Living Chain-Walking (Co)Polymerization of Propylene and 1-Decene by Nickel α-Diimine Catalysts

Homo- and copolymers of propylene and 1-decene were synthesized by controlled chain-walking (co)polymerization using phenyl substituted α-diimine nickel complexes activated with modified methylaluminoxane (MMAO). This catalytic system was found to polymerize propylene in a living fashion to furnish high molecular weight ethylene-propylene (EP) copolymers. The copolymerizations proceeded to give high molecular weight P/1-decene copolymers with narrow molecular weight distribution (Mw/Mn ≈ 1.2), which indicated a living nature of copolymerization at room temperature. The random copolymerization results indicated the possibility of precise branched structure control, depending on the polymerization temperature and time.


Introduction
Cationic late-transition metal catalyzed homo-and copolymerization of alkenes such as ethylene and propylene have received substantial attention in polymer science and materials chemistry [1][2][3][4][5]. Specifically, polypropylene (PP) is a very popular plastic material used for many applications, namely, in electrical devices, automotive parts, food packaging, household equipment, and many others [6]. At present, due to the epidemic situation of novel coronavirus pneumonia in the world, epidemic prevention materials in many countries are in short supply, especially for medical grade polypropylene raw materials for the production of medical masks.
Chain-walking alkene polymerization [44] can give polymers unique structures, which cannot be obtained by common vinyl polymerization. Chain-branching formation in the polymerization of linear 1-alkene using cationic nickel α-diimine catalysts is shown in Scheme 1 [49]. This mechanism involves 1,2-and 2,1-insertion followed by chain-walking behavior [44], in which the active metal undergoes chain-walking to the terminal or internal carbon followed by monomer insertion [49]. A 2,1-insertion followed by complete chain-walking installs a long methylene sequence [44], and 1,2-insertion gives the methyl branch in 2,ω-enchainment and the n-alkyl branch without chain-walking behavior. While some methyl and alkyl branches are also derived from a small amount of the partial chain-walking to the 1-alkene internal carbon [48], the product properties strongly depend on the microstructure of different branched polyolefins [1]. Generally, the highly branched polyolefins containing methyl and alkyl branches are amorphous, and the chain-straightened linear polymeric products are semicrystalline. For instance, Coates et al. demonstrated that 1-butene polymerization via stereoretentive chain-walking using cationic nickel α-diimine catalysts to produce semicrystalline isotactic 4,2-poly(1-butene) [50]. Cationic "sandwich" α-diimine nickel conducted accurate chain-walking polymerization of higher 1-alkenes to generate semicrystalline "polyethylene" [51].
Polymers 2020, 12, x FOR PEER REVIEW 2 of 11 based nickel catalysts has been reported only in a limited number of cases [46][47][48], and no example for propylene copolymerization with higher 1-alkene. Polymerization of propylene with α-diimine nickel complexes produced PPs with different structures depending on the ortho-substituent groups of the catalysts and the polymerization temperature [44]. Suitable catalysts polymerize propylene to a syndiotactic polymer at a low temperature [46] but to regioregular polymers at higher temperature. Chain-walking alkene polymerization [44] can give polymers unique structures, which cannot be obtained by common vinyl polymerization. Chain-branching formation in the polymerization of linear 1-alkene using cationic nickel α-diimine catalysts is shown in Scheme 1 [49]. This mechanism involves 1,2-and 2,1-insertion followed by chain-walking behavior [44], in which the active metal undergoes chain-walking to the terminal or internal carbon followed by monomer insertion [49]. A 2,1-insertion followed by complete chain-walking installs a long methylene sequence [44], and 1,2insertion gives the methyl branch in 2,ω-enchainment and the n-alkyl branch without chain-walking behavior. While some methyl and alkyl branches are also derived from a small amount of the partial chain-walking to the 1-alkene internal carbon [48], the product properties strongly depend on the microstructure of different branched polyolefins [1]. Generally, the highly branched polyolefins containing methyl and alkyl branches are amorphous, and the chain-straightened linear polymeric products are semicrystalline. For instance, Coates et al. demonstrated that 1-butene polymerization via stereoretentive chain-walking using cationic nickel α-diimine catalysts to produce semicrystalline isotactic 4,2-poly(1-butene) [50]. Cationic "sandwich" α-diimine nickel conducted accurate chainwalking polymerization of higher 1-alkenes to generate semicrystalline "polyethylene" [51]. Our group reported recently that the introduction of phenyl groups in α-diimine nickel system can efficiently improve catalytic performances with a fast chain-walking process [52][53][54][55]. Consequently, these catalysts conducted polymerizations of ethylene [52,53], 4-methyl-1-pentene [54] and linear 1-alkenes [55] to gain high molecular weight polymers and proved living behavior at low temperature. In this work, therefore, the propylene homopolymerization and copolymerization with 1-decene were conducted by phenyl substituted nickel α-diimine complexes activated by MMAO.

General Considerations
All manipulations, unless otherwise mentioned, were carried out using standard Schlenk or in glovebox. Nuclear magnetic resonance ( 1 H, 13 C NMR) spectra were recorded on a Varian 400 NMR Our group reported recently that the introduction of phenyl groups in α-diimine nickel system can efficiently improve catalytic performances with a fast chain-walking process [52][53][54][55]. Consequently, these catalysts conducted polymerizations of ethylene [52,53], 4-methyl-1-pentene [54] and linear 1-alkenes [55] to gain high molecular weight polymers and proved living behavior at low temperature. In this work, therefore, the propylene homopolymerization and copolymerization with 1-decene were conducted by phenyl substituted nickel α-diimine complexes activated by MMAO.

General Considerations
All manipulations, unless otherwise mentioned, were carried out using standard Schlenk or in glovebox. Nuclear magnetic resonance ( 1 H, 13 C NMR) spectra were recorded on a Varian 400 NMR instrument (Varian, Inc., Palo Alto, CA) at room temperature and 50 • C, respectively, using CDCl 3 as a solvent and tetramethylsilane (TMS) as internal standard for the compounds. Gel permeation chromatography (GPC) analyses of the molecular weight and molecular weight distribution of the polymers were performed on a Tosoh HLC-8320GPC chromatograph (Tosoh Asia Pte. Ltd., Tokyo, Japan) at 40 • C using THF as an eluent. Differential scanning calorimetry (DSC) was performed by a SII EXSTAR6000 system (Hitachi, Tokyo, Japan). Research-grade propylene was purified by passing it through dehydration column of ZHD-20 and deoxidation column of ZHD-20A. MMAO (6.5 wt % Al, 2.17 M in toluene) was donated by Tosoh-Finechem Co (Tokyo, Japan). Toluene was dried with sodium/benzophenone under nitrogen atmosphere and distilled before use. n-Hexane, CDCl 3 and CH 2 Cl 2 were purified over 4 Å molecular sieves. 1-Decene was purchased from Kanto Chemical Co on Aldrich Chemical Company (Tokyo, Japan) were dried over CaH 2 and distilled before use. Nickel complexes 1-5 were synthesized according to the literature [55]. Other chemicals were commercially obtained and purified with common procedures.

Polymerization Procedure
The propylene polymerization/copolymerization experiments were performed in a 100-mL glass reactor equipped with a temperature controller and a magnetic stirrer. A 30 mL amount of toluene was added to the dried reactor reactor under N 2 atmosphere.
Propylene polymerization: Propylene (1.2 bar) was introduced to the reactor kept at polymerization temperature after the toluene was saturated with propylene; MMAO was added and the solution was stirred for 10 min. Then, the catalyst solution (10 µmol) in toluene was injected into the polymerization system with a syringe to start polymerization. After the desired amount of polymerization time, the pressure vessel was vented and the reaction was quenched by addition of 5% acidic methanol (HCl/MeOH).
Propylene/1-decene copolymerization: 1-Decene was added to the toluene via a syringe, and the resulting solution was saturated with propylene (1.2 bar) under vigorous stirring for 10 min. Then MMAO was added and the solution was stirred for 10 min. Then, the catalyst solution (10 µmol) in toluene was injected into the polymerization system to start polymerization. After the desired amount of polymerization time, the pressure vessel was vented and the reaction was quenched by addition of 5% acidic methanol (HCl/MeOH).
The PPs and copolymers obtained were filtered from solution, washed with methanol, and dried in a vacuum oven to constant weight.

Time-Course of Propylene and 1-Decene Copolymerization
Copolymerization of propylene and 1-decene was performed in a 100-mL glass reactor equipped with a magnetic stirrer. A 30 mL amount of toluene was added to the fully dried reactor under N 2 atmosphere. After the reactor was set at polymerization temperature, 1-decene (12 mmol) was added to the toluene via a syringe, and the resulting solution was saturated with propylene (1.2 bar) under vigorous stirring for 10 min. Then MMAO was added and the mixture was stirred for 10 min. The catalyst 2 (12 µmol) solution in toluene was added to the reactor (total volume = 40 mL) to start polymerization. The reaction solution (6.6 mL) was sampled six times via a syringe at different polymerization time (5 min, 10 min, 30 min, 50 min, 70 min and 90 min), and the reaction mixture was terminated with 40 mL of a 3% HCl/MeOH solution. The copolymers obtained were filtered from solution, washed with methanol, and dried in a vacuum oven to constant weight.
Single-crystal of 3 was obtained by slowly diffusing of n-hexane into the CH2Cl2 solution of complex at room temperature, and its molecular structure was confirmed by X-ray diffraction ( Figure  1). Crystal data, data collection, and refinement parameters are listed in Table S1 (see the supplementary materials). The geometry at the Ni center is pseudo-tetrahedral, showing pseudo-C2symmetry. The bond length of N1−C20 [1.298(8) Å] and N2−C31 [1.264(8) Å] have typical imine double bonds character, respectively. In the solid state, the Ni1−N1 and Ni1−N2 bond distances of 2.036(6) and 2.017(6) Å in 3 are slightly narrower than the value of 2.041(5) Å for analogue 2 [56]. In addition, the N1-Ni1-Br3 angles of 117.99(7)° for 3 is also approximate to those for complex 2 (112.96°). The conjugation effect of phenyl substituents in the para-and ortho-aryl position of αdiimine ligand can be clearly observed in these molecular structures, expected to improve the catalyst performance for propylene (co)polymerization.

Catalytic Polymerization of Propylene
Upon activation with MMAO, polymerization of propylene was examined by complexes 1-5 with the [Al]/[Ni] ratio of 300 at 1.2 bar of propene for 30 min, and the results are listed in Table 1. The effect of reaction temperature was examined by varying the temperature from 0 to 50 °C ( Single-crystal of 3 was obtained by slowly diffusing of n-hexane into the CH 2 Cl 2 solution of complex at room temperature, and its molecular structure was confirmed by X-ray diffraction ( Figure 1). Crystal data, data collection, and refinement parameters are listed in Table S1 ( Single-crystal of 3 was obtained by slowly diffusing of n-hexane into the CH2Cl2 solution of complex at room temperature, and its molecular structure was confirmed by X-ray diffraction ( Figure  1). Crystal data, data collection, and refinement parameters are listed in  (6) and 2.017(6) Å in 3 are slightly narrower than the value of 2.041(5) Å for analogue 2 [56]. In addition, the N1-Ni1-Br3 angles of 117.99(7)° for 3 is also approximate to those for complex 2 (112.96°). The conjugation effect of phenyl substituents in the para-and ortho-aryl position of αdiimine ligand can be clearly observed in these molecular structures, expected to improve the catalyst performance for propylene (co)polymerization.

Catalytic Polymerization of Propylene
Upon activation with MMAO, polymerization of propylene was examined by complexes 1-5 with the [Al]/[Ni] ratio of 300 at 1.2 bar of propene for 30 min, and the results are listed in Table 1. The effect of reaction temperature was examined by varying the temperature from 0 to 50 °C (

Catalytic Polymerization of Propylene
Upon activation with MMAO, polymerization of propylene was examined by complexes 1-5 with the [Al]/[Ni] ratio of 300 at 1.2 bar of propene for 30 min, and the results are listed in Table 1. The effect of reaction temperature was examined by varying the temperature from 0 to 50 • C ( The effect of different catalyst precursors 1-5 was also studied at 25 • C ( Table 1, entries 2 and 4-7). Phenyl substituted 1-4 produced the PPs in moderate yields with high molecular weights of (11.9-19.1) × 10 4 g mol −1 and narrow molecular weight distribution (M w /M n = 1.16-1.48, Table 1, entries 2 and 4-6). The catalytic activities decreased in the order, 2 ≥ 1 ≥ 3 ≥ 4, and the M n values increased with increasing the ligand bulkiness (4 ≥ 3 ≥ 2 ≥ 1). Complex 2 produced higher molecular weight PP in highest yield than those of one methyl or phenyl groups (1, 3 and 4, Table 1, entries 4-6). The highest molecular weight PP obtained by 4 ( Table 1, entry 6), indicating the rate of chain propagation was greatly promoted by the bulky ortho-phenyl substituent on the N-aryl moiety, which should retard chain-transfer reactions [44]. Complex 2 with the conjugation ability of the para-phenyl substituent exhibited higher activity than the corresponding methyl-substituted 5 ( Table 1, entries 2 vs. 7), as already observed in polymerization of ethylene and higher 1-alkenes [55]. The higher molecular weight PP with narrower molecular weight distribution was obtained by 2 suggests that phenyl substituent should suppress chain-transfer reactions [44].
The effect of polymerization time in propylene polymerization was also studied by 2-MMAO at 25 • C (Table 1, entries 2 and 8-11). The yields and the M n value increased with the prolongation of polymerization time. Figure 2a shows the plot of M n increased linearly polymerization time accompanied with keeping narrow M w /M n = 1.10-1.25 ( Figure S8 from Supplementary Materials), and the N value was almost constant. This polymerization result verified that the propylene polymerization by 2-MMAO proceeded in a living manner at room temperature within a certain period of time. 1 H NMR spectroscopy analyses [57,58] showed that the obtained PPs proceeded only methyl group, while the total branching densities (245-267/1000 C, Table 2) were less than the expected value (333/1000 C), indicating the 1,3-enchainment of propylene via 2,1-insertion followed by chain-straightening. The 1,3-enchainment increased slightly with increasing reaction time and temperature (Table 2). Upon comparison with common polypropylene (PP, T g = −10~−5 • C), the produced PPs had low T g of approximately −29.4~−24.1 • C and no T m ( Table 2), indicating that the PPs were amorphous. The microstructure analyses of the polypropylenes were further studied by 13 C NMR spectroscopy [59,60]. 13 C NMR spectra of polypropylenes obtained by 2-MMAO at 0 and 25 • C are shown in Figure 3 (Table 1, entries 1 and 2), where only normal methyl branch was observed at 0 • C (Figure 3a), while 2-methylhexyl structure was observed at room temperature (Figure 3b). When nickel metal migrates forward along the newly 2,1-added propylene unit, ethylene-type unit coming from 1,3-enchainment can be produced, and backward along the polymer chain, longer branched chains can be formed [46]. Consequently, the microstructures of the polypropylenes were similar to the ethylene-propylene (EP) copolymers. Therefore, 2-MMAO was found to polymerize propylene in a living fashion to furnish EP copolymers.
Polymers 2020, 12, x FOR PEER REVIEW 6 of 11 nickel metal migrates forward along the newly 2,1-added propylene unit, ethylene-type unit coming from 1,3-enchainment can be produced, and backward along the polymer chain, longer branched chains can be formed [46]. Consequently, the microstructures of the polypropylenes were similar to the ethylene-propylene (EP) copolymers. Therefore, 2-MMAO was found to polymerize propylene in a living fashion to furnish EP copolymers.  Table 1) and copolymer yield for P/1-decene copolymerization ((b), Table 3) catalyzed by 2-MMAO at 25 °C.  (Table 1, entries 1 and 2).

Copolymerization of Propylene and 1-Decene
Copolymerization of propylene and 1-decene (D) was catalyzed by 2-MMAO at 0 and 25 °C with the [Al]/[Ni] ratio of 300, and the results are listed in Table 3. In comparison with the 1-decene homopolymerization under the same conditions [55], P/1-decene copolymerization exhibited higher activity and produced copolymers with higher molecular weight and narrow Mw/Mn values. In the P/1-alkene copolymerization study, similar trends were observed to those for propylene and 1-decene homopolymerization. For instance, 2-MMAO exhibited higher activity and higher molecular weight than the corresponding methyl-substituted 5 ( Table 3, entries 1 vs. 3).
Polymers 2020, 12, x FOR PEER REVIEW 6 of 11 nickel metal migrates forward along the newly 2,1-added propylene unit, ethylene-type unit coming from 1,3-enchainment can be produced, and backward along the polymer chain, longer branched chains can be formed [46]. Consequently, the microstructures of the polypropylenes were similar to the ethylene-propylene (EP) copolymers. Therefore, 2-MMAO was found to polymerize propylene in a living fashion to furnish EP copolymers.  Table 1) and copolymer yield for P/1-decene copolymerization ((b), Table 3) catalyzed by 2-MMAO at 25 °C.  (Table 1, entries 1 and 2).

Copolymerization of Propylene and 1-Decene
Copolymerization of propylene and 1-decene (D) was catalyzed by 2-MMAO at 0 and 25 °C with the [Al]/[Ni] ratio of 300, and the results are listed in Table 3. In comparison with the 1-decene  (Table 1, entries 1 and 2).

Copolymerization of Propylene and 1-Decene
Copolymerization of propylene and 1-decene (D) was catalyzed by 2-MMAO at 0 and 25 • C with the [Al]/[Ni] ratio of 300, and the results are listed in Table 3. In comparison with the 1-decene homopolymerization under the same conditions [55], P/1-decene copolymerization exhibited higher activity and produced copolymers with higher molecular weight and narrow M w /M n values. In the P/1-alkene copolymerization study, similar trends were observed to those for propylene and 1-decene homopolymerization. For instance, 2-MMAO exhibited higher activity and higher molecular weight than the corresponding methyl-substituted 5 ( Table 3, entries 1 vs. 3).  The P/1-decene copolymer obtained with 2-MMAO at 25 • C showed narrow molecular weight distribution of 1.14 ( Table 3, entry 1). Time-course of polymerization was investigated with 2-MMAO at 25 • C under the same conditions by sampling method, and the results are summarized in Table 4. The plot of M n against the P-D copolymer yield shows a good linear relationship accompanied by a very narrow polydispersity (M w /M n = 1.10-1.12, Figure 2b), keeping the number of polymer chains (N) constant. This result verified that the P/1-decene copolymerization with 2-MMAO proceeded in a living/controlled manner at 25 • C within a certain period of time. To the best of our knowledge, this is the first example of late transition metal catalyzed living copolymerization of propylene and 1-decene at room temperature. The copolymerization results indicated the possibility of precise microstructure control, depending on the polymerization temperature, which in turn strongly affects the physical polymer properties. The branching densities of the obtained P/1-decene copolymers thus obtained are shown in Table 3. P/1-decene copolymers possessed highly B values in comparison with the corresponding poly(1-decene)s [55]. The branching densities of the P/1-decene copolymer obtained at 0 • C (187/1000 C, Table 3, entry 2) was much higher than those of at room temperature (169/1000 C, Table 3, entry 1). Therefore, the branching density can be controlled by the polymerization temperature. The produced copolymers had T g of approximately −49 • C no T m ( Table 3), indicating that the P/1-decene copolymers were amorphous.

Conclusions
In summary, we conducted the living chain-walking polymerization of propylene using phenyl substituted α-diimine nickel catalysts in combination with MMAO to generate high molecular weights amorphous "ethylene-propylene (EP) copolymers" at room temperature. Copolymerization of propylene and 1-decene was successfully achieved using 2-MMAO, produced highly branched P/1-decene copolymers with high molecular weight and narrow molecular weight distribution. At room temperature, living copolymerization of propylene and 1-decene was observed. The 13 C NMR spectra of the P/1-decene copolymers obtained showed a microstructure almost exclusively composed of methyl and octyl (C8) branches.
Author Contributions: F.W. conceived and designed the experiments; P.L., X.L., M.X., F.Y. and F.W. performed the experiments; F.W. and G.X. analyzed the data and wrote the paper. S.B., X.L. and G.X. checked and revised the paper. All authors have read and agreed to the published version of the manuscript.

Conclusions
In summary, we conducted the living chain-walking polymerization of propylene using phenyl substituted α-diimine nickel catalysts in combination with MMAO to generate high molecular weights amorphous "ethylene-propylene (EP) copolymers" at room temperature. Copolymerization of propylene and 1-decene was successfully achieved using 2-MMAO, produced highly branched P/1-decene copolymers with high molecular weight and narrow molecular weight distribution. At room temperature, living copolymerization of propylene and 1-decene was observed. The 13 C NMR spectra of the P/1-decene copolymers obtained showed a microstructure almost exclusively composed of methyl and octyl (C 8 ) branches.
Author Contributions: F.W. conceived and designed the experiments; P.L., X.L., M.X., F.Y. and F.W. performed the experiments; F.W. and G.X. analyzed the data and wrote the paper. S.B., X.L. and G.X. checked and revised the paper. All authors have read and agreed to the published version of the manuscript.

Conflicts of Interest:
The authors declare no conflict of interest.