Selective branch formation in ethylene polymerization to access precise ethylene-propylene copolymers

Polyolefins with branches produced by ethylene alone via chain walking are highly desired in industry. Selective branch formation from uncontrolled chain walking is a long-standing challenge to generate exclusively branched polyolefins, however. Here we report such desirable microstructures in ethylene polymerization by using sterically constrained α-diimine nickel(II)/palladium(II) catalysts at 30 °C–90 °C that fall into industrial conditions. Branched polyethylenes with exclusive branch pattern of methyl branches (99%) and notably selective branch distribution of 1,4-Me2 unit (86%) can be generated. The ultrahigh degree of branching (>200 Me/1000 C) enables the well-defined product to mimic ethylene-propylene copolymers. More interestingly, branch distribution is predictable and computable by using a simple statistical model of p(1-p)n (p: the probability of branch formation). Mechanistic insights into the branch formation including branch pattern and branch distribution by an in-depth density functional theory (DFT) calculation are elucidated. Selective branch formation from uncontrolled chain walking is a longstanding challenge to generate exclusively branched polyolefins. Here the authors report such desirable microstructures in ethylene polymerization enabled by a nickel catalyst at 30 °C–90 °C that fall into industrial conditions and mimic ethylene-propylene copolymers.

A s the most important polymer by scale, polyolefins like polyethylene (PE) possess various architectures to form versatile products [1][2][3] . Branching is of great importance in these PEs to tune microstructures and thus determine material properties [4][5][6][7][8][9] . Via a traditional pathway, an exclusive branch in the PEs generally comes from a specific α-olefin, which is utilized as the comonomer in ethylene copolymerization. For instance, in the ethylene/propylene (EP) copolymer, linear low-density polyethylene (LLDPE), and polyolefin elastomer (POE) products, branch on the backbone of PEs is originated from C 3 , C 4 , C 6 , or C 8 α-olefins. By comparison, the generation of branch from ethylene as the sole feedstock in the polymerization catalysis is highly attractive with regard to a simplified industrial process and the avoidable use of high-cost α-olefins 10,11 .
Since Brookhart's seminal works in 1995 12-14 on α-diimine late transition metal such as nickel(II) and palladium(II) promoted olefin polymerization [15][16][17][18][19][20][21] , a unique chain walking mechanism that involves a successive β-H elimination followed by a reinsertion event with an opposite regiochemistry has emerged as a powerful tool to generate branches from ethylene alone, allowing for the production of a variety of polyethylene topologies 4,22,23 . For instance, the architecture of polyethylene could readily be adjusted by a simple variation of ethylene pressure 4,[24][25][26] . Over the past more than two decades, however, precise control on branches to produce well-defined polymer microstructures is a long-standing challenge, because of the uncontrolled chain walking process. A mixture of branch patterns including methyl branch and higher branches (C 2 , C 3 , C 4 , and C 4+ ) predominantly occur (Fig. 1). Therefore, selective branch formation is difficult in ethylene polymerization via a chain walking manner. This is particularly challenging when the reaction temperature is high or the degree of branching is very high, which easily induces the formation of a mixture of branch patterns. In view of the huge difficulty on controlling branch pattern, branch distribution (namely interval between two adjacent branches) on the backbone of PE is more elusive thus far. These events evidently inhibit the synthesis of ethylene-propylene copolymers from ethylene at industrial temperatures of 40-70°C 27 .
We now report that the low-cost nickel(II) catalyst shows a superior control on selective branch formation in ethylene polymerization, enabling the production of an exclusive methyl branch pattern with an ultrahigh number in a particularly broad temperature range of 30-90°C that meets the industrial process. Distribution of the formed branches in the obtained ethylenepropylene copolymers is highly selective based on a NMR analysis and is particularly predictable and computable based on a statistical model of probability. The underlying mechanism on the control of branch pattern and distribution is proposed and rationalized by an in-depth DFT calculation.

Results
Catalyst design and ethylene polymerization. As a rationale of catalyst design, the desired nickel precursor ipty-Ni was readily synthesized from the reaction of NiBr 2 (DME) with the rigid and sterically constrained α-diimine ligand 28,29 . The structure and purity of ipty-Ni was identified by multiple techniques including 1 H NMR spectrum, elemental analysis, and X-ray diffraction analysis ( Fig. 2; for detailed structural data, see Supplementary CIF file). As a comparison, ipty-Pd reported by us was also prepared (Fig. 2) 28 , which produced highly branched polyethylenes with unsolved microstructures at low temperatures of <30°C as well. With the activator of modified methylaluminoxane (MMAO), ipty-Ni exhibited high activities for ethylene polymerization to produce high to ultrahigh molecular weight polyethylenes (Table 1). Notably, the degree of branching was very high, which rose with elevating temperature from 30 to 90°C and further increased to reach an ultrahigh level of 200 brs/1000 C (calculated from 1 H NMR data) with decreasing ethylene pressure from 8 bar to 2 bar 30,31 . Dependence of degree of branching on pressure and temperature is in agreement with previous results from a superficial analysis of 1 H NMR spectroscopy [32][33][34][35] .
Exclusive branch pattern and selective branch distribution. To distinguish the branch pattern in these highly branched polyethylenes, instructive 13 C NMR spectra were in depth analyzed to provide insightful information (Table 2). Generally, high reaction temperature leads to the generation of long-chain branches, making selective branch more difficult. In contrast, whatever pressure (2 bar-8 bar) or temperature (30-90°C) varies in our nickel system, the smallest methyl branches are exclusively (99%) detected in all polyethylene samples of Table 1 (for all 13 C NMR spectra, see Fig. 3). This is unprecedented that the branch pattern is unaltered and exclusive at such a high level of >200 brs/1000 C and at such high temperature of 90°C. Importantly, the synthesis High degree of branching is attributed to high frequency of β-H elimination and subsequent re-insertion. Notably, the key exclusive formation of methyl branches means that 2,1-insertion of α-olefins predominates and no event occurs from 2,1-insertion of internal olefin that generates longer branches.
Proposed mechanism and computable branch distribution. To address this issue, we propose a mechanism for the formation of branch pattern and branch distribution (Fig. 4), which will be comprehensively discussed in the DFT calculation section (see below) 40 . Preliminarily, two distinct features should be noted: distribution of two neighboring methyl branches separated by an odd number of methylene (CH 2 ) group is impossible, this means 1,3-Me 2 unit, 1,5-Me 2 unit, and 1,(x-1)-Me 2 units (x = 8, 10, 12…) cannot be generated via a chain walking manner in ethylene polymerization; on the other hand, the formation of 1,2-Me 2 unit and long-chain branches such as 1,(x-1)-Me/Et units (x = 4, 6, 8, 10, 12…) is proposed, despite trace amounts.
In terms of branch distribution, we envision a statistical model of probability. We set up a simple calculation (Table 3, and for details, see Supplementary Excel file for statistic model) that assumes on each step of the proposed scheme ( Fig. 4) either formation of a branch occurs (with probability p) or chain growth occurs (with probability 1-p). That is the 1,4-Me 2 unit structure is formed by a branch formation (probability: p), the 1,6-Me 2 unit structure is formed by one growth step and a branch formation [probability: p(1-p)], and so on. Notably, this model that assumes the probability of branch formation vs. growth is independent of the distance from the previous branch. This calculated data based on the model agrees with experimental quantitative 13 C NMR data quite well (see branch distribution in Table 3). For example, in the Supplementary Excel file we set with p = 70.5% of 1,4-Me 2 unit (type into field B1 as 0.705), and then 20.8% of 1,6-Me 2 unit (21.0% from NMR data), 6.1% of 1,8-Me 2 unit, 1.8% of 1,10-Me 2 unit, 0.5% of 1,12-Me 2 unit and more units are automatically generated (also input the other p value in Supplementary Excel).
These highly branched polyethylenes with exclusive methyl branches produced by the palladium precursor ipty-Pd were further analyzed with regard to branch distribution ( Table 2, entries 8-10). At 0°C, the selectivity of methyl branch pattern is 99%, and most notably the selectivity of 1,4-Me 2 branch distribution reaches the highest value of 86%. As anticipated, distribution of these branches is also computable. Note that long-chain branches generate at elevated temperatures of >30°C in palladium species, indicating a reduced control on chain walking relative to nickel species. To the best of our knowledge, in chain walking polymerization selective branch pattern of methyl group generally occurs in the α-olefin reaction via a 1,2-insertion and subsequent ω, 2-enchainment 41-44 ; however, highly selective 1,4-Me 2 branch distribution is thus far only generated in 1-butene or trans-2-butene reactions at low temperatures such as −40°C [45][46][47][48][49] .  Focusing on the microstructure of these ethylene-propylene copolymers produced by ethylene polymerization, only 1,4-, 1,6-, 1,8-, and 1,(2n + 2)-(n = 1, 2, 3…) units with an even interval between two neighboring methyl branches are observed in chain walking polymerization with ethylene as the sole monomer. As a comparison, the microstructure of ethylene-propylene copolymer produced by copolymerization of ethylene and propylene is distinct (Fig. 3). 1,2-, 1,3-, 1,4 Mechanistic insight into branch formation (nickel system). For more detailed discussion, Cf. Supplementary Notes, and Figs. 1-10. All structures of transition states and intermediates can be found in Supplementary coordinates.XYZ file. The intriguing branching character stimulated us to study the related mechanism of ethylene polymerization by nickel species ipty-Ni through using DFT calculations (Fig. 5). At first, the reaction starts from the β-H elimination (BHE) based on a β-agostic species 1 β-T to give a propylenecoordinated complex 1. This process, which overcomes a free energy barrier (TS 1BHE ) of 12.9 kcal mol -1 , is endergonic by   Table 1, entire 1-7 and the in-depth analysis comes from Table 1, entry 7 (205 branches/1000 C). Comparison on the microstructure of ethylene-propylene copolymer produced either by ethylene polymerization or by copolymerization of ethylene (E) and propylene (P).
9.5 kcal mol −1 . This unstable 1 goes through a slightly lower TS 12reins with an energy barrier of 12.0 kcal mol −1 to complete propylene re-insertion with a 2,1-manner, generating a new βagostic intermediate 2. By comparison, 2 is the more both kinetically and themodynamically favorable than 1 β-T . Therefore, a new ethylene coordination separating β-agostic-H based on 2 is considered, and is found to be feasible via an energy barrier of 12.7 kcal mol −1 , leading to 3. To further theoretically access the origin of selectivity on exclusive methyl branches, that is why 1,4-Me 2 unit can be produced but longer-chain branch units such as 1,3-Me/Et unit are   3)) kcal mol -1 and is an apparently unfavorable process, which is consistent with experimental findings.

Discussion
In summary, we have demonstrated that challenging selective branch formation in ethylene polymerization via a chain walking manner is now achieved by a sterically constrained nickel catalyst at broad reaction temperatures (also by the corresponding palladium analog at low temperatures). This overcomes the general propensity of complex microstructures produced by uncontrollably successive β-H elimination and opposite re-insertion events. Thus, an exclusive pattern of ultrahigh methyl branches is accessible, along with a highly selective branch distribution of 1,4-Me 2 unit. Notably, branch distribution is unprecedentedly predictable and can be easily calculated by a simple statistical model of probability. The obtained polymers with methyl branches generated by ethylene alone mimic the commercial ethylene-propylene copolymers but possess tailored microstructures. Mechanistic insights by an in-depth DFT calculation on the selective branch pattern and distribution fully unravel the difference of nickel and palladium system. This work provides creative perspectives for chain walking polymerization and beyond, particularly shows how to generate, control, analyze, and predict branch in polymer synthesis. This work also for the first time demonstrates the precise synthesis of ethylene-propylene copolymer from ethylene alone at industrial temperatures.

Methods
Methods and detailed experiments are provided in Supplementary Information.