Combined Experimental and Theoretical Approach for Living and

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Combined Experimental and Theoretical Approach for Living and Isoselective Propylene Polymerization Gregory J Domski, James M Eagan, Claudio De Rosa, Rocco Di Girolamo, Dr. Anne M. LaPointe, Emil B Lobkovsky, Giovanni TALARICO, and Geoffrey W. Coates ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02107 • Publication Date (Web): 18 Aug 2017 Downloaded from http://pubs.acs.org on August 18, 2017

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Combined Experimental and Theoretical Approach for Living and Isoselective Propylene Polymerization Gregory J. Domski,† James M. Eagan,† Claudio De Rosa,‡ Rocco Di Girolamo,‡ Anne M. LaPointe,† Emil B. Lobkovsky,† Giovanni Talarico,‡* Geoffrey W. Coates†* † ‡

Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, NY, 14853, United States Dipartimento di Scienze Chimiche, Università degli Studi di Napoli Federico II, Via Cintia, 80126 Napoli, Italy

ABSTRACT: The controlled polymerization of propylene with living catalysts in an isoselective fashion with high activities is a continuing challenge for catalysis with at least one of these properties suffering in most existing systems. Through experimental and theoretical studies, the pyridylamidohafnium olefin polymerization catalysts have been optimized for living behavior, isoselectivity, and activity of propylene polymerization (Ð = 1.2, [m4] = 91%, TOF = 25,000 h-1). Substitution of the bridgehead position with geminal dimethyl substituents forces the stereoselective elements of the catalyst into closer proximity with the active site through a “buttressing effect” while simultaneously preserving symmetry in the catalyst after activation and the first monomer insertion into the Hf-Caryl bond of the ligand. Propagation was shown to be favored over termination through beta-hydrogen transfer to the monomer through a combination of theoretical modeling and experimental monitoring of the reaction which showed a linear increase in molecular weight as a function of polymer yield. Furthermore, through computational and 13C labeling NMR studies a different mechanism of stereocontrol involving a direct ligand-monomer interaction was confirmed.

KEYWORDS. Polymerization, living polymerization, metallocene and post-metallocene systems, stereoselective catalysis, homogeneous catalysis.

Introduction Since its discovery by Natta in 1955, isotactic polypropylene (iPP) has become one of the most important commercial polymers.1 Due to its strength, durability, and low cost, iPP has found widespread use as a commodity plastic and is found in products ranging from lawn furniture to automotive parts. There are many heterogeneous and homogeneous catalysts for the isoselective polymerization of propylene.2 However, most of these catalysts are prone to chain termination via -H transfer to the incoming monomer or to the metal, -methyl transfer, chain transfer to cocatalyst via transmetalation, or some combination thereof. The lack of living catalysts limits access to polymer architectures with iPP segments, such as block copolymers formed by sequential monomer addition.3 iPP block copolymers have been shown to have a range of applications including use as interfacial adhesives and compatibilizers for recycling.4 Similar, less-precise materials can be produced using reversible chain-transfer polymerization in combination with batch reactors.5,6 This strategy, however, has significant commercial advantage as more than one chain per catalyst is formed.

Several metallocene and constrained geometry catalysts (1, Figure 1) have been reported for living and isoselective propylene polymerization; however, isoselectivity for these systems is low ([mm] ≈ 40%),7 limiting their potential uses. Numerous non-metallocene catalyst systems have been reported for the living and isoselective polymerization of higher olefins (e.g. 1-hexene). However, for the majority of these systems, propylene polymerization is achieved with a concomitant decrease in stereoselectivity and/or the loss of living behavior. For example, Sita and coworkers reported cyclopentadienyl(acetamidinate)zirconium catalysts (2) which exhibited living behavior for 1-hexene polymerization furnishing highly isotactic poly(1-hexene) ([m4] > 95%).8 Polymerization of propylene using this catalyst also proceeds in a living and isoselective fashion; however, the iPP produced has an [m4]-value of 71%. The decreased stereoselectivity for propylene polymerization is proposed to result from reduced steric interaction between the less demanding propylene monomer and the ligand framework and/or growing polymer chain.8b Kol and coworkers reported diaminobisphenolatezirconium catalysts bearing ortho- and para-tert-butyl groups on the phenolate moiety. High

Figure 1. Catalyst precursors capable of isoselective propylene polymerization in a controlled or living fashion.

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degrees of isoselectivity are observed for both 1-hexene ([m4] ≥ 95%)9 and propylene polymerization ([m4] = 80%).10 1-hexene polymerization proceeds in a living fashion at room temperature furnishing polymers with narrow molecular weight distributions (Ð ≤ 1.15) and a linear increase in Mn with conversion over a 30-minute period. Busico and coworkers showed that propylene polymerization catalyzed by the same precatalyst activated with [PhNMe2H][B(C6F5)4]/iBu3Al/2,6t Bu2C6H3OH exhibits evidence of termination after 30 minutes, although it should be noted that different catalyst activation conditions were used.11 By increasing the steric demand of the diaminobisphenolate ligand (3), the period for which these catalysts are living was extended from 30 minutes to 3 hours and DFT calculations suggested that -H transfer to monomer is responsible for the loss of living behavior in propylene polymerization.11b The increase in the duration of the living period allowed for the preparation of iPP-block-PE.11 We have been interested in living, isoselective catalysts for propylene polymerization. The bis(phenoxyimine) supported group (IV) catalysts have been the subject of extensive studies by our group and others.12 Early work on the bis(phenoxyimine)titanium complexes focused on the development of living and highly syndioselective bis(phenoxyaldimine)titanium catalysts.12 Using rational modifications to the catalyst structure, isoselective catalysts which retained the living behavior were developed (4).13 A catalyst which produced iPP with [m4] of up to 73% at 0 °C was developed and used to prepare elastomeric multiblock (triblock, pentablock, heptablock) iPP-block-poly(ethylene-co-propylene) copolymers. In a separate report, we succeeded in synthesizing phenoxyaminezirconium complexes bearing an sp3 carbon donor (5) capable of polymerizing propylene in a living and isoselective fashion with [m4] up to 76% at 0 °C.14 It was also noted that catalyst initiation was low (25%) resulting in most of the catalyst forming inactive species for polymerization. Both of these catalysts represent a major advance in the development of living and isoselective propylene polymerization catalysts in that they facilitated the synthesis of well-defined iPP-multiblock copolymers, however, the degree of stereoselectivity is still rather modest compared to the nearly perfect isoselectivity exhibited by C2-symmetric ansa-zirconocenes.15 To date, the chiral -diiminenickel complexes (6) are the most isoselective catalysts for long-lived propylene polymerization.16 At temperatures of -60 °C or below, this catalyst furnishes highly isotactic PP with a melting transition temperature (Tm) of 140 °C. At temperatures above -40 °C, the regioselectivity of this catalyst decreases and a regioirregular (rir), amorphous rirPP is produced. Above room temperature, the living behavior of this catalyst subsides due to catalyst decomposition. Using this catalyst, we prepared multiblock iPP-block-rirPP copolymers that exhibit unprecedented mechanical properties by varying the temperature of the reaction during the course of the polymerization.17 Unfortunately, this catalyst suffers from extremely low activity at the low temperature required to form regioregular iPP (TOF = 1 h-1 at 78 °C) and is not practical for large-scale polymer production. Each of the catalysts reported to date for living and isoselective propylene polymerization are deficient in one or more of the following areas: degree of isoselectivity, activity, thermal stability, and lifetime. We set out to develop an active, thermally stable catalyst for the living and highly isoselective polymerization of propylene. Pyridylamidohafnium catalysts, originally reported by Symyx,18 show high isoselectivity, activity, and produce polymers with high molecular weights at elevated temperatures (> 75 °C). These catalysts have also been used to polymerize -olefins, synthesize functionalized polyolefins, and thermoplastic elastomers.19,20 In 2007, we reported that Cs-symmetric pyridylamidohafnium complexes (7, Figure 2) catalyzed the living polymerization of 1hexene and propylene.21 It was surprising that a catalyst with Cs-symmetry would afford polypropylene with isoselectivity and these catalysts

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have continued to attract interest regarding the mechanism of their stereoselectivity. The origin of stereocontrol by the pyridylamido catalysts is a result of two unique aspects of this family of catalysts. The first is the in situ ligand modification by monomer insertion into the metal-CAryl bond (Scheme 1). This has been demonstrated by a series of two-dimensional NMR and X-ray studies,22 as well as appended alkene precatalysts.21 As a result of this insertion, when R1 ≠ R2 multiple diastereotopic olefin polymerization catalysts can form resulting in multiple active sites and broad molecular weight distributions. The second unique attribute of pyridylamido stereocontrol is the deviation from the "chiral growing chain orientation mechanism of stereocontrol" (i.e. the Corradini model).15,23 Recently, density functional theory (DFT) methods were used to study the mechanism of stereocontrol for a pyridylamidohafnium catalyst containing a pendant alkene.24 Relative free energies revealed a chain skipping mechanism with stereocontrol determined by a direct ligand-propylene interaction rather than an interaction between the growing polymer chain and the ligand. However, there is little experimental evidence to support this hypothesis.

Scheme 1. Ligand Modification Through Monomer Insertion to the Hf-CAryl Bond.21,22 Given the high molecular weights (>100 kg/mol) of iPP produced at high temperature (>75 °C), and the fact that pyridylamidohafnium catalysts have been used to produce blocky copolymers via chain-shuttling polymerization,19 it is likely that the rate of termination is significantly lower than the rate of propagation for these catalysts. In our lab, the activation of a typical C1-symmetric pyridylamidohafnium dialkyl precatalyst (rac-10, Figure 2) with B(C6F5)3 under 2 atm of propylene, furnished high molecular weight iPP ([m4] = 80%; Table 1, entry 1). However, the resultant polymer had a broad, bimodal molecular weight distribution thereby indicating the presence of multiple active sites. In an earlier report,21 the propylene polymerization behavior of a Cs-symmetric pyridylamidohafnium dimethyl precatalyst (7, Figure 1, 2) was described. Upon activation with B(C6F5)3 under 2 atm of propylene, catalyst 7 furnished iPP ([m4] = 56%, entry 2) with no detectable melting endotherm via enantiomorphic site control. However, the iPP produced by 7/B(C6F5)3 possessed a narrow molecular weight distribution (Ð = 1.2) suggesting that this catalyst may be living for propylene polymerization. We hypothesize that the decrease in molecular dispersity is a consequence of catalyst symmetry at the methylene bridgehead and the unique mechanism of activation for this class of catalysts, but stereoselectivity still requires improvement. Overall, these data confirm that the living and isoselective polymerization of propylene is a complicated catalytic cycle influenced not only by the catalyst stereoselectivity but also by the rate of formation of active species, the ion-couple strength and the minimization of chain termination reactions. In this paper we report a systematic optimization of a pyridylamidohafnium catalyst for the living and isoselective polymerization of propylene using a combined experimental and computational approach (see details of computational

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methods in Supp. Info). In particular, this new mechanism of stereocontrol24 was experimentally tested and a successful catalyst design was obtained in order to afford living isoselective propene polymerization.

Results and Discussions The pyridylamidohafnium precatalysts synthesized in this study are shown in Figure 2. In an attempt to increase the isoselectivity, but retain the living behavior demonstrated by 7, we focused on increasing the steric bulk of the ortho-substituents on the N-aryl ring, a position hypothesized to dictate stereocontrol based on calculations.24 Compound 11, prepared from 2,4,6-tri-tert-butylaniline, exhibits Cssymmetry as evidenced by the a single Hf-CH3 singlet in 1H NMR and X-ray diffraction analysis (Figure 2). The solid-state structure further revealed the geometry of the hafnium center is best described as distorted trigonal bipyramidal. Consistent with theoretical studies (see Figure S1 and Table S1), the propylene polymerization of 11/ B(C6F5)3 furnished iPP ([m4] = 72%, entry 3) with the highest isoselectivity for pyridylamidohafnium catalysts bearing a methylene bridge linking the pyridine and amido moieties. The polypropylene produced by 11/ B(C6F5)3 was the only polymer produced by the methylene linked family of catalysts to exhibit a melting transition (Tm = 108 °C) in differential scanning calorimetry (DSC) thermogram. The molecular weight distribution was broadened relative to 7/ B(C6F5)3 (1.5 vs. 1.2 respectively), however, the Mn of iPP produced by 11/B(C6F5)3 is high and in good agreement with the theoretical value (see Figure S2 and Table S1). Therefore, we conclude the rate of propagation is significantly higher than the rate of termination, but the rate of initiation is decreased due to the steric crowding of the active site. Interestingly, the molecular weight and yield of iPP produced by 11/ B(C6F5)3 are significantly higher than that produced by 7/B(C6F5)3 indicating the catalyst derived from 11 is nearly twice as active for propylene polymerization. This result may seem counterintuitive considering the greater steric demand of 11, however, the increased crowding of the active site may force the [Me(C6F5)3B]- counter ion further from the metal center,25 and/or destabilize the transition state of coordination ef-

Scheme 2. Synthesis of quaternary carbon bridge dialkyl pyridylamidohafnium precatalysts fectively favoring insertion.26 In order to confirm this hypothesis, we performed DFT calculations on the ion-couple strength for the systems of Figure 2. The results reported in Figure S3 and Table S2 confirm that the system 11 shows a weaker ion-couple with respect to 7 for the coordination to both diasterotopic active sites. Unfortunately, we were unable to experimentally improve the isoselectivity of methylene bridged catalysts to a level comparable to those catalysts bearing a distal stereocenter at the bridge head (e.g. rac-10). The experimental and theoretical results suggest that catalysts with the methylene bridge are not as stereoselective desired. In order to address this, we chose to prepare Cs-symmetric pyridylamidohafnium dimethyl precatalysts with symmetric substitution on the carbon atom linking the pyridine and amido moieties to evaluate their potential for

Figure 2. Dimethyl pyridylamidohafnium precatalysts synthesized in this study and ORTEP drawings of the X-ray crystal structures, thermal ellipsoids projected at 40% probability and hydrogen atoms have been omitted for clarity.

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weight (entry 5). As proposed by Froese et al., a possible reason for the more rapid initiation in the case of an ortho-metalated naphthyl as opposed to an ortho-metalated phenyl is that insertion into the Hf-CAryl bond eliminates an eclipsing interaction between the proton of the naphthalene ring and a meta-proton on the pyridine ring.22a Assuming all other factors are equal, this eclipsing interaction would raise the ground state energy of the borane adduct of 13 relative to 12 thereby lowering the activation energy of insertion into the Hf-CAryl bond. Further support for this explanation can be found by comparing the solid state structures of 12 and 13. In the solid state, 13 exhibits a torsion angle of 16.5° (Figure 2) which is significantly larger than the torsion angle between the pyridine and ortho-metalated phenyl ring systems in 12 (2.4°). In any case, the DFT results reported in Table S2 confirm a lower ion-couple strengths of 13 with respect to 12 for both the diasterotopic active sites.

living isoselective propylene polymerization. The initial steps in the synthesis of the pyridylamine ligands bearing a quaternary carbon bridge are identical to those in the synthesis of their methylene-bridged analogues (Scheme 2). Suzuki coupling between the 6-bromo-2-pyridylketone furnished the corresponding 6-aryl-2-pyridylketones in moderate yields. Condensation of the 6-aryl-2-pyridylketones with 2,6-diisopropylaniline proceeded in moderate to excellent yields to furnish the corresponding 6-aryl-2-pyridylketimines (16 and 17) according to previous procedures.18-21 Attempts to alkylate the ketimines with MeLi were unsuccessful and only unreacted starting material was recovered, but an alternative approach reported by Gibson and Budzelaar succeeded in alkylating the ketimines with AlMe3.27 Hydrolysis of the resulting aluminum complex and metalation with HfCl4, afforded Cs-symmetric pyridylamidohafnium dialkyl complexes 12 and 13.28 These catalysts, bearing geminal dimethyl bridges, both adopt distorted trigonal bipyramidal coordination geometry, tending towards square pyramidal in the solid state as evidence in their X-ray crystal structures (Figure 2 and Figures S5-S7).

The propylene polymerization behavior of 13/B(C6F5)3 was evaluated at several temperatures. For the polymerization conducted at 0 °C, the polymer yield decreased slightly, however, no significant decrease in the molecular weight of the polymer was observed (entry 5 vs. entry 6). The molecular weight distribution of the iPP produced at 0 °C was slightly broadened compared to that produced at 20 °C, possibly due to lower solubility of the highly crystalline polymer at lower reaction temperature, resulting in polymer precipitation. Lowering the temperature of the polymerization had a minor effect on isoselectivity; 13C NMR spectroscopy (Figure S7) indicated that the [m4] of the iPP produced at 0 °C was essentially the same as that produced at 20 °C ([m4] = 91%) with the Tm rising slightly (137 °C vs. 133 °C). Raising the polymerization temperature to 50 °C lead to a significant increase in polymer yield with a decrease in the molecular weight and an increase in the molecular weight distribution (entry 7). Since no chain transfer agent was present, it is likely that -H transfer/elimination, which is favored at higher temperatures, was responsible for this deviation from living behavior.

Having Cs-symmetric pyridylamidohafnium dialkyl complexes bearing a quaternary carbon bridge in hand, we proceeded to evaluate their propylene polymerization behavior under our standard set of conditions. Upon activation with B(C6F5)3, 12 furnished highly isotactic iPP ([m4] = 92%, Table 1, entry 4). This result represents an increase of nearly 40% in the degree of isotacticity for the iPP prepared using 12/B(C6F5)3 compared to that prepared using the methylene-bridged analogue (7/B(C6F5)3) and an over 10% increase compared to that prepared with rac-10/B(C6F5)3. The polymer possessed a narrow molecular weight distribution (Ð = 1.2) indicative of living behavior. Although the molecular weights of the polymers produced by 12 were greater than polymers produced by 7 (Mn = 142 kg/mol vs. 94 kg/mol respectively), catalyst 12 was slightly less active (TOF = 5,200 h-1 vs. 7,000 h-1). The ortho-metalated naphthyl analogue 13 was also evaluated for polymerization behavior (entries 5-7). The degree of isoselectivity for 13/B(C6F5)3 was similar to that of the catalyst derived from 12 ([m4] = 91% and 92% respectively). The major differences in polymerization behavior between catalysts derived from 12 and 13 are in terms of activity and initiation efficiency. Not only does 13/B(C6F5)3 produce more polymer in the same amount of time, but the molecular weight of the polymer is significantly higher (Mn = 266 kg/mol vs. 142 kg/mol) and 13/B(C6F5)3 is more active than 12/B(C6F5)3 (TOF = 25,000 h-1 vs. 5,200 h-1). Unlike the catalyst derived from 12, naphthyl analogue 13/B(C6F5)3 exhibits a high degree of initiation efficiency as evidenced by the excellent agreement between the theoretical molecular weight (Mntheo = molecular weight calculated based on the assumption of one chain per hafnium center) and the experimentally determined molecular

Compared to the methylene-bridged analogues, compounds 12 and 13/B(C6F5)3 both display significantly higher isoselectivity. Indeed, 12 exhibits the highest isoselectivity of any pyridylamidohafnium catalyst of the set (Tm = 140 °C). A possible explanation for the increased stereoselectivities of 12 and 13/B(C6F5)3 relative to the methylene bridged catalysts 7 and rac-10/B(C6F5)3 may be the increased steric demand on the active site which results from a “buttressing effect” brought about by the interaction between the methyl substituents on the backbone and the 2,6-diisopropylphenyl group. These trends suggest isoselectivity in the pyridylamidohafnium catalysts is a result of a complex interplay between bridgehead substitution, ortho-aniline substitution, and the HfCaryl substituent.

Table 1. Propylene polymerization data for dialkyl pyridylamidohafnium/B(C6F5)3 catalysts.a

rac-10

Trxn (°C) 20

Yield (g) 3.31

TOFb (h-1) 32,000

M nc (kg/mol) 284

M nd (theo.) 331

Ðc (Mw/Mn) 3.7

[ m4 ]e (%) 80

Tgf (°C) -7

Tmf (°C) 124

2

7

20

0.73

7,000

94

73

1.2

56

-9

n.d.f

3

11

20

1.47

14,000

163

147

1.5

72

-4

108

4

12

20

0.55

5,200

142

66

1.2

92

-7

140

5

13

20

2.62

25,000

266

262

1.2

91

-5

133

6

13

0

1.97

19,000

257

197

1.3

91

-7

137

7

13

50

3.43

33,000

76

343

1.5

85

-4

127

Entry

Cat.

1

a

b

Polymerization conditions: 10 mol Hf, [Hf]/[B] = 1.0, 30 mL toluene, saturated at 2 atm of propylene, 15 minutes. Average turnover frequency = mol propylene/mol Hf∙h. cDetermined using gel permeation chromatography in 1,2,4-C6H3Cl3 at 140 °C versus polyethylene standards. dTheoretical molar mass = mass of polypropylene/mol Hf. eDetermined by integration of the methyl region of the 13C NMR spectrum. fDetermined via differential scanning calorimetry, 2nd heating cycle. gn.d. = not detected.

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To better understand these factors, we performed DFT calculation on these catalysts in regards to their si/re facial selectivity for propylene insertion. Consistent with previous DFT studies of an alkene pendant pyridylamidohafnium catalysts,24 the systems described herein showed a strong preference for site epimerization so we used the energetically preferred chain orientation for insertion for all of the calculations. We found that the geminal dimethyl substitution of system 13 resulted in an energetic difference for the two transition states (TS) of facial selectivity (free energies) of 2.8 (2.0) kcal/mol (see Figure 3, A and B), whereas a methylene bridged system was 0.7 (1.2) kcal/mol (see Figure 3, C and D). This energy difference results from a smaller C-C-N bond angle in the geminal dimethyl catalyst 13 relative to 7 (109° vs. 112°) which forces the ortho-substituents of the aniline into closer proximity (3.6 Å vs. 3.8 Å) to the active site, increasing the steric ligand-monomer interactions. Exchanging the naphthyl ring of 13 with a phenyl ring (12), the facial selectivity rose to 3.8 (3.0) kcal/mol, an energetic difference that is consistent with the slight increase in isoselectivity of the iPP produced experimentally. Together, these calculations indicate that the bridgehead substituents create a Thorpe-Ingold-like “buttressing effect” in combination with the ortho substituents of the aniline, and this is the dominant factor in stereoselective propylene polymerization. The identity of the aryl group has less influence on stereocontrol, although it does influence on the relative rate of initiation, as well as the ion-couple strength (see Table S2) and the molecular mass. This is in contrast with rac-10 and its derivatives, which show much higher stereocontrol for naphthyl versus phenyl. Furthermore, the stereochemistry of initial propylene insertion into the Hf-Caryl bond was found to not have an impact on the preferred si facial selectivity (Figure S9). Interestingly, when the polymeryl chain was exchanged for a methyl group (i.e. first monomer insertion) in the theoretical models, the catalysts still exhibited a high facial selectivity of 4.3 (4.0) kcal/mol (Figure S8). This is contrary to the typical Corradini model for metallocene and post-metallocene stereoselective systems where polymeryl chain orientation determines the incoming propylene monomer’s facial coordination.15,23 We recently reported a similar proposal for a model pyridylamidohafnium catalyst bearing a pendant alkene.24 A novel mechanism of direct ligand-monomer interaction mechanism was proposed, akin to enantioselective small molecule catalysis. We were inspired by the experiments performed by Zambelli where 13C labeled end-groups revealed that for metallocene catalysts, the first monomer insertion is not selective, affording a near 1:1 ratio of threo and erythro 13C label by NMR spectroscopy.29 13-Hf(13CH3)2 was prepared using 13CH3MgI. Activation and polymerization of a small amount of propylene afforded iPP with detectable end-groups. Quantitative 13C NMR spectroscopy of

Figure 3. Transition state (TS) geometries for the facial selectivity of geminal dimethyl bridge (A, B) and methylene bridge (C, D) mono-inserted catalysts with polymeryl chain. DFT calculations indicate propene si face (A, C) was favored by a free energy of 2.0 and 1.2 kcal/mol respectively. Red arrows indicate ligand-monomer steric interactions for the disfavored re face (B, D). the iPP produced by 13-Hf(13CH3)2 /B(C6F5)3 (Figure 4) showed only a single end-group peak consistent with highly selective threo facial selectivity for the first insertion ( = 22.7 ppm).15 This experimental result combined with the computational work leads us to conclude that this class of catalysts operate under a distinctly different mechanism of stereocontrol than that for stereoselective metallocenes and post-metallocenes reported up to now. Since the iPP produced by 13/B(C6F5)3 at room temperature exhibited a narrow molecular weight distribution, good agreement between Mntheo and MnGPC and a high degree of isoselectivity for the polymerization conducted at room temperature, the polymerization behavior of this catalyst was investigated in more detail. The relationship between Mn and polymer yield was determined for 13/B(C6F5)3 and is represented in Figure 5. For propylene polymerization catalyzed by

Figure 4. Quantitative 13C NMR (125 MHz, 1,1,2,2-C2D2Cl4, 135 °C) spectra of methyl region of iPP produced by 13/B(C6F5)3 at 20 °C and from 13 CH3 enriched 13-Hf(13CH3)2 /B(C6F5)3; 2,1-insertion regioerrors are also labeled.

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over molar mass distribution ([m4] = 91%, TOF = 25,000 h-1, Ð = 1.2). The pyridylamidohafnium catalysts all operate under site control and we have experimentally confirmed that 13/B(C6F5)3 operates by a different mechanism of stereocontrol than isotactic polymerization catalysts reported up to now. This study applied advances in organometallic and polymer chemistry while simultaneously relying on the quantitative and predictive power of computation. Together, these disciplines will enable the design and discovery of new stereoselective catalysts for olefin polymerization.

ASSOCIATED CONTENT Supporting Information

Figure 5. Living plot of iPP produced by 10 mol of 13/B(C6F5)3 at 20 °C. 13/B(C6F5)3, the relationship between Mn and polymer yield is linear throughout the course of a 12-minute polymerization and the molecular weight distribution remains narrow (Ð ≤ 1.20) consistent with the behavior of living polymerization. Interesting insights are obtained by DFT calculations on the geometries of the transition states (TSs) for the -H transfer to monomer (HT). These catalysts show two distinct TSs for HT that differ mainly in the Hf-H distance (from 2.1 to 3.2 Å) and C-Hf-C angle (from 85 to 130 °) depending on the nature of the aryl substituent.30 Decreasing the amount of space by using a sterically demanding naphthyl ring (13) results in a shift towards the HT TS with a longer Hf-H distance and shorter C-Hf-C angle. The mono-inserted catalyst derived from 13 showed, in the lowest energy TS, a remarkable 9.2 kcal/mol preference for propagation over -H transfer and a TS with a Hf-H distance = 3.2 Å, combined with a C-Hf-C of 87° (Figure 6, A). Replacing the naphthyl ring of 13 with a less demanding phenyl ring (e.g. 12) reduced the energetic difference between propagation and -H transfer to 8.6 kcal/mol with a preference for the more common HT geometry (see Figure 6, B). Overall, the theoretical calculations correspond well with the experimental data for living character.

Figure 6. Lowest energy TS of -hydrogen transfer to monomer for the naphthyl catalyst (13) (A) and the analogous phenyl substituted catalyst 12 (B). The free energy difference of -H transfer and propagation for (A) and (B) was 9.2 and 8.6 kcal/mol, respectively. The lower energy HT TS promoted by 13 corresponds to the structure with longer Hf-H distance and shorter C-Hf-C angle (A) relative to (B).

Conclusions In conclusion, by using a combined experimental and theoretical approach, we have uncovered several features of the pyridylamidohafnium catalysts that govern activity, stereoselectivity, and living behavior. We developed a catalyst for propylene polymerization, 13/B(C6F5)3, which produced iPP with high levels of isoselectivity, activities, and control

Computational details, Figures S1-S9 and Tables S1-S17, experimental procedures, NMR spectra, and X-ray crystallography data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT Funding for this project was provided by the Center for Sustainable Polymers, an NSF Center for Chemical Innovation (CHE-1413862) and Mitsubishi Chemicals. G.T. wishes to thank the University of Naples Federico II (Ricerca di Ateneo 2017 of University of Naples Federico II, DR_409_2017) for financial support.

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