Catalyst Nuclearity Effects on Stereo- and Regioinduction in

Mar 15, 2018 - In comparison to monometallic controls, bimetallic olefin polymerization catalysts often exhibit superior performance in terms of highe...
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Catalyst Nuclearity Effects on Stereo- and Regioinduction in Pyridylamidohafnium-Catalyzed Propylene and 1‑Octene Polymerizations Yanshan Gao,† Xia Chen,†,‡ Jialong Zhang,†,§ Jiazhen Chen,† Tracy L. Lohr,*,† and Tobin J. Marks*,† †

Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113, United States School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, China § State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China ‡

S Supporting Information *

ABSTRACT: In comparison to monometallic controls, bimetallic olefin polymerization catalysts often exhibit superior performance in terms of higher polyolefin Mw, higher comonomer incorporation, and higher polar comonomer tolerance. However, using cooperating catalyst centers to modulate stereoselectivity in α-olefin polymerizations is relatively unexplored. In this contribution, the monometallic Hf(IV) complex, L1-HfMe2 (catalyst A, L1 = 2,6-diisopropylN-{(2-isopropylphenyl)[6-(naphthalen-1-yl)pyridin-2-yl]methyl}aniline), and homo-bimetallic Hf(IV) complexes, L2-Hf2Me5 (catalyst B) and L2-Hf2Me4 (catalyst C) (L2 = N,N′-{[naphthalene-1,4diylbis(pyridine-6,2-diyl)]bis[(2-isopropylphenyl)methylene)]bis(2,6-diisopropylaniline}), are activated with Ph3C+B(C6F5)4− and investigated in propylene and 1-octene homopolymerizations. In propylene polymerizations, the conformationally flexible catalyst B-derived bimetallic dicationic catalyst produces higher Mw polypropylene (up to 7.8×), higher total stereo- and regiodefect densities (up to 3.5×), and lower Tm (by as much as ∼14 °C) versus the monometallic catalyst A-derived control. In 1-octene polymerizations, the conformationally flexible catalyst B-derived bimetallic dicationic catalyst induces greatly reduced isotacticity (23% reduction in [mmmm]) versus the catalyst A-derived monometallic control ([mmmm] > 99%). Interestingly, conformationally flexible catalyst B-derived cationic bimetallic Hf catalysts are also known to undergo rapid intramolecular/ intermetal methyl exchange and to exhibit strong Hf···Hf cooperative enchainment effects in ethylene homo- and copolymerizations. Steric effects and intramolecular intermetal chain transfer likely both contribute to the increased isotactic polyolefin stereo- and regiodefect content.

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olefin homopolymerizations (e.g., 1-hexene and 1-octene).27−32 Since only one monomer is employed, comonomer selectivity is irrelevant; however, stereochemical control becomes of interest in such polymerizations. Thus, Sita et al.30,32 and Agapie et al.28,29 recently reported stereochemical effects in propylene polymerizations mediated by bimetallic Zr2 catalysts (Figure 1), and Marks et al.27 recently reported stereochemical effects in 1-octene polymerizations by Ti2 catalysts. While these results suggest that bimetallic catalysis represents a new and promising strategy for modulating polyolefin stereochemistry, the range of metals and ligand scaffolds available for bimetallic α-olefin polymerization has scarcely been explored and may provide much new and generalizable structure−function information. This laboratory recently reported a homobimetallic Hf precatalyst (catalyst B, Figure 1), which when activated (2(B+) and 3(B2+), Figure 2.1), exhibits remarkable

INTRODUCTION Polyolefins represent one of the most extensively used plastics worldwide. Intense research has been focused on developing new single-site homogeneous olefin coordination polymerization catalyst systems.1−6 In this regard, bimetallic cooperative catalysts have attracted significant attention in coordination polymerization catalysis where metal···metal cooperative effects dramatically impact catalytic behavior and the resulting polyolefin microstructures (see references for recent reviews,7−9 examples of early10−14 and late15−20 transition metal catalysis, and examples of other bimetallic catalytic reactions21−25). Bimetallic cooperative effects are established by comparing bimetallic catalysts with similarly configured monometallic controls. Ethylene homopolymerization and ethylene/α-olefin (or polar comonomer) copolymerizations are well-studied processes in which bimetallic catalysts exhibit cooperativity effects in terms of higher product Mw, higher comonomer enchainment selectivity,8,9 and enhanced polar comonomer tolerance.18,26 In contrast, there are limited reports of bimetallic cooperative catalysis in propylene and higher α© XXXX American Chemical Society

Received: January 25, 2018 Revised: March 6, 2018

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DOI: 10.1021/acs.macromol.8b00181 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. Examples of bimetallic stereoselective propylene or 1-octene polymerization precatalysts; mono- and bimetallic hafnium propylene polymerization precatalysts.

Figure 2. (1) Structures of activated catalytic ion pairs for propylene and 1-octene polymerizations. (2) Operative low-barrier structural dynamics.

Table 1. Organohafnium-Catalyzed Propylene Polymerization Dataa entry

cationic species

T (°C)

t (min)

PP (g)

act.b

Mwc

Đc

[mmmm]d (%)

1 2 3 4 5 6 7 8g 9g 10h

1(A+) 3(B2+) 2(B+) 5(C2+) 4(C+) 1(A+) 3(B2+) 1(A+) 3(B2+) 1(A+)

70 70 70 70 70 30 30 70 70 70

1.0 1.0 3.0 1.0 1.0 0.5 0.5 1.0 2.0 6.0

1.449 0.447 0.198 1.909 0.449 1.545 0.408 1.032 0.186 5.350

290 89 13 382 90 617 163 206 19 184

395 993 695 555 415 325 2547 440 445 36

2.0 3.4 2.4 3.9 2.6 2.0 4.1 2.8 4.7 1.9

93.8 85.3 90.7 91.1 91.8 95.6 89.9 95.3 87.0 93.2

total defects (regiodefect)d (%) 1.8 6.3 3.8 2.6 1.9 1.3 3.7 1.9 3.6 2.7

(0.8) (2.0) (1.8) (0.9) (0.9) (0.6) (1.1) (1.0) (1.7) (0.9)

Lisoe

Tmf (°C)

56 16 26 39 53 75 27 53 28 37

151.0 137.3 140.3 141.5 151.5 155.7 146.0 150.8 139.1 143.7

Conditions: catalyst, 5 μmol of catalyst A or 2.5 μmol of catalyst B or catalyst C; cocatalyst, Ph3C+B(C6F5)4−; propylene, 1 atm; toluene, 50 mL. Each entry performed in duplicate. bUnits (kg PP) (mol of metal)−1 min−1. cGPC vs polystyrene standards in (kg mol−1). dDetermined by 13C NMR and calculated according to literature procedure. Total defects = rr defect + 2,1-defect.44 eLiso (isotactic sequence length) = 1/∑total defects.52 f Determined by DSC. gSolvent: toluene (48 mL) + 1,2-difluorobenzene (2 mL). h40 equiv. of Et2Zn (0.2 mmol) added. r defects included in total defects calculation in this case. a

modify stereochemical control in bimetallic propylene and 1octene homopolymerizations. Note that activated monometallic 1(A+) is a highly effective catalyst for isotactic propylene homo- and copolymerizations.36−38 It will be seen here that conformationally flexible catalyst B-derived dicationic bimetallic Hf catalysts, which are known to undergo rapid intramolecular/ intermetal methyl exchange and exhibit strong Hf···Hf cooperative enchainment effects in ethylene homo- and copolymerizations, significantly modify the product defect densities/stereoselection and Mw in propylene and 1-octene homopolymerizations while also affecting the polypropylene melting point (Tm).

cooperativity in ethylene/1-octene copolymerization in terms of both polymer Mw and 1-octene enchainment selectivity vs monometallic control 1(A+) (Figure 2.1).33 In addition, variable-temperature and EXSY NMR studies reveal rapid intramolecular, methyl−methyl exchange in the activated bimetallic 2(B+) and 3(B2+) mono- and dicationic catalytic species (Figure 2(2)).33 This process is rapid in the absence of traditional transmetalation/chain transfer agents such as ZnEt2.33−35 The above observations raise the intriguing question of how the previously established bimetallic cooperative effects in ethylene homo- and copolymerizations might perturb or B

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RESULTS Propylene. Propylene polymerizations were carried out under rigorously anhydrous/anaerobic conditions at 30 or 70 °C under a constant propylene pressure of 1.0 atm with rapid stirring (≥1000 rpm). Catalyst loadings and polymerization yields were adjusted to minimize the influences of reaction exotherms and mass transfer effects.39−41 Polymerization results are summarized in Table 1. To simplify the discussion, we also comment on the nature of the cationic species obtained after activating the Hf precatalysts with the same Ph3C+B(C6F5)4− cocatalyst in the absence of olefin monomer. Cationic species are labeled with numbers 1−5 together with the catalyst precursors A−C and cationic charges of 1+ or 2+. Since all cationic species have the same B(C6F5)4− anion, it is not included in the labeling for clarity (Figure 2(1)). According to literature reports on monometallic 1(A+), these cationic complexes undergo an initial olefin monomer insertion into the Hf−Cnaph bond to form the actual polymerization-active species.42−44 In the present study, both mono- and bimetallic Hf catalysts give highly isotactic polypropylenes (PP) under the present conditions. GPC analysis shows that all polypropylene products obtained are monomodal; some GPC traces are slightly broadened, which may reflect the unique activation mechanism of the present pyridylamido Hf catalysts, possibly leading to more than one active species after monomer insertion into the Hf−CNaph bond.34,42,43,45 The tacticity and defect contents of all polypropylenes were analyzed by established high-temperature 13C NMR techniques.44 All spectra exhibit qualitatively similar resonances/stereodefects but with varying peak intensities and are similar to those reported by Busico and Stevens,46 and Coates47 using similar monometallic catalysts. The principal stereodefects present in these polypropylene samples correspond to mmmr, mmrr, and mrrm pentads (Figure 3) in a roughly 2:2:1 ratio, arguing that

total defect density (rr + 2,1-defects) to approximate the average length of the regular isotactic sequences.52 Liso = 56 using 1(A+) indicates an average of 56 consecutive isotactic propylene monomer insertions until a defective insertion occurs. With bimetallic catalyst 3(B2+) the activity is somewhat lower than that of the monometallic control. Furthermore, the isotacticity falls with a 3.5× increase in total defect content from 1.8% to 6.3% (Liso = 16) along with a fall in Tm by 14 °C, from 151.0 to 137.3 °C, and the polymer Mw is increased by 2.5×, from 395 to 993 kg/mol using 3(B2+) versus the monometallic control (Table 1, entries 2 vs 1). Surprisingly, using the bimetallic monocationic catalyst 2(B+) under the same conditions to ensure comparable results affords only small quantities of polymer. Running the reaction 3× longer yields small quantities of polypropylene with a defect content and Tm of 3.8% and 140.3 °C, respectively (Table 1, entry 3), intermediate between 1(A+) and 3(B2+). Bimetallic dicationic catalyst 5(C2+) and bimetallic monocationic species 4(C+) derived from catalyst C (entries 4 and 5) produce polymers with total defect densities and Tms between that of 1(A+) and 3(B2+). Thus, the Tms of the present PP samples decrease in the order: 1(A+) ≈ 4(C+) > 5(C2+) > 2(B+) > 3(B2+). The total defect density trend is opposite to that of Tm (Figure 4), in good agreement with the general relationship between total defect content and Tm.52

Figure 4. Relationship between total PP stereodefect + regiodefect content vs polymer Tm for the present active monometallic and bimetallic Hf catalysts.

Octene. Next, 1-octene homopolymerizations were investigated to determine if tacticity and microstructure control trends similar to propylene are operative with this sterically bulkier monomer. Cationic species 3(B2+) produces polymers with reduced poly(1-octene) (PO) isotacticity versus 1(A+) with [mmmm] = 77% vs >99% (Table 2, entries 2 vs 1). In

Figure 3. Defects in isotactic polypropylene: (1) mx(rr)my defect, (2) mx(r)my defect, and (3) 2,1-regiodefect.

Table 2. Organohafnium-Catalyzed 1-Octene Homopolymerization Dataa

an enantiomorphic site-control enchainment mechanism is operative.48 Microstructure analysis of the present PPs obtained from the mono- and bimetallic catalysts exhibit NMR resonances corresponding to head-to-head and tail-to-tail miss-insertions (2,1-regiodefects, Figure 3(3)).49,50 Negligible regiodefects assignable to 3,1- misinsertions49 are detected in these PPs. The relationship between PP stereodefect/regiodefect contents and polymer Tm is well-established.51−54 Thus, Tm assay is used here as complementary metric of PP defect content. Polymerizations with monometallic monocationic species 1(A+) at 70 °C (Table 1, entry 1) were first studied. Cationic catalyst 1(A+) exhibits high activity and produces highly isotactic polypropylene (93.8%) with a high Tm = 151.0 °C. The isotactic sequence length (Liso) was calculated from the

entry

cationic species

t (min)

polymer (g)

act.b

Mwc

Đc

mmmmd (%)

1 2 3 4 5

1(A+) 3(B2+) 2(B+) 5(C2+) 4(C+)

1.0 20 20 2.0 2.0

3.349 0.729 0.151 1.635 0.591

335 3.7 0.8 82 30

532 696 1049 135 716

2.0 2.2 2.3 3.2 2.5

>99 77 >99 >99 >99

a Conditions: catalyst, 10 μmol of catalyst A or 5 μmol of catalyst B or catalyst C; cocatalyst, Ph3C+B(C6F5)4−, 1 or 2 equiv; 1-octene, 5 mL (3.58 g), toluene, 45 mL, 25 °C. Each entry performed in duplicate. b Units (kg PO) (mol of metal)−1 min−1. cGPC versus polystyrene standards in (kg mol−1). dDetermined by 13C NMR and analyzed according to the literature procedure.55

C

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Macromolecules contrast to propylene polymerizations, the 1-octene polymerizations with catalysts such as 4(C+), and 5(C2+) all give highly isotactic polymers with [mmmm] > 99% (Table 2, entries 5 and 4, respectively). Cationic species 2(B+) is also active for 1-octene polymerization but with considerably lower activity than 3(B2+).

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DISCUSSION As noted in the Results section, the present propylene and 1octene homopolymerization results indicate that dicationic bimetallic Hf catalysts 3(B2+), 2(B+), and 5(C2+) produce polymers with higher total defect contents than does monometallic catalyst 1(A+). Furthermore, the conformationally flexible, dicationic catalyst 3(B2+) yields polypropylenes with a greater polypropylene Tm depression and Mw enhancement (Figure 2). In regard to the mechanism, few studies have been reported on tacticity induction in binuclear group 4 polymerization catalysts.27−29 Two possible scenarios (or both) appear to be the most plausible explanations for these bimetallic effects. The first possibility is that steric effects may slow the rate of site epimerization and hence erode isoselectivity. A second possible scenario is that these effects arise from intramolecular coordinative chain transfer, occurring in the absence of a chain transfer agent (vide inf ra). Steric Effects on Tacticity. For C1-symmetric catalyzed isotactic α-olefin polymerizations, single site enantiomorphic site control (also known as site epimerization or the back-skip mechanism),56 in which α-olefin insertion occurs predominantly at one of the C1-symmetric catalyst’s two diastereotopic sites, is often invoked as the source of isotacticity.57,58 After αolefin insertion, the growing polymer chain is oriented toward the more hindered and less favorable diastereotopic site on the catalyst. Prior to the next insertion, the steric hindrance (between the polymer chain and the ancillary ligand framework) forces a site epimerization process to reposition the polymer chain toward the less-hindered diastereotopic site,57,59 and repeated insertion/epimerization eventually yields highly isotactic polymer (Figure 5(2)).48,56,60−62 Talarico et al. analyzed the mechanism of stereochemical control in C1symmetric pyridylamido Hf catalyst 1(A+)-mediated isotactic propylene polymerization by DFT methods using a “naked cation” (counteranion-free) computational model (Figure 5(1)).63 Their DFT studies reveal a strong preference for propylene insertion at site 1, implying chain propagation via a back-skip mechanism (Figure 5(2)).58 Note that PP pentad analysis and literature46,47,64 on the C1-symmetric pyridylamido Hf catalyst-mediated isotactic propylene polymerizations all support a back-skip mechanism. In the back-skip mechanism, fast site epimerization relative to monomer insertion is required to minimize misinsertions.4,56,58,65 Steric effects play an important and unique role in modulating the dynamics of the epimerization/coordination/ insertion process in the back-skip mechanism, which determines poly(α-olefin) isotacticity. If steric effects enhance the preference of site 1 over site 2, which favors site epimerization back to site 1 from site 2, this makes monomer coordination/insertion from site 1 more favorable (Figure 5(2) and Scheme 1(1)). In this case, steric effects tend to increase isotacticity. In contrast, if steric effects weaken the enantioselectivity of site 1 over site 2, this makes site epimerization back to site 1 from site 2 less competitive than coordination/ insertion before the site epimerization; i.e., insertion at site 2 becomes more likely (Scheme 1(1)). In this case, steric effects

Figure 5. (1) Preferred transition states for 1,2-propene insertion into growing polymeryl chain P at activated monometallic catalyst 1(A+) site 1 (a) and site 2 (b).63 Sites 1 and 2 are defined by the orientation of the Hf−polymeryl bond relative to the chiral ligand framework. Preferred 1,2-propene insertion transition states into the growing polymer chain at site 1 with si (c) and re (d) enantiofaces. (2) Backskip mechanism of isotactic propylene polymerization with C1symmetric monometallic catalyst 1(A+).

would decrease isotacticity.56−58 In the present catalyst system, steric congestion around the active metal center in bimetallic catalyst 3(B2+), arising from the borate anion, and the steric influence of the other Hf center and supporting ligation likely weaken the enantioselectivity at site 1 by retarding the rate of site epimerization,5,32,56,57,66−70 which increases the probability of monomer insertion at site 2 before epimerization and thereby increases the PP defect density (Scheme 1(1)). Note that steric effects might also lead to decreased stereoselectivity because of less effective interaction between the catalyst and the preferred monomer enantioface. In principle, the perturbation of the site control model due to steric or other effects may lead to a change in isotacticity which can be modeled by stochastic analysis of the polypropylene isotacticity at the pentad level and compared to the deviation from perfect site control models.44,46,56,57,71,72 Unfortunately, in the present system this analysis does not provide statistically significant resolution of this issue, likely due to the inherent limitations of the site control models (see Supporting Information for full details).56,57 Although steric effects must be considered in any explanation for the present observed bimetallic effects on polymer isotacticity, some data are not consistent with a sterics-only model. All results to date, albeit limited, indicate that for bimetallic catalyst-mediated α-olefin polymerizations steric congestion around a catalytic center, doubtless arising from the borate counteranion and the steric influence of the other metal center and supporting ligation, enhances the enantioselectivity of one diastereotopic site over the other, thereby increasing product isotacticity. Agapie et al.29 argued that the D

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Macromolecules Scheme 1. Plausible Mechanistic Scenarios for Increased Total Defect Densities in Bimetallic Catalysisa

a

Note: site labeling refers to the polymerization active Hf center only.

operative beyond steric factors alone. Here we propose the intriguing mechanistic scenario that unassisted intramolecular alkyl/polymeryl exchange is a plausible origin of the lower isoselectivity in the bimetallic catalyzed propylene and 1-octene polymerizations. Intramolecular Chain Transfer and Tacticity. As noted above, fast methyl exchange is observed in species 3(B2+) and 2(B+) by variable temperature 1H-EXSY NMR (Figure 2(2)) and is supported by DFT calculations.33 This indicates that methyl groups rapidly and reversibly transfer between structurally distinct Hf centers. We tentatively suggest that the increased isoselection defects observed here reflect such intramolecular alkyl/polymeryl exchange processes. Such intramolecular coordinative chain transfer polymerization (CCTP) would be analogous to the Hf-methyl scenario (Figure 2(2)) but would involve polymeryl units transferring between nonequivalent polymerization sites. Note that monometallic precatalyst 1(A+) (Figure 2) is used in conjunction with an organozirconium catalyst and ZnEt2 as a chain transfer agent by Dow Chemical researchers in an intermolecular CCTP process to yield ethylene-based block copolymers (Figure 6).35 In this case, negligible alkyl group

enhanced PP activity and isoselectivity of the bis[amine bis(phenolate)] Zr2 catalyst in Figure 1 versus the monometallic analogue ([mmmm] = 31.0% vs 22.0%, respectively) arises from distal steric interactions with the second Zr center. Sita et al.30,32 reported that tethered mononuclear Zr catalysts (Figure 1) act as propagators to produce isotactic PPs ([mmmm] = 69.7%), similar to the analogous monometallic Zr complex ([mmmm] = 69.4%). Also, Marks et al.27 recently reported that a meso-Ti2 catalyst (Figure 1) shows significantly enhanced isoselectivity in α-olefin polymerizations. In the present organohafnium system, the more encumbered bimetallic 3(B2+) yields PP with significantly lower isotacticity than monometallic 1(A+) (85.3% vs 93.8%; Table 1, entries 2 vs 1) in the absence of ZnEt2. This result differs from the aforementioned steric effects on tacticity induction using bimetallic catalysts reported by Agapie et al.29 and Marks et al.27 Regarding 1-octene versus propylene as a probe of steric factors, although tacticity analysis of longer chain poly-α-olefins is less informative than PP,55,73−81 polymerizations of different α-olefins often show similar stereoselectivity,78,79,82,83 and the well-established models for stereoselective propylene polymerizations, such as enantiomeric site and chain-end control mechanisms,76,77,80 are commonly invoked in the stereoselective polymerizations of longer chain α-olefins, both suggesting similarity with propylene in stereocontrol mechanisms.76,77,80 Note that while 3(B2+) and 2(B+) have similar Hf center steric characteristics (Figure 2), 1-octene polymerization mediated by 2(B+) gives highly isotactic polymer with [mmmm] > 99%, whereas 3(B2+) gives polymer with a significantly lower isotacticity, [mmmm] = 77% (Table 2, entries 3 vs 2), suggesting effects other than steric encumbrance may be operative in stereoselection during 1-octene polymerizations. Similar trends for propylene polymerization are observed as well ([mmmm] = 90.7% vs 85.3%; Table 1, entries 3 vs 2). Similar trends for both 1-octene and propylene homopolymerizations plausibly suggest that they follow the same stereocontrol mechanism; the dramatic difference in isoselection for bimetallic catalyst B-derived dicationic 3(B2+) versus monocationic 2(B+), each of which bears similar Hf center steric characteristics, suggests that another effect is likely

Figure 6. Organozinc chain transfer agent (CTA)-assisted intermolecular shuttling between two mononuclear catalysts to produce olefin block copolymers (OBCs).35

polymeryl fragment exchange/transfer between Hf and Zr catalytic centers is observed in the absence of ZnEt2.35 Note also that chain transfer processes effected with chain transfer agents such as Et2Zn or Me3Al46 can significantly erode the isoselectivity in such propylene polymerizations.30,32,46 On the basis of the model proposed by Talarico et al.,63 we propose a E

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catalyzed propylene polymerization without added ZnEt2 (Tm = 137.3 °C; total defect content 6.3%; Table 1, entry 2). Regarding molecular structure considerations, note that in contrast to the two enantiomers of monometallic catalyst 1(A +) present, bimetallic dicationic 3(B2+) exists as four diastereomers.33 For 1(A+), the racemic mixture still produces isotactic polypropylene, and isotacticity falls when ZnEt2 is introduced, which in turn produces stereoblock PP with r defects46 (Figure 3) from Zn-mediated intermolecular chain transfer.46 Since no chain transfer agent is used in the present experiments (except in the experiment mentioned above with 40 equiv. of ZnEt2) and since no significant r defects are observed with the present catalysts, intermolecular chain transfer to a different diastereomer seems unlikely, and the mixed diastereomers should not significantly influence the polymer microstructure in these polymerizations.

similar mechanistic scenario for stereocontrol in bimetallic catalysts undergoing intramolecular CTA-unassisted CCTP in Scheme 1(2). It will be seen below that the polypropylene total defect contents and ease of alkyl exchange are correlated. As discussed above, bimetallic dicationic complex 3(B2+) first undergoes monomer insertion into the Hf−CNaph bond. Note that without a Hf−CNaph bond the second cationic Hf center is not polymerization-active.43 The active catalytic Hf center has local C1-symmetry, and sites 1 and 2 were defined by Talarico et al., as shown in Figure 5(1) (a and b). Regarding the polymerization-active Hf center in dicationic species 3(B2+), which was designed to reproduce the coordination environment of 1(A+), it is reasonable to assume that it has a qualitatively similar energetic preference for site 1 (Scheme 1(2)). However, in this bimetallic case, the rapid intramolecular chain transfer process (Figure 2(2) and Scheme 1(2), ktr) may compete with the normal insertion−epimerization−insertion scenario (Scheme 1(2), kepim and kprop). Since site 2 is energetically disfavored in comparison to site 1, the growing chain likely epimerizes back to site 1 again. However, if the intramolecular chain transfer rate (ktr) is comparable to the rate of site epimerization (kepim), there will be a higher content of the growing chain at site 2 relative to site 1. Propagation from site 2, which has a lower monomer enantioface selectivity relative to site 1, would be expected to increase the propensity for introduction of stereo- and regiodefects. Two stereochemical events are involved in the above proposed process, i.e., polymeryl exchange and site epimerization. Site epimerization relates to inversion of stereochemistry at the metal center while retaining stereocenters on the polymer chain;57 the polymeryl exchange process may be accompanied by configurational inversion of the polymeryl carbon atom that is σ-bonded to metal,56,84 but this will not influence isoselection. However, the net effect would be to disturb the epimerization process, resulting in increased relative concentrations of site 2. Note that polymeryl chain exchange is likely to be somewhat slower than methyl exchange as was reported by Sita et al.32,85,86 in intermolecular alkyl exchange unassisted by any exogenous agent. We attempted to obtain unequivocal NMR evidence for rapid polymeryl/polymeryl intramolecular exchange between the two Hf centers. However, due to the complicated activation chemistry and complex multi-isomer catalyst structures, hence very complex NMR spectra, attempts to draw unambiguous conclusions by 13C-labeling experiments as reported by Sita et al.85,86 were inconclusive. Note that although non-orthometalated pyridylamido Hf cation in 2(B+) is a relatively dormant species,33,43 it still produces polymer with low activity in both propylene (Table 1, entry 3) and 1-octene polymerizations (Table 2, entry 3). In principle, the non-orthometalated pyridylamido Hf cation in 3(B2+) may produce polymer and contribute to its high total defect content. However, the high isotactic polypropylene obtained by 2(B+) likely suggests that the contribution of the non-orthometalated pyridylamido Hf cationic species is not toward polymers with an increased total ratio of defects. For the present Hf catalysts, when 40 equiv. of ZnEt2 are used in combination with monometallic 1(A+) for propylene polymerization, a decrease in PP Tm is observed together with a slight increase in total defect content (Tm from 151.0 to 143.7 °C; total defect content from 1.8% to 2.7%; Table 1, entries 1 vs 12), possibly due to CTA-assisted intermolecular CCTP. A similar or greater increase in total defect density and reduction of polymer Tm are observed for bimetallic dication 3(B2+)-

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CONCLUSIONS Propylene and 1-octene homopolymerizations were investigated in detail with monometallic and homo-bimetallic organohafnium catalysts. Conformationally flexible dicationic bimetallic Hf catalyst 3(B2+), which is known to undergo rapid intramolecular/intermetal methyl group exchange and to exhibit strong Hf···Hf cooperative enchainment effects in ethylene homo- and copolymerizations, introduces the largest total density of structural defects in the polypropylene and poly(1-octene) products in addition to the greatest polypropylene Mw enhancement and Tm depression. The enhanced defect densities and Tm depressions rival or exceed those of polymerizations mediated by the corresponding monometallic Hf catalysts in the presence of a Et2Zn chain transfer agent (CTA), i.e., produced by CTA-assisted intermolecular chain transfer/exchange. Two possible scenarios (or both) appear to be most reasonable explanations of these bimetallic effects. Besides steric effects, which may slow the rate of site epimerization and hence erode isoselectivity, intramolecular coordinative chain transfer, occurring in the absence of a chain transfer agent, may also be operative.

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EXPERIMENTAL SECTION

Materials and Methods. All manipulations of air-sensitive materials were performed with rigorous exclusion of O2 and moisture in oven-dried Schlenk-type glassware on a dual manifold Schlenk line, interfaced to a high-vacuum line (10−6 Torr), or in an argon-filled MBraun glovebox with a high-capacity recirculator (