 3. Molecular structure of 1,2-insertion product 8 (thermal ellipsoids are drawn at 50% probability; solvent molecules and hydrogen atoms have been omitted for clarity).

Figure 2. P−H HMBC spectrum of res-I in DMSO-d6 (500 MHz, 298 K).

cis-coordination with an acute C10−Pd−O4 angle of 91.37°. Thus, the molecular structure of inserted species unambiguously confirmed the existence of compound 8 with amide carbonyl coordination and ruled out the possibility of ester carbonyl coordination to the metal center. The 2,1-insertion product 9/9′ was the second major detectable species, which was isolated, and its identity was fully established using combination of spectroscopic and analytical tools. A characteristic singlet at 0.77 ppm can be assigned to terminal methyl (−CH3) group, while the diastereotopic methylene (−CH2) protons give rise to multiplets at 0.35 and 0.16 ppm (Figure S15). The molecular structure of compound 9 was fully established using a combination of 1−2D NMR spectra. These MAAA insertion investigations suggest that 1,2-isertion is preferred over 2,1-insertion, although the preference is only marginal. Insertion of 2-(Trifluoromethyl)acrylic Acid. Palladiumcatalyzed insertion copolymerization of vinyl fluoride (VF) was investigated by Jordan49 and co-workers, and vinyl fluoride was C


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NMR spectroscopy, and the data are consistent with literature reports for methyl ester of 17(E/Z)54,55 (Figures S29−S33). Along the same line, the presence of 13 was established by a combination of 1−2D NMR spectroscopy and literature precedence.55 The elimination intermediates 12 and 16 can potentially initiate further insertion of TFMAA into Pd−H and Pd−F bond leading to species 14/15 and 18/19, respectively. The existence of 18/19 was confirmed using mass spectrometry which established the presence molecular ion peak at m/z = 648.98 Da [M + H]+ (Figure S38). In our attempts to detect the presence of 14, 15, 18, and 19, we recorded 1H, 31P, 19F (Figure 4), 13C, 1H−19F HMBC, and a host of 2D NMR spectra. Among the four probable products (14, 15, 18, and 19), our combined NMR studies indicated presence of compound 15 (Figures S19−S28). The C−H correlation spectral observations were further supported by a long-range F−H (HMBC) correlation spectrum, which revealed a 0.35 ppm proton cross-peak to a −132 ppm fluorine resonance (Figure 4). Thus, the above insertion investigations conclusively establish that 1,1,-disubstituted olefin MAAA favors 1,2-insertion mode, while electron-poor TFMAA almost exclusively yielded 2,1-insertion products. Reactivity of of MAAA and TFMAA. The rate of insertion of functional olefin is a detrimental factor in insertion copolymerization. Because of increased steric around the double bond, 1,1-disubstituted olefins might encounter slower insertion than their monosubstituted counterpart. In order to test this assumption and shed light on the rate of insertion, stoichiometric insertion studies were carried out with a more reactive metal precursor, acetone dimer 6. Acetone dimer 6 was exposed to AgBF4, leading to cloudy gray solution in CD2Cl2. The resultant solution was filtered through a syringe filter into a NMR tube containing functional olefin (20 equiv). The mixture was heated to 60 °C for desired time. The consumption of PdMe was quantitatively monitored with dibromomethane (DBM) as an internal standard. MAAA and TFMAA revealed pseudo-first-order consumption of Pd−Me as indicated by 1H NMR spectroscopy. The pseudo-first-order rate constants of kobs(60 °C) = 2.4 × 10−3 s−1and kobs(60 °C) = 3.5 × 10−3 s−1 were observed for MAAA and TFMAA, respectively (Figure 5). These consumption rates are close to those observed for acrylic acid (1.4 × 10−3 s−1) and methyl acrylate (1.2 × 10−3 s−1) at 25 °C. These rates indicate that the two monomers are amenable to insertion. Copolymerization of Difunctional Olefins. As anticipated from above reactivity studies, 1,1-disubstituted difunctional olefins might undergo insertion with ethylene to produce copolymers, and the important findings have been summarized in Table 1 (Scheme 3). Typically, copolymerization experiments were carried out in a Buechi high-pressure reactor using 5.L (L = CH3CN) as catalyst precursor under desired ethylene pressure at 95 °C. After suitable time, the reactor content was transferred to a flask, and the volatiles were stripped off. Insertion copolymerization of electron-rich MAAA was investigated first. At a very low (0.06 M) concentration, copolymer yields were high, but MAAA incorporation (Table 1, entries 1 and 2) could not be observed. While at higher concentration of MAAA (3.0 M), an enhanced incorporation of 5.8 mol % was observed (Table 1, entry 4). Incorporation was determined by proton NMR using a known literature method.38 A high-temperature proton NMR revealed a characteristic −CH2 resonance at 1.71 ppm (Figure S45),

found to strongly inhibit the copolymerization. Formation of [Pd−F] species via β-F elimination was considered to be one of the reasons for low VF incorporation and reduced reactivity. On the contrary, 8.9 mol % of trifluoropropene (TFP) was found to be incorporated with a very high (15%) fluorine content.50 It was noted that TFP prefers 1,2-insertion, whereas acrylic acid is known to favor 2,1-insertion.51 Therefore, it will be very appropriate to investigate insertion of TFMAA (which hosts both the groups in the same monomer) and shed light on the possible similarity/differences between various insertion modes in VF, TFP, and TFMAA. A reaction between complex 652 and TFMAA was monitored using NMR spectroscopy.53 It has been observed that the DMSO can compete with the monomer for insertion and might reduce the rate of insertion.51 To circumvent the possibility of DMSO competition and to increase the rate of insertion, reactive precursor 6 was employed.51 A reaction between 6 and TFMAA was carried out at 70 °C for 3 h, and NMR was recorded. The possibility of the existence of 1,2-insertion product 10 and 2,1- insertion product 11 was considered. Since 10 cannot decompose via further β-hydride or β-fluoride elimination (Scheme 1), the chances of detecting 10 by NMR were anticipated to be higher than detecting 11. However, the absence of characteristic methylenic (Pd−CH2−) protons ruled out the possibility of formation of 10. In contrast, 11 is vulnerable to both β-hydride and β-fluoride elimination, and in fact, β-H elimination species 16 (Scheme 2) was detected by mass spectroscopy which Scheme 2. Possible β-Hydride/Fluoride Elimination and Subsequent Reinsertion Products of 2(Trifluoromethyl)acrylic Acid

revealed pseudomolecular ion peak at m/z = 506.96 Da [16H]+ (Figure S40). Similarly, the β-F elimination product 12 was detected by mass spectrometry as a pseudomolecular ion peak at m/z = 526.97 [12 + H]+ Da. Although these two species could not be detected by NMR, observation of elimination (from 11) products 13 and 17 (E/Z) unambiguously supported the intermediacy of 11. The existence of 17 (E/Z) as a major product was confirmed by proton NMR, which revealed a methyl resonance at 2.16 (17E) and 2.26 (17Z) ppm (Figure S29). Further, the corresponding olefinic protons appeared at 7.18 and 7.63 ppm for E and Z isomers, respectively. The existence of 17 (E/Z) has been confirmed by using a combination 1−2D D


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Figure 4. Expanded view of 19F NMR spectrum of a reaction of TFMAA with 6 after 3 h at 70 °C.

indicated ethylene insertion into 11 is faster than β-H/β-F elimination reactions from 11 to yield compound 12 or 16 (Scheme 2), while presence of methyl (−CH3) end groups revealed that insertion of TFMAA into Pd−H species (16) might be operative. The ratio of ethylene-terminated end groups to TFMAA-terminated end groups indicates competitive insertion of both monomers into Pd−alkyl/Pd−H species. Polymers containing an amide unit are of particular interest due to specific hydrogen-bonding interactions which render enhanced material properties. Copolymerization of ethylene with various monosubstituted acrylamides has been recently reported.56 However, insertion copolymerization of ethylene with disubstituted acrylamides is not trivial and remains unattended. Reversible addition−fragmentation chain transfer polymerization (RAFT) of acetamidoacrylic acid (AAA) has been achieved using specially designed RAFT reagents.57 The insertion copolymerization of ethylene with acetamidoacrylic acid (AAA) was investigated. A broad proton resonance at 1.81 ppm revealed incorporation of AAA in ethylene chain (Figure S54). While at lower concentration (0.06 M) only traces of AAA could be incorporated, the highest incorporation of 3.4% was observed at 3.0 mol/L concentration of AAA at ambient (1 bar) ethylene pressure. Insertion of vinyl bromide (VB) was investigated by Wolczanski and co-workers, and the rate of insertion of VB was found to be higher than vinyl chloride.58 Though the insertion of VB was studied, insertion copolymerization of VB with ethylene has not been attempted. Along the same line, radical polymerization of 2-bromoacrylic acid (BAA) is known for quite some time;59,60 however, to the best of our knowledge, insertion (co)polymerization is unattempted. Insertion copolymerization of 2-bromoacrylic acid will be particularly challenging, as the monomer can undergo βbromide elimination leading to catalyst poisoning.61 At a concentration of 0.06 mol/L and 1 bar ethylene pressure, only 1.3% BAA incorporation was observed (Table S7). A characteristic proton resonance at 2.02 ppm revealed incorporation of BAA in the polymer chain (Figure S62). Increasing the BAA concentration to 3.0 mol/L resulted in

Figure 5. First-order consumption of Pd-Me by TFMAA and MAAA on a reaction with 6 in the presence of AgBF4 for 1 h at 60 °C (CD2Cl2, 500 MHz, 333 K).

which can be assigned to methylene protons next to the carbon carrying the functional group. Reduced activity at a higher concentration can be ascribed to the strong amide chelation to metal center (Table 1, entries 3 and 4). Trifluoromethylacrylic acid was selected as a representative electron-poor 1,1-disubstituted difunctional olefin, and insertion copolymerization with ethylene was investigated (Table 1, entries 5−8). Incorporation of TFMAA in the polyethylene backbone was attested by the observation of a characteristic proton signal at 2.1 ppm. In addition, existence of the copolymer was unambiguously demonstrated by a combination of spectroscopic and analytical methods. The highest TFMAA incorporation observed was 3.0% at 3 mol/L concentration at 1 bar ethylene pressure (entry 8). In addition to mechanistic findings discussed earlier, chain-end analysis further corroborated intermediacy of species 11 and 16 (Figure S51). The presence of TFMAA-terminated polymer chain ends E


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Macromolecules Table 1. Copolymerization of Difunctional Polar Olefins with Ethylene Using Catalyst 5.ACNa entry

comonomer

conc (mol/L)

P (bar)

yield (g)

activity (×103) (mol of ET/mol of Pd/h)

% incorpb

PDI

Mn × 103 c

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

MAAA MAAA MAAA MAAAd TFMAA TFMAA TFMAA TFMAA AAA AAA AAA AAA BAA BAA DMAM DMAM DMAM DMAM AMA AMA AMA AMA

0.06 0.06 3.0 3.0 0.06 0.06 3.0 3.0 0.06 0.06 3.0 3.0 3.0 3.0 0.06 0.06 3.0 3.0 0.06 0.06 3.0 3.0

5 1 5 1 5 1 5 1 5 1 5 1 5 1 5 1 5 1 5 1 5 1

0.85 0.11 0.11 0.13 0.32 0.18 0.16 0.07 0.57 0.27 0.3 0.1 0.23 0.18 0.73 0.17 0.15 0.03 1.52 0.13 0.29 0.03

1.50 0.20 0.20 0.05 0.51 0.32 0.28 0.12 1.01 0.48 0.53 0.18 0.41 0.32 1.30 0.30 0.27 0.05 2.7 0.32 0.52 0.05

tr tr 0.65 5.8 0.3 0.7 2.3 3.0 tr 0.79 1.6 3.4 1.1 1.3 0.5 1.8 8.4 11.8 tr 0.37 0.37 6.51

2.60 2.38 1.86

10.7 10.3 6.0

131 127

2.33

3.9

1.17 1.24 2.39 1.36 2.25

2.0 2.7 9.6 6.2 4.8

129 120 119

1.44

3.5

2.0

4.2

1.26 2.5 1.54 1.34 1.63 1.83

4.1 3.0 5.1 3.2 2.6 1.1

Tm (°C)e

127

126 120 118 98/115

111

Conditions: P = ethylene pressure, solvent = toluene (50 mL), temperature = 95 °C, time = 1 h, catalyst (5.ACN) = 20 μmol, BHT = 169 μmol. b% incorporation = determined by high-temperature 1H NMR, tr = traces. cMn = determined by HT-GPC in 1,2,4-trichlorobenzene at 160 °C. d Polymerization was carried out for 5 h. eDetermined by DSC. a

allylmalonate (DMAM) and allylmalonic acid (AMA) were selected. Copolymerization of dimethyl allylmalonate with ethylene was performed under optimized conditions. Proton resonance at 1.40 and 1.95 ppm (Figure S68) indicated incorporation of DMAM into the polymer backbone. The proton NMR observation was further confirmed by 13C NMR which revealed a carbon resonance at 36.5 ppm (Figure S69). Thus, at lower concentration (0.06 M) and 5 bar ethylene, 0.5% DMAM incorporation was observed (entry 15). While at the same concentration (0.06 M) but at lower ethylene pressure (1 bar), 1.8% DMAM incorporation was witnessed. The percentage incorporation further increased to 11.8% at 1 bar ethylene pressure and 3.0 mol/L DMAM concentration. Though higher incorporation of comonomer was observed at higher comonomer concentration, this came at the expense of lower catalytic activity and lower molecular weight (entry 18). The above findings suggest that like their parent monosubstituted allyl monomers, difunctional allyl monomers are equally tolerated by 5.ACN, and better incorporation could be achieved. The scope of the disubstituted allyl monomer was expanded to allylmalonic acid. In chain incorporation of AMA was detected by NMR spectroscopy, which revealed a characteristic methylene (−CH2) proton resonance at 1.41 and 2.35 ppm (Figure S76). At a lower concentration of 0.06 mol/L of AMA at 5 bar ethylene pressure, only trace incorporation of AMA (entry 19) was observed. Increasing concentration of AMA led to surged incorporation of AMA with the highest enchainment of 6.51% (entry 22) but at the cost of reduced activity. Thus, it is demonstrated that 1,1-disubstituted difunctional olefins and difunctional allyl monomers can be incorporated into a linear polyethylene chain in a direct approach to functional polyethylene.

Scheme 3. Insertion Copolymerization of Various 1,1Disubstituted Difunctional Monomers with Ethylene

almost similar incorporation of 1.3% (Table 1, entries 13 and 14). Apparently, among the four 1,1-disubstituted functional olefins investigated, lowest incorporation was obtained in the case of BAA. Although β-Br elimination is a competitive side reaction, ethylene insertion appears to be faster, leading to BAA incorporation. Motivated by the successful insertion copolymerization of 1,1-disubstituted difunctional olefins with ethylene, we set out to assess the insertion polymerization of 1,1-disubstituted allyl monomers. Although insertion copolymerization of monofunctional allyl monomers has been reported by Nozaki and coworkers,62 insertion copolymerization of disubstituted allyl comonomers has never been attempted before. As a representative of 1,1-disubstituted allyl monomers, dimethyl F


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Figure 6. Water contact angle for pure polyethylene and copolymers containing 1,1-disubstituted functional olefins and difunctional allyl monomers (an error of ±3° is applicable).

Determination of Hydrophilicity. As stated in the Introduction, incorporation of even small amount of functional groups in the polyethylene backbone can drastically alter the physical properties of functional polyethylene. For example, incorporation of functional groups would render hydrophilicity to polyethylene. The hydrophilicity of these copolymers can be deduced by measuring water contact angle (WCA). A copolymer film was prepared by melting the polymer sample on a glass slide and spreading it uniformly. The water contact angle of various copolymers was determined by a sessile drop method. As a reference, polyethylene was prepared using the same catalyst, and water contact angle was recorded, which was found to be 112°. MAAA-derived copolymer with 1.8 mol % MAAA incorporation displayed slightly reduced WCA of 97° (Figure 6). Along the same line, TFMAA with 2.3% incorporation revealed a WCA of 91°, while a dramatic reduction in water contact angle to 76 ± 3° was observed for ethylene copolymers containing 11.8% of incorporated DMAM. These findings clearly demonstrate that the hydrophilicity of polyethylene copolymers has increased (as compared to pure polyethylene) due to the incorporation of functional groups.

difunctional olefin, 2-(trifluoromethyl)acrylic acid was investigated using metal precursor 6. Interestingly, only 2,1insertion/elimination products could be observed by multinuclear NMR and mass spectroscopy. The first insertion intermediate 11 is unstable and undergoes β-hydride and βfluoride elimination to give elimination products 13, 17 (E/Z), 12, and 16. The former elimination products (13 and 17) could be fully mapped by 1−2D NMR spectroscopy.64 Among the subsequent insertion products 14, 15, 18, and 19, NMR findings indicate the presence of 15. The rate of insertion of MAAA and TFMAA into the Pd−Me bond was investigated by proton NMR. These NMR investigations revealed pseudo-firstorder consumption of Pd−Me. As MAAA and TFMAA were found to be amenable to insertion, we investigated insertion copolymerization of MAAA with ethylene. Complex 5.ACN tolerates both ester and amide groups and successfully copolymerizes MAAA with ethylene. High-temperature NMR measurements revealed an unprecedented incorporation of 5.8% at 3.0 mol/L concentration of MAAA and 1 bar ethylene pressure. Along the same line, various difunctional olefins (TFMAA-3%, AAA-3.4%, and BAA1.3%) and allyl monomers (DMAM-11.8% and AMA-6.5%) could be incorporated in the polyethylene chain. The practical significance of incorporation of doubly functional olefins in ethylene copolymer was demonstrated by measuring the water contact angle. As evidenced, the water contact angle is reduced from 112° for PE to 76° for the ethylene copolymer of DMAM (Figure 6). Thus, a comprehensive picture of insertion of difunctional olefins, their copolymerization with ethylene, and surface properties of the resultant functional polyethylene is presented.

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CONCLUSIONS In summary, insertion of 1,1-disubstituted difunctional olefins was investigated in great detail, and a comprehensive mechanistic picture has been presented for the first time. Exposure of a relatively electron-rich difunctional olefin, methyl 2-acetamidoacrylate (MAAA), to palladium complex 5 revealed existence of 1,2- and 2,1-insertion products 8 and 9 with a slight preference to the former (ca. 1:0.7). In contrast to the preferred 2,1-insertion of methyl acrylate48 and acrylamide,56 MAAA was found to favor 1,2-insertion to yield 8.63 Both modes of insertion favor amide chelate over ester chelation. In fact, amide chelation was evidenced in the solid state structure of compound 8 (Figure 3). Insertion of electron-deficient

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EXPERIMENTAL SECTION

General Methods and Materials. Unless noted otherwise, all manipulations were carried out under an inert atmosphere using G


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P NMR (500 MHz, CDCl3, 298 K): δ = 25.39. 13C NMR (125 MHz, CDCl3, 298 K): δ = 173.6 (s, Cf), 172.7 (s, Cd), 160.5 (s, Ar−C), 134.2 (s, Ar−C), 133.8 (m, Ar−C), 133.5 (m, Ar−C), 130.4 (m, Ar− C), 128.1 (s, Ar−C), 120.7 (s, Ar−C), 116.6 (m, Ar−C), 115.0 (m, Ar−C), 111.4 (s, Ar−C), 58.9 (s, Cb), 53.3 (s, Ce), 26.8 (s, Ca), 26.3 (s, Hc), 23.5 (s, Cg). 31

standard Schlenk line techniques or m-Braun glovebox. Toluene was distilled from sodium, diethyl ether, and THF from sodium/ benzophenone under an argon atmosphere. Acetonitrile and methylene chloride were distilled on calcium hydride. Ethylene (99.995%; 5 grade) was supplied by Ms. Vadilal Chemicals Ltd., Pune, India. Methyl acetamidoacrylate, methyl acetamidoacrylic acid, trifluoromethylacrylic acid, 2-bromoacrylic acid, 2-allylmalonic acid, and dimethyl allylmalonate were supplied by Sigma-Aldrich and were used as received. [(COD)PdMeCl], 65 [2-(2-methoxyphenyl)phosphino]benzenesulfonic acid,66 and phosphinesulfonato palladium complexes 57 and 621 were synthesized following known procedures. The copolymerization was run in a Buechi glasuster cyclone 075 highpressure reactor equipped with overhead mechanical stirrer, heating/ cooling jacket, and pressure regulators. Solution NMR spectra were recorded on a Bruker Avance 200, 400, 500, and 700 MHz instruments. Chemical shifts are referenced to external reference TMS (1H and 13C) or 85% H3PO4 (31P). Coupling constants are given as absolute values. Multiplicities are given as follows: s = singlet, d = doublet, t = triplet, and m = multiplet. Hightemperature NMR of the copolymers was recorded in C2D2Cl4 or C6D6 + TCB (10:90) solution. Mass spectra were recorded on a Thermo Scientific Q-Exactive mass spectrometer; the column specification is Hypersil gold C18 column 150 × 4.6 mm diameter; 8 μm particle size; mobile phase used is 90% methanol + 10% water + 0.1% formic acid. A differential scanning colorimeter (DSC) was carried out on DSC Q-10 from TA Instruments at a heating and cooling rate of 10 K min−1. High-temperature gel permeation chromatography (HT-GPC) of the polymers was recorded in 1,2,4trichlorobenzene at 160 °C on a Viscotek GPC (HT-GPC module 350A) instrument equipped with the triple detector system. The columns were calibrated with linear polystyrene standards, and the reported molecular weights are with respect to polystyrene standards. However, due to low molecular weight copolymers, only refractive index detector was considered while determining the molecular weight of the copolymer. Water contact angle measurements were carried out using GBX model from DIGIDROP instruments. Stoichiometric Insertion Reaction of Methyl Acetamidoacrylate with 5. 110 mg (0.18 mmol) of 5 and 52 mg (0.36 mmol, 2.0 equiv) of methyl acetamidoacrylate were weighed in a glove box and transferred to a Schlenk tube with a magnetic needle. To this mixture, 5 mL of tetrachloroethane was added, and the reaction mixture was heated for 6 h at 90 °C. After this time period reaction mixture was cooled down to room temperature, and diethyl ether (30 mL) was added upon which solid precipitate was observed. The resultant solid residue was separated by cannula filtration, washed with diethyl ether (5 mL × 2), and dried under reduced pressure at room temperature to obtain res-I (49 mg). 31P NMR shows three distinct species in a ratio of 1:8.5:0.7 at 26.06, 20.95, and 19.08 ppm. Filtrate (tetrachloroethane + diethyl ether layer) was evaporated under reduced pressure and washed with diethyl ether to yield res-II, which was found to contain compounds 8 and 9. Compound 8 was isolated from Res-II upon multiple extraction−reprecipitation cycles (dissolution in 2 mL of DCM, followed by precipitation with 10 mL of diethyl ether). The dichloromethane insoluble fraction (from res-II) left behind was found to be pure compound 9. The possible products of MAAA insertion have been depicted in Scheme S1. Formation of intermediate complexes 8 and 9 could be detected by 2D NMR spectroscopy of res-I. ESI-MS (+ve) C27H30NO8PPdS (8), calculated m/z = 665.05; observed m/z = 666.05 [M + H]+; m/z = 688.04 [M + Na]+; C26H28NO8PPdS (elimination and reinsertion product of 9; see Scheme S1) calculated m/z = 651.03; observed m/z = 652.04 [M + H]+. Key Resonances of Insertion Products. 7n: 1H NMR (700 MHz, DMSO-d6, 298 K): δ = 0.09 (s, 16H, H), 3.60 (s, 36H, H). 31P NMR (500 MHz, DMSO-d6, 298 K): 20.95. 13C NMR (175 MHz, DMSO-d6 298 K): 55.9 (s, OCH3), 0.6 (s, CH3). 8: 1H NMR (500 MHz, CDCl3, 298 K): δ = 8.15 (s, 1H, Hi), 8.02 (s, 1H, NH), 7.52 (m, 4H, Ar−H), 7.39 (m, 2H, Ar−H), 7.03 (m, 3H, Ar−H), 6.91−6.86 (m, 2H, Ar−H), 3.59 (s, 3H, He), 3.56 (s, 6H, Hh), 2.16 (s, 3H, Hg), 1.41 (s, 3H, Hc), 1.37 (s, 1H, Ha), 1.28 (bs, 1H, Ha).

9: 1H NMR (500 MHz, DMSO-d6, 298 K): δ = 8.71 (s, 1H, Hx), 7.74 (m, 1H, Ar−H), 7.56 (m, 2H, Ar−H), 7.46 (m, 2H, Ar−H), 7.36 (m, 2H, Ar−H), 7.10 (m, 4H, Ar−H), 3.56 (s, 3H, Hw), 3.49 (s, 3H, Hw), 3.33 (s, 3H, Ht), 2.09 (s, 3H, Hv), 0.77 (s, 3H, Hr), 0.35 (m, 1H, Hq), 0.16 (m, 1H, Hq). 31P NMR (500 MHz, DMSO-d6, 298 K): δ = 19.0. 13C NMR (125 MHz, DMSO-d6, 298 K): δ = 179.0 (s, Cu), 174.1 (s, Cs), 160.2 (s, Ar−C), 136.2 (s, Ar−C), 134.3 (s, Ar−C), 133.6 (s, Ar−C), 130.1 (s, Ar−C), 128.2 (s, Ar−C), 120.3 (s, Ar−C), 112.1 (m, Ar−C), 64.1 (s, Cp), 50.7 (s, Ct), 28.9 (s, Cq), 17.7 (s, Cv), 11.5 (s, Cr).

Stoichiometric Insertion Reaction of TFMAA with 6. Acetone dimer 6 (22.3 mg, 0.021 mmol) and AgBF4 (8.3 mg, 0.043) were added to the vial in glove box. 0.3 mL of CD2Cl2 was added to above vial leading to cloudy gray reaction mixture, which was shaken for 5 min and filtered through syringe filter into a high-pressure NMR tube containing 60.2 mg (0.43 mmol, 20.5 equiv) of TFMAA. The resulting clear yellow solution was heated to 70 °C for 3 h, and NMR was recorded. The possible insertion products have been depicted in Scheme S2. Among various possible insertion/elimination products, some of the products were traced using 1−2D NMR spectroscopy and the others by mass spectroscopy. In-depth analysis of TFMAA insertion/ elimination products almost ruled out the possibility of 1,2-insertion, as neither 1,2-insertion product 10 nor the elimination/insertion products derived from 10 could be observed. On the other hand, 2,1insertion product 11 and subsequent β-H and β-F elimination intermediates 12 and 16 could be detected by mass spectroscopy. The mass analysis was further supported by NMR spectroscopy which revealed the presence of elimination product 13 and 17 (E/Z). The existence of 17 (E/Z) and 13 was fully established by a combination of spectroscopic and analytical methods, and the observed characteristic features comply with known literature data for 17.54,55 Key Resonances for Insertion and Elimination Products. 15: 1H NMR (500 MHz, CD2Cl2, 298 K): δ = 0.35 (t, 2H, Ha). 13C NMR (125 MHz, CD2Cl2, 298 K): δ = −2.9 (t, Ca), 117.9 (s), 162.5 (s). 19F NMR (376 MHz, CD2Cl2, 298 K): δ = −132.0 (s). 17E: 1H NMR (500 MHz, CD2Cl2, 298 K): δ = 2.16 (s, 3Ha), 7.18 (s, 1Hb). 13C NMR (125 MHz, CD2Cl2, 298 K): δ = 15.6 (Ca), 135.2 (Cc), 144.6 (Cb), 121.4 (Cd). 19F NMR (376 MHz, CD2Cl2, 298 K): δ = −59.4 (s, CF3).

17Z: 1H NMR (500 MHz, CD2Cl2, 298 K): δ = 2.26 (s, 3Ha′), 7.63 (s, 1Hb′). 13C NMR (125 MHz, CD2Cl2, 298 K): δ = 16.2 (Ca′), 135.9 H


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(Cc′), 149.6 (Cb′), 122.2(Cd′). 19F NMR (376 MHz, CD2Cl2, 298 K): δ = −59.2 (s, CF3). 13: 1H NMR (500 MHz, CD2Cl2, 298 K): δ = 0.88 (s, 3H), 2.43 (s, 2H). 13C NMR (125 MHz, CD2Cl2, 298 K): δ = 16.2 (Ca), 28.9 (Cb), 118.8 (Cc). 19F NMR (376 MHz, CD2Cl2, 298 K): δ = −71.0 (s, =CF2). Copolymerization of Methyl Acetamidoacrylate with Ethylene. The ethylene−methyl 2-acetamidoacrylate copolymerization was carried out in a 50 mL high pressure glass reactor (Buechi) equipped with mechanical stirrer and heating/cooling jacket. Prior to the experiment, the reactor was heated in vacuum to 80 °C for 30 min, cooled to room temperature and was filled with argon. The reactor was flushed with ethylene (3 times, 10 bar) and was charged with an appropriate quantity of toluene under positive ethylene stream. Next, the reactor was pressurized to 5 bar and saturated with ethylene for 30 min at desired reaction temperature before it was cooled to room temperature. A solution of butylated hydroxyl toluene (BHT, 37 mg, 0.16 mmol), the calculated amount of methyl 2-acetamidoacrylate (diluted in 10 mL of toluene) and catalyst solution (12 mg, 20 μmol in 10 mL DCM) was introduced into the reactor at room temperature. The reactor was then pressurized to 1 or 5 bar with constant stirring and appropriate temperature (95 °C) was reached within 1−5 min. The polymerization was routinely carried out for 60 min, the excess ethylene was slowly vented off and the reactor was allowed to cool down to room temperature. The resultant solution was transferred to round-bottom flask, evaporated in vacuum to obtain solid mass, which was further dried under reduced pressure at 50 °C for 8 h or until constant weight is obtained. The unreacted monomer and BHT were washed with excess of diethyl ether and copolymer was further dried for several hours. The resultant copolymers were characterized using high temperature 1H NMR spectroscopy and methyl 2-acetamidoacrylate incorporation was determined using the 1 H NMR. Representative 1H NMR spectra of the copolymer is depicted in Figure S45. The reactivity of catalyst 5.ACN in the copolymerization of other difunctional monomers was tested and the results are summarized in Table 1. Key Resonances. 1H NMR (500 MHz, TCB + C6D6, 403 K): δ = 1.30 (s, Hβ,γ, ), 1.71 (s, Hb, −COCH3), 3.63 (s, He, OCH3). 13C NMR (125 MHz, TCB + C6D6, 403 K): δ = 27.2 (Cδ), 29.7 (Cγ), 30.5 (Cβ), 37.2 (Cα), 52.0 (Ce, OCH3), 164.2 (Cc), 170.0 (Cd, COOMe).

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ACKNOWLEDGMENTS Financial support from SERB-DST (SR/S2/RJN-11/2012 and EMR/2016/005120), India, is gratefully acknowledged. We thank Mrs. D. Dhoble for HT-GPC analysis. S.R.G., S.S.D., and V.S.K. thank CSIR and UGC for the senior research fellowship. CSIR-National Chemical Laboratory is gratefully acknowledged for additional support. S.H.C. is indebted to AvH foundation Bonn, Germany, for equipment grant.

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DEDICATION Dedicated to Dr. Swaminathan Sivaram on the occasion of his 70th birthday. REFERENCES

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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.7b01356. Synthetic procedure, characterization of copolymers, key insertion products, spectroscopic and analytical data (PDF) Structure of 8 (CCDC number: CCDC 1532879) (CIF) CheckCIF file (PDF)

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AUTHOR INFORMATION

Corresponding Author

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

Rajesh G. Gonnade: 0000-0002-2841-0197 Samir H. Chikkali: 0000-0002-8442-1480 Notes

The authors declare no competing financial interest. I


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Macromolecules (52) For comparative investigations Pd−DMSO complex 5 was used for initial insertion studies. Later more reactive Pd−acetone dimer 6 was utilized for insertion experiments. (53) See Supporting Information for experimental details. (54) Methyl esters of (17E/Z) have been reported by: Palecek, J.; Kvicala, J.; Paleta, O. Fluorinated Butanolides and Butenolides: Part 9. Synthesis of 2-(trifluomethyl)butan-4-olides by Witting Reaction Using Methyl 3,3,3-trifluopyruvate. J. Fluorine Chem. 2002, 113, 177−183. These reported chemical shifts exactly match with the observed NMR data, indicating presence of 17 as the major product. (55) The observed spectroscopic data for 13 comply with the literature report; see: Rangarajan, T. M.; Sathyamoorthi, S.; Velayutham, D.; Noel, M.; Singh, R. P.; Brahma, R. Products formed at Intermediate Stages of Electrochemical Perfluorination of Propionyl and n-Butyryl Chlorides. Further Evidence in Support of NiF3 Free Radical Pathway. J. Fluorine Chem. 2011, 132, 107−113. (56) For insertion copolymerization of ethylene with various monosubstituted acrylamides, see: Friedberger, T.; Wucher, P.; Mecking, S. Mechanistic Insights into Polar Monomer Insertion Polymerization from Acrylamides. J. Am. Chem. Soc. 2012, 134, 1010− 1018. (57) For RAFT polymerization of AAA, see: Dedeoglu, B.; Ugur, I.; Degirmenci, I.; Aviyente, V.; Barcin, B.; Cayli, G.; Acar, H. Y. First RAFT Polymerization of Captodative 2-acetamidoacrylic acid (AAA) Monomer: An Experimental and Theoretical Study. Polymer 2013, 54, 5122−5132. (58) Strazisar, S. A.; Wolczanski, P. T. Insertion of H2CCHX (X = F, Cl, Br, OiPr) into (tBu3SiO)3TaH2 and β-X-Elimination from (tBu3SiO)3HTaCH2CH2X (X = OR): Relevance to Ziegler−Natta Copolymerizations. J. Am. Chem. Soc. 2001, 123, 4728−4740. (59) Balakrishnan, T.; Kasilingam, E. K. Kinetics of Polymerization of Vinyl Monomers Initiated by Lead tetraacetate. J. Polym. Sci., Part A: Polym. Chem. 1986, 24, 1747−1752. (60) Rivas, B. L.; Canessa, G. S.; Pooley, S. A. Co-oligomerization Without Initiator of α-Bromoacrylic Acid and 2-Oxazolie Monomers. Eur. Polym. J. 1992, 28, 43−47. (61) Insertion of vinyl chloride was recently investigated and βchloride elimination was found to be the major termination pathway, see: Leicht, H.; Goetteker-Schnetmann, I.; Mecking, S. Incorporation of Vinyl Chloride in Insertion Polymerization. Angew. Chem., Int. Ed. 2013, 52, 3963−3966. (62) Ito, S.; Kanazawa, M.; Munakata, K.; Kuroda, J.; Okumura, Y.; Nozaki, K. Coordination−Insertion Copolymerization of Allyl Monomers with Ethylene. J. Am. Chem. Soc. 2011, 133, 1232−1235. (63) This observation is in line with insertion of methyl methacrylate, which prefers 1,2-insertion, though 2,1-insertion was also observed; see ref 39. (64) The β-fluoride elimination and ethylene insertion into Pd−F bond have been recently reported; see: Wada, S.; Jordan, R. Olefin Insertion into a Pd−F Bond: Catalyst Reactivation Following β-F Elimination in Ethylene/Vinyl Fluoride Copolymerization. Angew. Chem., Int. Ed. 2017, 56, 1820−1824. (65) Salo, E. V.; Guan, Z. Late-Transition-Metal Complexes with Bisazaferrocene Ligands for Ethylene Oligomerization. Organometallics 2003, 22, 5033−5046. (66) Drent, E.; van Dijk, R.; van Ginkel, R.; van Oort, B.; Pugh, R. I. The First Example of Palladium Catalysed Non-Perfectly Alternating Copolymerisation of Ethene and Carbon Monoxide. Chem. Commun. 2002, 964−965.

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Reactivity of Difunctional Polar Monomers and Ethylene

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