Tandem vinyl insertion-/ring-opening metathesis copolymerization with ansa-6-[2-(dimesitylboryl)- phenyl]pyrid-2-ylamido zirconium complexes: role of trialkylaluminum and MAO†

The novel dialkylzirconium complexes L’ZrR2, (R = CH3, Bn = benzyl, CH2SiMe3, L’ = Me2Si{η-tetramethylcyclopentadienyl}{6-[2-(dimesitylboryl)phenyl]pyrid-2-ylamido}) were synthesized. Upon activation with 1 equiv. of [Ph3C] [B(C6F5)4] , both L’Zr(CH3)2 and L’Zr(Bn)2 are quantitatively converted in situ into [L’Zr(CH3)] [B(C6F5)4] − and [L’Zr(Bn)][B(C6F5)4] − while only 28 mol% conversion is observed with L’Zr(CH2SiMe3)2. The aluminum-free cationic catalysts [L’Zr(CH3)] [B(C6F5)4] −, [L’Zr(Bn)][B(C6F5)4] −

This switch can be tuned via the temperature-dependent dissociation of the N-B bond 9 for which detailed variable-temperature 1 H and 11 B NMR measurements have been carried out.
With the aid of a variety of different pre-catalysts, the following key features have already been identified: [8][9][10][11][12][13][14] (i) a crowded ligand sphere around the metal favors the α-H + elimination process, i.e. the switch from VIP to ROMP, (ii) high NBE concentrations are required to support this switch from VIP to ROMP, and (iii) the propensity of the pyridine group to coordinate to boron can be governed by the sterics of the substituents at boron.
Unfortunately, the catalytic system L′ZrCl 2 /MAO displays a low activity in E-NBE copolymerization (≤6 kg of polymer mol −1 catalyst h −1 bar −1 ), which has been attributed to the bulky ligand sphere and the propensity of the alkylidene to undergo cross metathesis with ethylene. 12 Generally, cationic M-alkyl catalysts derived from L′MR 2 via borane-or borate-activation have been demonstrated to display higher polymerization activity than the corresponding cationic M-alkyl catalysts derived from L′MCl 2 /MAO. This can be explained by the fact that catalysts of the type [L′MR] + [BAr F ] − are truly catalytic species while the systems L′MR + /MAO in fact exist in the form of ion pairs or even adducts of the general formula [L′M (μ-R) 2 AlR 2 ] + [RMAO] − . 7,[15][16][17] Here, we report on structural modifications on the halfsandwich zirconium dichlorides (L′ZrCl 2 ) by replacement of both chloro ligands by dialkyl groups (alkyl = CH 3 , benzyl, CH 2 SiMe 3 ). The desired cationic complexes were then prepared via treatment of the dialkyl complexes with a stoichiometric amount of [Ph 3 C] + [B(C 6 F 5 ) 4 ] − . The catalytic performance of the cationic complexes in the homopolymerization of both NBE and E as well as in the copolymerization of E with NBE was explored in comparison to the parent dichloro-complex L′ZrCl 2 , activated by MAO.
Upon in situ activation with [Ph 3 C] + [B(C 6 F 5 ) 4 ] − and triisobutylaluminum (Al i Bu 3 ), 51-60 L′Zr(CH 3 ) 2 , L′Zr(Bn) 2 and L′Zr (CH 2 SiMe 3 ) 2 showed moderate catalytic activity in E-NBE copolymerization ranging from 15 to 80 kg of polymer mol −1 catalyst h −1 bar −1 , which is significantly higher than the activity of L′ ZrCl 2 /MAO (≤4 kg of polymer mol −1 catalyst h −1 bar −1 ) 12 under similar conditions (Table 3, entries 2-6). It is worth noting that L′Zr(Bn) 2 exhibited the highest productivity (80 kg of polymer mol −1 catalyst h −1 bar −1 ) in E-NBE copolymerization, in line with its highest activity in NBE homopolymerization (vide supra). This high activity of [L′Zr(Bn)] + [B(C 6 F 5 ) 4 ] − might be attributed to a reversible η 2 -η 1 rearrangement of the benzylic group, which stabilizes the active sites and reduces deactivation. 15 Generally, NBE incorporation into the copolymers was low (≤2 mol%) compared to the one obtained with L′ ZrCl 2 /MAO (28 mol%) under similar conditions. 12 Solely VIPderived poly(NBE) moieties were observed in poly(E)-co-poly (NBE) (Fig. 5). The characteristic resonances at δ = 47.0 (C 2 /C 3 ), 41.5 (C 1 /C 4 ) and 32.9 (C 7 ) ppm are assignable to alternating syndiotactic (alt-st, E-NBE-E-NBE)/isolated (E-NBE-E-E) VIPderived sequences while the signal at δ = 29.5 ppm corresponds to PE sequences. 61,62 Also, no signals for alternating isotactic (alt-it, E-NBE-E-NBE) units, NBE diads (E-NBE-NBE-E) or triads (E-NBE-NBE-NBE-E) were observed. Although the altst and isolated NBE sequences cannot be distinguished as a result of overlapping signals, it is with respect to the low NBE incorporation and the bulky ligand sphere reasonable to propose isolated NBE sequences. 13 C NMR analysis proved that PE sequences in the copolymers were mostly mainly linear with low degrees of branching (∼1 branch per 1000 carbons), indicative for a low β-hydride elimination and chain-walking propensity. Table 2 Results for NBE homopolymerization (ROMP) by the action of L'Zr(CH 3 ) 2 , L'Zr(Bn) 2 and L'Zr(CH 2 SiMe 3 ) 2 activated by [Ph 3   In addition, a terminal vinyl group was observed in all PE sequences of copolymers as clearly demonstrated by multiplets at δ = 5.9 and 5.0 ppm in the 1 H NMR spectrum (Fig. S5, ESI †) and at δ = 139.3 and 114.1 ppm in the 13 C NMR spectrum 14 (Fig. 5). Similarly, the PE prepared from L′Zr(CH 3 ) 2 / [Ph 3 C] + [B(C 6 F 5 ) 4 ] − /Al i Bu 3 showed vinyl terminals (Fig. S6, ESI †); the PE itself was mainly linear with 2 branches per 1000 carbons as estimated by 13 C NMR (Fig. S7, ESI †). In combination with the melting point (T m ) of poly(E)-co-poly(NBE), which ranged from 129 to 131°C and a single peak in the GPC, the structure of the copolymers produced by L′Zr(CH 3 ) 2 , L′Zr(Bn) 2 and L′Zr(CH 2 SiMe 3 ) 2 activated with [Ph 3 C] + -[B(C 6 F 5 ) 4 ] − and Al i Bu 3 was VIP-type poly(E)-co-poly(NBE) with few branches (Fig. 5 and Fig. S8-S9, ESI †). With L′Zr(CH 3 ) + /Al i Bu 3 , an increase in polymerization temperature from 30 to 80°C resulted in a dramatic decrease in molecular weight (M n ) from 570 000 to 32 000 g mol −1 and a significant increase in the polydispersity index (Đ) from 2.6 to 7.6, indicative for substantial β-hydride elimination/chain transfer to monomer. Generally, AlR 3 plays an essential role in the polymerization process. In VIP, heterobimetallic complexes termed as [L′M(μ-R) 2 AlR 2 ] + are considered to be the dormant species. 15,[63][64][65] Consequently, subsequent dissociation of AlR 3 to form metal alkyl cations is the key step in olefin polymeri- zation. 15,66 The reversible coordination/decoordination of the pyridyl group in L′ZrCl 2 has been shown to play a crucial role in the α-hydrogen abstraction process. 12 Unlike L′ZrCl 2 , L′Zr (CH 3 ) 2 displays only the "open" structure with no pyridyl coordination to boron as evidenced by 11    and CH 2 SiMe 3 groups and the solid state structures it is reasonable to assume that the pyridine group is not coordinated to boron in the temperature range of polymerization. The finding that [L′ZrR] + [B(C 6 F 5 ) 4 ] − (R = CH 3 , Bn, CH 2 SiMe 3 ) forms ROMP-derived poly(NBE) in the absence but not in the presence of Al i Bu 3 and that [L′ZrR] + [B(C 6 F 5 ) 4 ] − (R = CH 3 , Bn, CH 2 SiMe 3 ) does form VIP-derived poly(NBE)-co-poly (E) in the presence but not in the absence of Al i Bu 3 strongly suggests that Al i Bu 3 binds to the pyridyl-moiety in [L′ZrR] + , thereby terminating its capability to induce α-hydrogen abstraction, a process that occurs in the absence of Al i Bu 3 . Accordingly, the cationic complexes are capable of forming VIP-derived poly(NBE)-co-poly(E) in the presence of Al i Bu 3 , but not in its absence. In the absence of Al i Bu 3 , α-H + elimination dominates and poly(NBE) ROMP  For the copolymers produced by L′Zr(CH 3 ) 2 /MAO and L′Zr (Bn) 2 /MAO, only VIP-derived poly(NBE)-co-poly(E) was obtained. Low NBE incorporation, i.e. 1.1 and 2.0 mol%, respectively, was observed along with vinyl terminals and few long chain branches (Fig. S12-S13, ESI †). By contrast, L′Zr (CH 2 SiMe 3 ) 2 /MAO produced copolymers with both ROMP-and VIP-derived NBE units in the same polymer chain, that is poly (NBE) ROMP -co-poly(NBE) VIP -co-PE. At 30°C, the ratio of poly (NBE) ROMP : poly(NBE) VIP : PE was 1 : 2 : 97 ( Fig. S14-S15, ESI †) and increased to 22 : 3 : 75 with increasing temperature (50°C) (Fig. 6a and Fig. S16-S18, ESI †). Again, terminal vinyl groups and few long chain branches were observed. A further increase in the temperature (80°C) resulted in a decrease in the proportion of ROMP-type poly(NBE) units in the copolymers (14 : 5 : 81) (Fig. S19-S20, ESI †).
Signals at δ = 47.0 (C 2,3 ), 41.5 (C 1,4 ), 32.9 (C 7 ) and 30.2-29.9 (C 5,6 ) ppm can be attributed to alt-st/isolated VIP-derived poly (NBE) units; those at δ = 29.7 ppm are assignable to PE. As outlined above, isolated VIP-type NBE sequences interrupted by PE sequences are reasonable. This incapability to form consecutive VIP-derived NBE-NBE sequences also accounts for the finding that no VIP-derived sequences are visible in poly (NBE) ROMP . The signals at δ = 134.2, 133.2 (C 2,3 ), 43.0, 38.8 (C 1,4 ), 42.8, 42.1, 41.4 (C 7 ) and 33.5, 32.5 (C 5,6 ) are unambiguously assignable to ROMP-derived NBE sequences. In contrast to poly(NBE) ROMP -co-poly(NBE) VIP -co-PE prepared by a structurally similar Ti-catalyst, 8 copolymers had a substantially higher trans-content with more tttt and cccc sequences (t = trans, c = cis). Notably, these signals prevail even after extensive hot extraction with THF. This together with the finding that poly (NBE) ROMP is a high cis polymer but poly(NBE) ROMP -co-poly (NBE) VIP -co-PE is predominantly trans and the unimodal GPC traces (Fig. S21 †) strongly suggest that the ROMP-derived sequences are part of the entire polymer chain. Further evidence for the proposed copolymer structure (Fig. 6a) comes from the absence of any glass transition (Fig. S22 †) attributable to pure poly(NBE) ROMP . This absence of any poly (NBE) ROMP -derived T g in combination with the high incorporation of ROMP-derived poly(NBE) sequences (vide supra) also points towards a multi-block structure, however, without real proof.
Our plausible explanation for the formation of this unique polymer structure is an α-H + elimination/addition process as outlined in Scheme 2. According to our proposed scheme, L′Zr (CH 2 SiMe 3 ) 2 is activated by MAO to form the cationic species L′Zr(CH 2 SiMe 3 ) + , which through α-H + elimination yields a Zr-alkylidene (L′ZrvCHSiMe 3 ) that promotes ROMP of NBE. As outlined earlier, 8,12,13 substantial steric congestion is required to induce the switch from VIP to ROMP. In that regards, the CH 2 SiMe 3 group acts similar, though less effective, than a NBE group between the metal and the polymer chain. In the presence of ethylene, L′ZrvCHSiMe 3 yields a Zr-methylidene (L′ZrvCH 2 ) via cross metathesis, which is also ROMP-active, too. The previous observation that with catalysts containing the 6-(2-BR′ 2 -phenyl)pyrid-2-ylamido motif (R′ = ethyl, mesityl) high NBE concentrations are needed to form any ROMP-derived sequences 8,10-13 strongly suggests that cross metathesis of any Zr-alkylidene with E to form Zr-methylidene is the predominant reaction in these systems. This is in line with the high diffusivity of E. The transient Zr-methylidene is presumably exhausted to most extent as a result of its low stability, e.g. via bimolecular decomposition, 12 which accounts for the low ROMP propensity and the low productivity of these pre-catalysts upon activation with MAO in E-NBE copolymerization. As already outlined, this accounts for the finding that no polymer is obtained in the copolymerization of E with NBE by the action of [L′Zr(CH 3  to the ROMP-active species can re-establish the VIP-active species, which now incorporates E and NBE. During E-NBE insertion copolymerization, E incorporation is favored as evidenced by the high E content in the resulting E-NBE copolymers. The proposed switch from ROMP to VIP, i.e. α-H + addition, is remarkable, since for such a step the pyridinium (Py-H + ) moiety formed in course of α-H + abstraction must be stable in the presence of MAO at least for a short time. In contrast to Al i Bu 3 and most probably because of its size, MAO neither effectively blocks the pyridine moiety nor instantaneously deprotonates the Py-H + moiety, which explains both for the ROMP propensity in the presence of MAO and the ROMP-inactivity in the presence of Al i Bu 3 . In fact, as outlined earlier, 8 higher MAO concentrations result in larger fractions of ROMPderived units in E-NBE copolymers. Any additional α-H + elimination in course of the copolymerization would regenerate the ROMP-active species, however, in view of the high propensity of the system to undergo cross metathesis with E (vide supra), only very few additional ROMP-derived poly(NBE) sequences can be expected to form ( presumably <1% with respect to the initial amount of pre-catalyst). Instead, because of the instability of the Zr-methylidenes at elevated temperature, polymerization quickly comes to an end, which is indeed observed.
What would be needed to boost productivity is an internal olefin that forms a more stable alkylidene in course of the cross metathesis with the Zr-alkylidene. Unfortunately, internal olefins are unable to undergo VIP.

Conclusions
Three new complexes, L′Zr(CH 3 ) 2 , L′Zr(Bn) 2 and L′Zr (CH 2 SiMe 3 ) 2 , based on modifications of L′ZrCl 2 have been synthesized. These compounds are thermally stable and do not allow for the generation of alkylidenes, neither via thermolysis nor via the addition of phosphines. The corresponding cations were prepared in situ upon activation with [Ph 3 C] + [B(C 6 F 5 ) 4 ] − . The aluminum-free monoalkyl cations are able to produce predominantly cis, ROMP-derived poly(NBE) instead of VIPderived poly(NBE) in NBE homopolymerization. The formation of alkylidenes in the absence of any aluminum reagent is proposed. Dialkyl complexes activated with [Ph 3 C] + [B(C 6 F 5 ) 4 ] − and Al i Bu 3 exhibit moderate catalytic activity in E-NBE copolymerization, producing only VIP-type poly(NBE) and branched PE with vinyl terminals. Polymer structures are considered to be a result of the high propensity of Al i Bu 3 to react with the pyridyl- moiety in [L′ZrR] + (R = CH 3 , Bn, CH 2 SiMe 3 ). Similar to L′ZrCl 2 / MAO, which affords E-NBE copolymers containing both ROMP-and VIP-derived poly(NBE) in the PE chain with low productivity, L′Zr(CH 2 SiMe 3 ) 2 /MAO allows for the synthesis of copolymers containing both (mostly trans) ROMP-and VIPderived NBE sequences within the same polymer chain, i.e. poly(NBE) ROMP -co-poly(NBE) VIP -co-poly(E), through an α-H + elimination/addition process. By contrast, L′Zr(CH 3 ) 2 /MAO and L′Zr(Bn) 2 /MAO produce poly(NBE) VIP -co-poly(E) without any ROMP-derived poly(NBE) sequences. We attribute the incapability of L′Zr(CH 3 ) 2 /MAO and L′Zr(Bn) 2 /MAO to promote the ROMP of NBE to the low stability of L′ZrvCHR (R = H, Ph) and their high propensity to undergo cross-metathesis with E. Finally, implications on the copolymerization of α-olefins (E) with NBE using "standard" metallocenes are clear. Apart from high NBE concentrations (catalyst : NBE > 1 : 5000), copolymerizations must be MAO-co-catalyzed and the catalyst, which ever, must allow for α-H + elimination in order to observe ROMPderived structures. This is in most copolymerization systems not the case. In fact, particularly industrial large-volume systems sometimes contain substantial amounts of aluminum alkyls, which not only promote chain transfer and increase productivity [70][71][72] but also effectively prevent the formation of any ROMP-active sites by blocking any Lewis-basic groups.

Experimental
All manipulations were carried out using standard Schlenk or dry box techniques under an argon or nitrogen atmosphere unless specified otherwise. Deuterated solvents for NMR measurements were degassed by several freeze-pump-thaw cycles and stored inside a glove box. Benzene-d 6 and toluened 8 were dried and distilled from sodium/benzophenone; CD 2 Cl 2 was distilled from P 2 O 5 ; C 2 D 2 Cl 4 and o-C 6 D 4 Cl 2 were distilled from CaH 2 . THF, diethyl ether, toluene, n-pentane and CH 2 Cl 2 were deoxygenated by sparging with N 2 and passed through a triple-column solvent purification system (MBraun, Garching, Germany). Commercially available reagents for synthesis, i.e. methylmagnesium bromide solution  4 ] − were used without further purification. Methylalumoxane (MAO, 10 wt% solution in toluene) was purchased from Sigma-Aldrich, Germany. The toluene was removed in vacuo and the remaining white powder was dried in vacuo at 80°C overnight to remove any free AlMe 3 . Celite was dried in vacuo at 180°C for two days prior to use. Before charging the autoclave, ethylene (E) gas was purified by passing it through columns filled with Cu-catalyst (BASF R3-11G) and 3 Å molecular sieves. All homopolymerization reactions of NBE were performed in Schlenk tubes under N 2 atmosphere. The homopolymerization of E and all copolymerizations of E with NBE were carried out in a Büchi-Uster pressure reactor equipped with a Huber thermo-stat. The ethylene pressure was kept constant and E-consumption was monitored with the aid of a Büchi pressflow bpc 6010 flow controller. 1 H and 13 C NMR spectra were recorded at 400 and 100 MHz, respectively, on a Bruker Avance III 400 spectrometer at 25°C unless noted otherwise. Chemical shifts are reported in ppm and referenced to tetramethylsilane (TMS). All 1 H and 13 C NMR data of the ethylene homo-and E-NBE copolymers were measured at 100°C except where noted. Molecular weights and molecular weight distributions were obtained by high-temperature gel permeation chromatography (HT-GPC) on an Agilent PL-GPC 220 system equipped with three consecutive PL gel 5 μm MIXED-C 300 × 7.5 mm columns with 1,2,4-trichlorobenzene as the solvent at 160°C. The flow rate was set to 1 mL min −1 . The GPC system was calibrated with narrow polystyrene (PS) standards purchased from Polymer Labs with the molecular weights in the range of 162-6 035 000 g mol −1 (Easi Vial-red, yellow and green, Fig. S23 †). Melting points and glass transition temperature were measured by differential scanning calorimetry (DSC) under N 2 atmosphere on a Perkin-Elmer DSC 4000 at a heat rate of 10°C min −1 .

L′Zr(CH 3 ) 2
To a solution of L′ZrCl 2 (300 mg, 0.396 mmol) in diethyl ether (20 mL) at −35°C was added MeMgBr (3.0 M in diethyl ether, 0.30 mL, 0.911 mmol). The solution became a suspension within 10 min and was stirred overnight at room temperature in the dark. After the removal of the solvent, the yellow residue was dissolved in toluene (10 mL) and the insoluble solid was filtered off through a pad of celite.

General procedure for NBE homopolymerization
All preparations were carried out inside a glove box. Defined amounts of NBE were dissolved in toluene (40 mL) inside a Schlenk tube. Then a toluene solution (5 mL) of a defined amount of the pre-catalyst was added. The mixture was stirred for 5 min before the addition of a defined amount of [Ph 3 C] + [B(C 6 F 5 ) 4 ] − in toluene (5 mL) and heated to the desired temperature. Polymerizations were quenched after 1 h by the addition of methanol (10 mL). The mixture was then poured into methanol (500 mL) containing concentrated HCl (10 mL). The polymer was collected by filtration and adequately washed with methanol and then dried in vacuo at 50°C for 2 days.

General procedure for E homo-and E-NBE copolymerization
Samples were prepared inside a glove box. Polymerization reactions were conducted by using a Büchi glass reactor (500 mL), which was dried at 120°C in vacuo for 2 h, cooled to 30°C and purged with Ar gas before use.  ) and a solution of a defined amount of MAO in toluene (5 mL) were quickly introduced into the reactor and stirred (300 rpm) for 5 min at 30°C before the addition of a solution of pre-catalysts in toluene (5 mL). The reactor was pressurized with ethylene gas once the mixture had reached the desired temperature. The polymerization reaction was quenched after 1 h by the addition of methanol (10 mL). The resulting mixture was then poured into methanol (500 mL) containing concentrated HCl (10 mL). The polymer was collected by filtration and washed with methanol. All the E-NBE copolymers were extracted extensively with THF at 50°C overnight before filtration. The resulting polymers were adequately washed with methanol and then dried in vacuo at 50°C for 2 days.

X-ray measurements and structure determination
Data were collected on a Bruker Kappa Apex 2 duo diffractometer at 100 K. Structures were solved using direct methods with refinement by full matrix least-squares of F 2 , with the program system SHELXL 97 in connection with a multi-scan absorption correction. 73 All non-hydrogen atoms were refined anisotropically.