Direct Synthesis of Partially Chain-Straightened Propylene Oligomers and P-MA Co-Oligomers Using Axially Flexible Shielded Iminopyridyl Palladium Complexes

In this study, a series of partially chain-straightened propylene oligomers and functional propylene–methyl acrylate (P-MA) co-oligomers were synthesized with 8-alkyl-iminopyridyl Pd(II) catalysts. The molecular weight and polar monomer incorporation ratio could be tuned by using Pd(II) catalysts with various 8-alkyl-naphthyl substituents (8-alkyl: H, Me, and n-Bu). In propylene oligomerization, all the 8-alkyl-iminopyridyl Pd(II) catalysts convert propylene to partially chain-straightened (119–136/1000 C) oligomers with low molecular weights (0.3–1.5 kg/mol). Among the catalysts, Pd1 with non-substituent (H) on the ligand showed the highest activity of 5.4 × 104 g/((mol of Pd) h), generating oligomers with the lowest molecular weight (Mn: 0.3 kg/mol). Moreover, polar-functionalized propylene-MA co-oligomers with very high incorporation ratios (22.8–36.5 mol %) could be obtained in the copolymerization using these 8-alkyl-iminopyridyl Pd(II) catalysts. Additionally, Pd1 exhibited the best performance in propylene-MA copolymerization as it displayed the highest MA incorporation ratio of up to 36.5 mol%. All the three catalysts are capable of generating partially chain-straightened P-MA co-oligomers and the activities decrease gradually while the molecular weight increases with the increasing steric hindrance of the alkyl substituent (H < Me < n-Bu). Compared to Pd4 with the rigid 8-aryl substituent, the flexible 8-alkyl-iminopyridyl Pd(II) catalysts (Pd1-3) not only showed much higher activities in the propylene oligomerization, but also yielded P-MA co-oligomers with significantly higher incorporation ratios in the propylene co-oligomerization.


Introduction
Since polypropylene was first synthesized in 1950s, this type of thermoplastic polymer has become one of the most used plastic products worldwide [1,2]. The excellent chemical and mechanical properties of polypropylene enable it to be used in packaging, manufacturing, the automotive industry, and household appliances [3]. Regardless of its widespread usage, the nonpolar properties of polypropylene limits its utilization. To improve the polarity of polypropylene for adhesion, dye-ability and compatibility, functional groups were always introduced into the material [4,5]. A post-modification technique was commonly used in industry. However, a deterioration of the accompanying properties always happened due to the processes occurring under harsh conditions, such as extreme heat, irradiation with high energy, and excessive etching. Additionally, the uncontrollable functional group distribution and side reactions would always diminish the material's quality [4,5]. Therefore, propylene and functional vinyl monomers copolymerization using transition metal catalysts is the most straightforward and economic strategy to generate functional polypropylenes. Early-transition-metal catalysts exhibit highly efficient propylene polymerization. Additionally, Ziegler-Natta catalysts, metallocene catalysts, and other early-transition-metal catalysts were always easily deactivated by functional-groupcontaining polar monomers, although several cases have shown they could be applied in polar functional monomer and olefin copolymerization when the incorporated polar monomers were protected or tied up by Lewis acid as a "masking reagent" [6][7][8][9][10][11][12][13]. Recently, several cases showed a few cationic metallocene and (pyridylamido)Hf complexes could directly copolymerize propylene and polar monomers bearing the special structure without a "masking reagent" [14][15][16][17].

General Procedures and Materials
All the reactions and polymerizations were carried out using standard Schlenk techniques or a glovebox under a dry nitrogen atmosphere. Anhydrous n-hexane, tolu-

General Procedures and Materials
All the reactions and polymerizations were carried out using standard Schlenk techniques or a glovebox under a dry nitrogen atmosphere. Anhydrous n-hexane, toluene, dichloromethane (DCM) and methyl acrylate were purchased from Ennegy Chemicals (Shanghai, China) and Sinopharm (Shanghai, China). All the solvents and monomers are dried with CaH 2 . The propylene gas was obtained from Nanjing Special Gas Co. (Nanjing, China) and applied in reaction without purification. The complexes Pd1-4 were synthesized according to the methods reported before [52,54]. The deuterated solvents were dried and distilled with CaH 2 . Nuclear magnetic resonance (NMR) spectra ( 1 H: 600 MHz, 13 C: 150 MHz) were recorded on a JNM-ECZ600R NMR instrument (Nippon Electronic Company, Tokyo, Japan) at ambient temperature unless otherwise stated. The chemical shifts observed in the 1 H and 13 C NMR spectra were referenced to the residual resonance of deuterated solvents (the coupling constants are reported in Hz, CDCl 3 : 1 H, 7.26 ppm, 13 C, 77.16 ppm). The molecular weight and molecular-weight distribution of polypropylene (PP), propylene oligomers, and propylene-MA co-oligomers (P-MA) were determined by size exclusion chromatography (SEC) analyses, which were carried out with two linear Styragel SEC columns containing Tosoh equipment (Tosoh Corporation, Tokyo, Japan) and eluted with THF. The system was calibrated with the polystyrene standards and operated at 40 • C at a flow rate of 0.35 mL/min with THF.

Propylene Polymerization
A 350 mL thick-walled pressure vessel together with a magnetic stirrer was charged with 40 mL DCM in the glovebox. The pressure vessel was connected to a Schlenk line bearing high pressure. A solution of Pd1-4 (10 µmol) with NaBArF (20 µmol, 2.0 equiv.) in 2 mL of CH 2 Cl 2 was introduced to the vessel via a syringe at room temperature. The reactor was pressurized and maintained at 4 atm propylene pressure with rapid stirring. The mixture was stirred for 3 h at the reaction temperature. After cooling down, the reactor was quenched in air. Then, the synthesized polymer was obtained via evaporation under vacuum until the weight remained constant.

Copolymerization of Propylene and MA
A 350 mL thick-walled pressure vessel with a stirring bar was charged with an appropriate amount of DCM, MA, and NaBArF in the glovebox. The pressure vessel was connected to a Schlenk line bearing high pressure. The Pd catalysts (20 µmol) in 2 mL CH 2 Cl 2 were syringed into the well-stirred solution with the total reaction volume kept at 20 mL. The reactor was pressurized with propylene and maintained at 4 atm pressure. The copolymerization was terminated by passing through a pad of silica after continuous stirring for 12 h. Then, the formed copolymers were obtained under vacuum to an unchanged weight. X MA % = 3/(I all − 3)×100%. I all : total H integral; I OCH3 = 3: OCH 3 hydrogen integral (ca. 3.5-5.0 ppm).

Results and Discussions Propylene Polymerization
All of the Pd(II) complexes (Pd1-4) (Chart 2) were active in propylene polymerization after in situ dechlorination with 2.0 equiv. sodium tetrakis(3,5-bis(trifluoromethyl)phenyl)borate(NaBArF). The polymerization results are summarized in Table 1. All the 8-alkylnaphthyl Pd(II) complexes showed moderate catalytic activities (21.7-54 kg mol −1 h −1 ), and highly branched propylene oligomers (119-136/1000 C) with various molecular weights (0.3-1.5 kg mol −1 ) were obtained at given conditions (Table 1, Figure 1). Compared to Pd4 with the rigid 8-arylnaphthyl structure, Pd1-3 complexes bearing the 8-alkylnaphthyl structure with higher flexibility exhibited up to ten times higher activities with significantly lower molecular weights. This result is close to the ethylene polymerization results, in which palladium complexes bearing the 8-alkyl group also exhibited activities that were orders of magnitude higher [54]. The axial steric hindrance provided by the flexible substituents would probably facilitate the coordination and insertion of the propylene molecule [35,36,57]. The further comparison of Pd1, Pd2 and Pd3 revealed the higher molecular weight of propylene oligomers could be obtained using the complexes bearing the longer the alkyl group chain (entries 5-6 vs. 3-4 vs. 1-2, Table 1). This may be due to the axial steric effect. A more effective axial steric hindrance provided by longer alkyl substitutes would retard the chain transfer reaction more effectively in polymerization [38,54]. In addition, the activities of all the complexes, Pd14, increased 1.6-2.7 times when the polymerization temperature increased, while the molecular weight decreased (entries 1-8, Table 2). The significantly reduced energy barrier of propylene insertion into the active palladium species with these alkyl groups, and the increased ratio of chain transfer rate to chain growth rate are responsible for the results observed above.
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Copolymerization of Propylene and MA
A 350 mL thick-walled pressure vessel with a stirring bar was charged with an appropriate amount of DCM, MA, and NaBArF in the glovebox. The pressure vessel was connected to a Schlenk line bearing high pressure. The Pd catalysts (20 μmol) in 2 mL CH2Cl2 were syringed into the well-stirred solution with the total reaction volume kept at 20 mL. The reactor was pressurized with propylene and maintained at 4 atm pressure. The copolymerization was terminated by passing through a pad of silica after continuous stirring for 12 h. Then, the formed copolymers were obtained under vacuum to an unchanged weight. XMA% = 3/(Iall -3)×100%. Iall: total H integral; IOCH3 = 3: OCH3 hydrogen integral (ca. 3.5−5.0 ppm). All of the Pd(II) complexes (Pd1-4) (Chart 2) were active in propylene polymerization after in situ dechlorination with 2.0 equiv. sodium tetrakis(3,5bis(trifluoromethyl)phenyl)borate(NaBArF). The polymerization results are summarized in Table 1. All the 8-alkylnaphthyl Pd(II) complexes showed moderate catalytic activities (21.7−54 kg mol -1 h −1 ), and highly branched propylene oligomers (119-136 / 1000 C) with various molecular weights (0.3−1.5 kg mol −1 ) were obtained at given conditions (Table 1, Figure 1). Compared to Pd4 with the rigid 8-arylnaphthyl structure, Pd1−3 complexes bearing the 8-alkylnaphthyl structure with higher flexibility exhibited up to ten times higher activities with significantly lower molecular weights. This result is close to the ethylene polymerization results, in which palladium complexes bearing the 8-alkyl group also exhibited activities that were orders of magnitude higher. [54] The axial steric hindrance provided by the flexible substituents would probably facilitate the coordination and insertion of the propylene molecule. [35,36,57] The further comparison of Pd1, Pd2 and Pd3 revealed the higher molecular weight of propylene oligomers could be obtained using the complexes bearing the longer the alkyl group chain (entries 5−6 vs. 3−4 vs. 1-2, Table 1). This may be due to the axial steric effect. A more effective axial steric hindrance provided by longer alkyl substitutes would retard the chain transfer reaction more effectively in polymerization. [38,54] In addition, the activities of all the complexes, Pd1−4, increased 1.6−2.7 times when the polymerization temperature increased, while the molecular weight decreased (entries 1−8, Table 2). The significantly reduced energy barrier of propylene insertion into the active palladium species with these alkyl groups, and the increased ratio of chain transfer rate to chain growth rate are responsible for the results observed above.        Most interestingly, a chain walking phenomenon was also found in iminopyridyl palladium-catalyzed propylene polymerization ( Figure 2). In contrast to the chain-walkinglead branch formation in ethylene polymerization, chain walking drives partial chain straightening in propylene polymerization ( Figure 3). Theoretically, being derived from the methyl group containing monomer, polypropylene has a branching density of 333/1000 C. All the obtained propylene oligomers or polypropylenes in this study are 119-136/1000 C, which are significantly lower than 333/1000 C. This indicates that a remarkable chain straightening occurs during the polymerization. The calculated value of 1,3 chain straightening is around 0.49-0.54 (Table 1) for all the obtained propylene oligomers or polypropylenes, which indicates almost half of the propylene molecules experience 1,3 chain straightening. The chain walking in polymerization would facilitate the propylene insertion ( Figures S1-S13). The driving force for chain walking is mainly due to the reduction in the steric hindrance during the insertion of propylene molecules in the polymerization process [59]. The 13 C NMR spectrum analysis of a propylene oligomer yield with Pd3 at 30 • C is shown in Figure 3 [59,60]. Branch structures observed include methyl, 2-methylpropyl+, 2-methyl(CH 2 )n+, and 2,4-dimethylpentyl+. The adjacent-methyl and isolated-methyl branches are also observed.   The performance of the iminopyridyl Pd(II) complexes in propylene−MA copolymerization was also explored ( Table 2). In this study, polar-functionalized propyl- The performance of the iminopyridyl Pd(II) complexes in propylene-MA copolymerization was also explored ( Table 2). In this study, polar-functionalized propylene-MA co-oligomers with relatively high values of incorporation ratios (up to 36.5 mol %) were obtained ( Table 2). The Pd(II) catalysts showed low activities (level of 10 3 g mol −1 h −1 ), which were remarkably lower than those in propylene homo-polymerization. As expected, the higher MA concentration resulted in higher MA incorporation, but reduced activities. Compared to propylene homo-polymerization, the molecular weight was dramatically decreased when MA was supplied. However, the molecular weight was not further decreased with the extra MA supplement (Table 2, entries 1, 3, 5, 7 vs. 2, 4, 6, 8). The substantial reduction in molecular weights implied the insertion of excess polar monomers might greatly suppress the growth of the polymer chain and facilitate chain transfer during polymerization (Figures S14-S19) [49,52]. The propylene-MA co-oligomers obtained with Pd2-4 containing substituted alkyl/aryl group tended to produce co-oligomers with higher molecular weights compared to Pd1 (entries 1-2 vs. 3-8, Table 2). It demonstrates the steric effect is crucial for polymer chain propagation over chain transfer, which is similar to the propylene homo-polymerization result. The propylene-MA co-oligomer obtained with the 8-alkylnaphthyl Pd1-3 tended to generate co-oligomers with higher incorporation ratios compared to 8-arylnaphthyl Pd4 (entries 1-6 vs. 7-8, Table 2), which suggests the complexes with flexible alkyl substituents have the advantage of MA insertion.

Conclusions
In conclusion, we described propylene (co)polymerization using a series of 8-alkyl-1-naphthyl iminopyridyl Pd(II) catalysts. Compared to the exhibition of 8-arylnaphthyl Pd4 in propylene polymerization, all the 8-alkylnaphthyl Pd1-3 catalysts showed up to ten times higher activities, but generated propylene oligomers with significantly lower molecular weights. In the corresponding Pd(II)-catalyzed propylene-MA copolymerization, the 8-alkylnaphthyl Pd1-3 catalysts displayed higher MA incorporation ratios with similar molecular weights to 8-arylnaphthyl Pd4. Most notably, all the produced polypropylenes and propylene (co)oligomers are partial chain straightening, with almost half of the inserted propylene monomers undergoing 1,3 chain straightening. These propylene oligomers and