Nickel-catalyzed thermally induced polyethylene (PE), Poly(methyl acrylate) (PMA) diblock copolymerization

We disclose a new route for the preparation of linear polyethylene (PE), poly(methyl acrylate) (PMA) diblock copolymers. The PE segment is synthesized by coordination, insertion polymerization (CIP), and the PMA segment is polymerized through radical polymerization (RP). Notably, no solvent is used during the process. Linear alkyl, nickel species synthesized by the CIP of ethylene under a solid−gas two-phase reaction are thermally activated at room temperature in neat methyl acrylate (MA), which triggers RP. Diblock copolymer formation and its composition are supported by a unimodal SEC shift and 1 H/ 13 C NMR analyses. The presence of small PE crystals in amorphous PMA is implied by powder XRD analysis. The difference in the reactivities of MA and methyl methacrylate (MMA) suggests that RP initiates through the thermal homolytic scission of the C−Ni bond of α-carbonyl Ni species.

Polyethylene (PE) is among the largest classes of plastic materials and is indispensable for large-scale industrial applications. However, its nonpolar, saturated C, C backbone has a low surface energy, which leads to difficulties in certain applications [ 1 ]. For example, polar coatings do not wet the surface of polyethylene well unless a pretreatment is applied [ 2 , 3 , 4 , 5 ]. Furthermore, when polyethylene is blended with polar polymers, the large difference in polarity often leads to macrophase separation. For the same reason, the design and control of stable interfaces with polar fillers are also challenging. The introduction of polar groups onto polyethylene can be realized either by postpolymerization functionalization or copolymerization with polar monomers. Radical copolymerization of ethylene with polar monomers is typically used to synthesize copolymers rich in polar groups [ 6 ]. Owing to the inherent nature of radical polymerization, the products tend to be highly branched structures [ 7 ]. Random coordination insertion polymerization (CIP) of ethylene with polar monomers, which are represented by alkyl acrylates, is an emerging technology for adding polar character to PE. In this case, the PE block can be a linear structure, which is indispensable for the characteristic physical properties of high-density polyethylene (HDPE) [ 8 ]. Notably, there is a trade-off relationship between the acrylate incorporation ratio and “PE-like” properties because the more polar groups that are incorporated, the shorter the PE segmental persistence length becomes [ 9 ]. The PE, acrylate ester diblock polymer retains the crystallinity and mechanical strength of PE while having ester functionalities such as adhesion, surface properties, and solvent resistance [ 10 ].

In the earliest trials for PE-methyl(meth)acrylate ester diblock copolymerization, a stepwise sequence was adopted [ 11 ]: A PE block was first prepared through CIP mediated by a zirconocene, and then the alkylzirconium species was used as an initiator for subsequent group transfer polymerization (GTP) of MMA (Fig. 1a ). After the CIP of PE, the radical polymerization (RP) of a polar monomer was also adopted as a practical route. In 2006, Kawahara and coworkers achieved the first synthesis of a polyethylene, poly(methyl methacrylate) (PE, PMMA) copolymer via reverse addition-fragmentation chain-transfer (RAFT) polymerization of MMA using polyethylene equipped with a chain-transfer agent (Fig. 1b ) [ 12 ]. Building on this concept, in 2008, Subramanian and coworkers reported PE, poly( n -butyl acrylate) (PBA) diblock formation through the CIP/ATRP sequence. These findings demonstrated that radical growth from polyolefin precursors can furnish well-defined block architectures (Fig. 1c ) [ 13 ]. These approaches bypassed catalyst poisoning by polar monomers, preserved distinct structures, and enabled elaborate growth of the polar block. In this lineage, Harth and coworkers proposed light-induced “switchable” concepts for diblock copolymerization in 2018 (Fig. 1d ) [ 14 ]. In such systems, the same palladium complex functions as a CIP catalyst under no-light conditions and as an initiator for the RP of acrylate esters under light conditions. They also reported that the same methodology can be applied to Ni-catalyzed systems [ 15 , 16 , 17 , 18 , 19 ]. In both cases, they used the Brookhart-type late-transition-metal catalysts that favor the catalysis of branched polymer growth as CIP segments [ 20 ]. Although a CIP-block microstructure strongly governs bulk physical properties, forming linear CIP blocks and coupling them to a radical-grown segment have remained elusive. Moreover, within coordination, insertion systems designed to produce linear polyolefins, the formation of radical homo-oligomers of MMA has been reported; in 2015, Mecking and Monteil elucidated the underlying mechanism in detail [ 21 , 22 ]. Here, we report a new reaction manifold that integrates a radical polymer block with a linear polyethylene prepared by a sequence of a CIP process for linear PE and RP for PMA to form linear PE → PMA diblock copolymers by a thermal radical initiation process. No solvents were used during the process. Insertion of one molecule of methyl acrylate (MA) to form an alkyl, Ni complex is the key step in the initiation of the radical process.

This work and the historical landscape of approaches for combining polyethylene with acrylate esters: Stepwise strategies ( a ) The first PE, PMMA diblock copolymer via CIP, GTP sequence, ( b ) The first PE-PMMA polymer hybrid via RAFT polymerization, ( c ) PE, PBA diblock formation through a CP/ATRP ( d ) Metal, organic insertion/light initiated radical polymerization, and This work: PE, PMA termo-induced deblock copolymerization

Nickel complex Me, Ni was synthesized according to a reported procedure⁵ with slight modifications. A reddish-black solution of NiMe₂(py)₂ (194 mg, 785 µmol) in Et₂O (10 mL) was treated dropwise with a solution of the ligand (369 mg, 785 µmol) and pyridine (633 µL, 7.85 mmol) in toluene (5.0 mL). The reaction mixture was stirred overnight, after which the solvent was removed under reduced pressure to yield a dark green residue. The residue was suspended in THF and filtered, and the filtrate was concentrated under reduced pressure to yield a yellow residue. Recrystallization from ether/toluene yielded the desired nickel Me, Ni as a bright yellow solid (430 mg, 691 µmol, 88% yield).

Yellow Me, Ni powder (30.3 µmol: 18.8 mg) was charged into an autoclave in a glovebox under an Ar atmosphere, and then ethylene (1 MPa) was introduced into the same autoclave (with the filling and purging conditions applied 3 times), which had been heated to 150 °C in advance. The system was stirred with a magnetic stirrer under solid/gas biphase conditions at room temperature for 1 h. After the reaction, the autoclave was opened in a glove box (Ar atmosphere). Alkyl, Ni complex was obtained as a film-like yellow solid (48.9 mg). This solid was used in the following reactions without further purification.

After 2.2 , 16.0 mg (approximately 10 µmol) of the obtained solid was then mixed with 1.0 ml of dry MA, which was distilled to preliminarily remove the radical inhibiter and stored in the refrigerator at -30 °C. The yellow suspension was mixed for 1 h at room temperature, and the resulting reddish-brown viscous oil was dried under vacuum at room temperature until the liquid completely evaporated. A total of 48.3 mg of a pale orange solid (solid A ) was obtained (PE, PMA diblock copolymer).

Me, Ni (60.0 µmol: 37.3 mg) was charged into an autoclave (predried at 150 °C) in a glovebox under an Ar atmosphere, and ethylene (1 MPa) was then introduced into the autoclave (after being filled and purged three times), after which magnetic stirring was conducted under solid/gas biphase conditions at room temperature for 1 h. After the reaction, the autoclave was opened in the glove box (Ar atmosphere). Alkyl, Ni complex was obtained as a film-like yellow solid (78.6 mg). This alkyl, Ni complex (13.1 mg ) was then treated with HCl (100 µl, 1 M of ether solution) in 1.0 ml of THF for 12 h. The product was subsequently washed with acetone 3 times and dried in vacuo to yield 17.5 mg of solid B (PE) as a white solid.

A portion of the alkyl-Ni obtained in Section 2.3.2 (65.5 mg, approximately 50 µmol) was pulverized into a fine powder. Afterward, it was mixed with 1.0 ml of dry MA, which was distilled to preliminarily remove the radical inhibiter and stored in a freezer at -30 °C. The yellow suspension was mixed for 12 h at room temperature, and the resulting reddish brown viscous oil was dried under vacuum at room temperature until the liquid completely evaporated. A total of 529 mg of a pale orange solid (solid C ) was obtained (PE, PMA diblock copolymer).

Solid C (501 mg) was then further purified through Soxhlet extraction with 200 ml of 2-butanone at 120 °C for 2 days, which yielded an insoluble white solid D (378 mg).

The 1 H NMR spectrum of the product shows signals from both the PE backbone and the PMA segment (O, CH₃ and α/β-CH) in a single spectrum (Fig. 3a ). The ratio of the PE fragment to the PMA fragment was evaluated on the basis of the integration of the methylene and methoxy signals in the ¹H NMR spectrum. The PE/PMA ratio was estimated to be approximately 2:1. The composition was estimated from the mass balance under the assumption of ideal initiation and complete conversion and thus may deviate from the actual composition of synthesized polymer chains (see Fig. S6 ). The 13 C NMR spectrum (Fig. 3b ) displays peaks from both PE and PMA units. Notably, a small resonance at approximately 32 ppm (B′) is detected, which can be assigned to the carbon atom at the junction between the two blocks [ 23 ]. As expected, the 13 C NMR spectrum of the PE segment shows a single intense resonance for the internal methylene carbons of a linear polyethylene chain at 29 ppm. This result is in contrast to previous examples in which a PE block was connected by CIP with an MA block by RP using a palladium complex of a Brookhart-type diimine ligand, which yielded a branch structure for PE [ 15 , 16 , 17 , 18 , 19 ].

a 1 H NMR (TCE- d 2 , 1 H: 500 MHz, 16 scans, 403 K) and b 13 C NMR ( 13 C: 126 MHz, 8192 scans, 403 K) spectra of solid A of Fig. 2

A size-exclusion chromatography (SEC) chart of the PE/PMA copolymer obtained by sequential addition was compared with that of the first PE block (Fig. 4a ). After ethylene polymerization, protonolysis of the resulting alkyl, Ni yielded solid B . In parallel, the same alkyl, Ni was treated with MA for an additional 12 h, which yielded solid C . Solid C was purified by continuous extraction using a Soxhlet apparatus with 2-butanone at 120 °C for 3 days, which yielded insoluble solid D . SEC measurements were carried out for solids B, C , and D at 140 °C using o -dichlorobenzene (ODCB) as the eluent. Quantitatively, for the PE solid B , M n = 2.5 × 10 5 and M w = 6.6 × 10 5 (Đ = 2.64), whereas after the MA radical polymerization stage, the peaks for PE, PMA solid D shifted to M n = 7.0 × 10 5 and M w = 9.1 × 10 5 (Đ = 1.30), respectively [ 24 ]. The observed higher-molecular shift in the SEC trace of solid B compared to that of solid D is consistent with block copolymer formation, although the shift is relatively modest considering the expected increase in molecular weight upon PMA block formation. In addition, the removal of low-molecular-weight PE and PMA fractions during the purification from solid C to solid D is also suspected. Therefore, the increase in apparent molecular weight from B to D alone is not taken as definitive evidence for block copolymer formation. For solid D , the results of the quantitative 13 C NMR analysis are described in the Supporting Information. SEC analysis revealed a molecular weight of approximately 250,000. This weight corresponds to an initiation efficiency of less than 1%, which implies that most of the Ni, Me precursor remained unreacted during ethylene polymeri