Titanium and Vanadium Ethylene Polymerization Catalysts Containing Tris ( Pyrazolyl ) Borate Ligand : Effects of Polymerization Parameters on Activity and Polymer Properties

O comportamento nas reações de polimerização de etileno envolvendo uma série de complexos de titânio(IV) e vanádio(V), [TpMXCl 2 ] (1, M= Ti, Tp ́= Tp, X = Cl; 2, M = Ti, Tp ́= Tp, X = Cl; 3, M = V, Tp ́= Tp, X = NBu; 4, M = V, Tp ́= Tp, X = NAr) (Tp= HB(3-mesitilapirazolil) 2 (5-mesitila-pirazolyl), Tp = HB(3-mesitial-pirazolil) 3 , Ar = 2,6-Pr 2 -C 6 H 3 ) foi investigado em tolueno ou hexano na presença de MAO ou TiBA/MAO (1:1) como cocatalisadores e variando a temperatura de polimerização. Os resultados de polimerização mostraram que independentemente do tipo de precursor catalítico, altas atividades foram obtidas usando o sistema TiBA/MAO/hexano. Para as espécies de titânio, maxima atividade catalítica foi alcançada à 60 oC enquanto que para as espécies de vanádio as mais altas atividades foram obtidas à 30 oC. Estudos relacionados à influência da razão molar [Al]/[M] (M = Ti, V) mostraram que estes precursores catalíticos apresentaram altas atividades catalíticas utilizando concentrações de Al tão baixas quanto 50:1. As curvas de DSC mostraram a produção de polietilenos lineares com temperaturas de fusão entre 123 e 140 °C. Os pesos moleculares viscosimétricos dos polietilenos estão no intervalo de 7 x 10-16,4 x 10 g mol.


Polymerization procedures
All polymerization reactions were performed in a Büchi reactor (2L) equipped with a magnetic driven mechanical stirrer and temperature measurement indicator.The reactor was rinsed with 1 L of a 0.04 mol L -1 Al( i Bu) 3 solution in hexane prior to use.Under an ethylene atmosphere the correct amounts of solvent (hexane or toluene) and activator solution (MAO or TiBA and MAO) were introduced sequentially.After complete thermal equilibration, the catalyst precursor (1-4) was added as solid and the reactor was quickly pressurized (5.2 atm) under stirring.The total volume of the reaction mixture was 1 L for all polymerization runs.The total pressure was kept constant by a continuous feed of ethylene.The polymerization runs were stopped by cooling, degassing the reactor, and introducing 1 mL of methanol.The polymers were washed with acidified ethanol, then ethanol and water, and dried in a vacuum oven at 60 °C for 12 h.On the basis of the results of multiple runs, we estimate the accuracy of these numbers to ± 8%.

Polymer characterization
Polymer melting points (T m ) and crystallinities were determined on a Thermal Analysis DSC-2010 instrument calibrated with Indium, using a heating rate of 10 o C min -1 in the temperature range 40 -200 °C.The heating cycle was performed twice, but only the results of the second cycle are reported, since the former is influenced by the mechanical and thermal history of the samples.The intrinsic viscosity for titanium PE samples was determined in a modified Ostwald capillary viscometer, at 135 °C in decaline, using concentrations of 0.1 g dL -1 .In the case of the polyethylenes obtained using the vanadium catalysts, a modified procedure 25 was used in which the PE was firstly dissolved in decaline at 165 ºC (concentrations of 0.02 g dL -1 ) and then the PE solution transferred quickly to a modified Ubelohde viscometer at 135 °C.Based on the experimental intrinsic viscosity values [η] it was possible to calculate the viscometric molar mass according to the formula [η] = k( -M v ) α for k = 6.7×10 -4 dL g -1 and the α = 0.67. 26

Polymerization studies involving tris(pyrazolyl)borate titanium catalysts
Ethylene polymerization studies involving [Tp´TiCl 3 ] (1, Tp´= Tp Ms* ; 2, Tp´= Tp Ms ) were carried out in toluene or hexane at 60, 75 and 90 ºC using methylaluminoxane (MAO) or a mixture of triisobutylaluminum (TiBA) and MAO (1:1) as activator system.The polymerization results are summarized in Table 1.As expected, the more sterically open catalyst 1 showed higher activities relative to the catalyst precursor 2 independently of the polymerization conditions used in this study.It is interesting to point out that for the polymerization reactions performed in hexane the activities displayed by 1 and 2 are closer than those ones obtained in toluene.For instance, in toluene at 60 ºC the catalyst precursor 1 was ca.2.2 times more active than 2 (Table 1, entry 1 vs 10) while the use of hexane instead of toluene determined a difference of only 1.4 times (Table 1, entry 4 vs 13).
The influence of the activator is better visualized in Figures 1 and 2. In general, higher activities were found in hexane using TiBA/MAO (1:1) as activator system, mainly in the case of catalyst precursor 2. It seems that under these polymerization conditions, the combination of MAO in suitable Al content, combined with the scavenger properties of TiBA affords better conditions to stabilize the active species.Besides, TMA, which is present in commercial MAO, can acts as a poison in the polymerization reaction.8][29] Therefore, the higher activity observed in the case of TiBA/MAO (1:1) could also be due to the lower nominal amount of MAO which was employed in the polymerization reaction.
The influence of polymerization temperature on activity has been evaluated in the range of 60-90 °C, at [Al]/[Ti] = 1000.For the polymerization reactions carried   out in the presence of MAO, higher activities were obtained at 75 ºC, with a slight decrease at 90 ºC suggesting that higher temperatures can result in higher rates of catalyst deactivation as can be seen in Figures 1 and 2. On the other hand, for the polymerization reactions performed using a combination of activators (TiBA/MAO), higher activities were found at 60 ºC, which decrease at higher polymerization temperatures (75 and 90 °C).Based on these results above mentioned we can speculate that the catalytic species generated under TIBA/MAO activation are more thermal sensitive than those produced in the presence of MAO.Nevertheless, we cannot neglect the higher solubility of TMA in hexane at higher temperature, which would be responsible to initiate a deactivation process of the catalytic species.
The polymer properties have been evaluated by means of differential scanning calorimetry (DSC), and intrinsic viscosity measurements.Characteristics of the resulting polymers are shown in Table 1.
According to Table 1, most of the melting temperatures (T m ) of the polyethylenes are higher than 130 ºC, indicating the formation of high-density polyethylene (HDPE).In addition, most of the resulting polymers present high crystallinities.According to viscosity measurements, molecular weight remained comprised between 6 and 830 × 10 3 g mol -1 .Higher viscosity average molecular weights ( -M v ) were found using the most sterically crowded titanium catalyst 2 in hexane and using TiBA/MAO as activator system as consequence of lower degree of chain transfer process to Al as demonstrated previously. 15Figure 3 represents the correlation between solvent system and ( -M v ) varying the polymerization temperature using catalyst precursor 2.
Polymerization runs in which the temperature was varied reveal that an increase in temperature results in large decreases in molecular weight.For instance, the viscosity average molecular weight ( -M v ) is considerably reduced [from 100 000 at 60 ºC to 7 000 at 90 ºC (1, toluene/MAO catalyst system, entries 1-3); from 710 000 at 60 ºC to 26000 at 90 ºC (2, hexane/MAO catalyst system, entries 13-15)].For these cases, the lower molecular weight products obtained at elevated temperatures can be reasonably ascribed to the higher chain-transfer rates.
To investigate the effects of the [Al]/[Ti] molar ratio on activity, polymerizations of ethylene were performed with catalyst precursor 1 at 60 °C, varying the [Al]/[Ti] ratio from 100 to 2000.Representative data for ethylene polymerizations are summarized in Table 2.
The polymerization results show that the use of a ratio of at least 100:1 is necessary to produce a highly active catalyst system.Nevertheless, it is worth to point out that the use of higher [Al]/[Ti] molar ratios (500 and 1000) did not cause an accentuated increase in activity (Table 2, entry 19 vs 20-21).The maximum activity was achieved at 2000:1 (Table 2, entry 22, 21.4 x 10 3 kg of PE mol[Ti] -1 h -1 ).The DSC results have shown that the [Al]/[Ti] molar ratio does not seem to have a relevant effect on T m of the polyethylenes (T m = 132-134 °C).However, it should be pointed out that the appearance of two peaks in the  polyethylene produced using high concentration of Al (Table 2, entry 22) suggests the formation of at least two different catalytic species.

Polymerization studies involving tris(pyrazolyl)borate vanadium catalysts
To get a survey of the polymerization activity of tris(pyrazolyl)borate vanadium catalysts, [Tp Ms* VCl 2 (NR)] (3, R = t Bu; 4, R = 2,6-i Pr 2 -C 6 H 3 ) ) the ethylene polymerizations have been investigated at a temperature range from 0 to 60 ºC and using different activator systems (MAO or TiBA/MAO).The results are summarized in Table 3. Figures 4 and 5 show the influence of the temperature, solvent, and activator system on the performance of 3 and 4.
Higher activities were obtained using hexane as solvent (Table 3, entries 23-25 vs 26-28 or 32-34 vs 35-37).The available data for these catalytic systems are very limited at present, however, based on the studies performed by McConville et al. related to non-metallocene titanium compounds, 30,31 we speculate that the lower activity of these catalytic species in toluene could be associated to the coordination capacity of toluene to vanadium, which might engender a competition between the coordinating olefin and the solvent itself as presented in Scheme 1.
The vanadium complex 3 was somewhat more active than 4, and very high activities (> 10.1 x 10 3 kg of PE mol[V] -1 h -1 ) can be achieved utilizing TiBA/MAO as activator system (Table 3, entries 29-31).As observed for titanium catalyst precursors 1-2, the optimized catalytic activity for 3-4 has been found using hexane as solvent in the presence of TiBA and MAO as activator system.
In general, the influence of the polymerization temperature on the activity of 3 and 4 demonstrates a

Scheme 1.
Plausible competition between toluene and ethylene monomer towards coordination to the vanadium atom using the catalyst system [Tp Ms* VCl 2 (N t Bu)]/activator.significant increase of the polymerization conversion by an enhancement of the temperature from 0 to 30 ºC as a consequence of higher solubility of the activator at this temperature (30 ºC) (Figures 4 and 5).A further increase to 60 ºC leads to a decrease of the polymerization conversion, indicating reduced complex stability, resulting in a lower activity of the vanadium catalyst systems in the presence of TiBA and MAO at higher temperatures, mainly for catalyst precursor 4.
Polyethylenes produced by 3 and 4 were firstly characterized by differential scanning calorimetry (DSC).DSC curves of the PE showed T m in the range from 123 to 140 ºC.Exceptionally, the DSC curve of the PE produced by 4 with MAO in toluene at 60 ºC (Table 3, entry 34) shows clearly the appearance of two peaks in distinct temperature regions suggesting formation of two different catalyst species in the polymerization medium (Figure 6).
Resulting polyethylenes were insoluble in decaline using standard viscosimetric analysis procedures (135 °C/ concentrations of 0.1 g dL -1 ).Thus, a modified procedure was adopted wherein the PE sample was dissolved in decaline at 165 ºC using 0.02 g dL -1 and the solution was quickly transferred to a viscosimeter at 135 ºC.The viscosity-average molecular weights ( -M v ) of the PE's produced with 3-4 are in the range of 1.4-16.4x 10 6 g mol -1 indicating the production of ultra-high molecular weigh polymers (UHMWPE).The lower ( -M v ) obtained for the PE's produced with 4 related to those ones obtained using 3 (entry 24 vs 33 and entry 30 vs 39) can be associated to higher activity of this latter catalyst.
Considering the higher activity of 3, an additional study was carried out aiming at evaluating the influence of the [Al]/[V] molar ratio on activity.One set of experiment was performed using hexane as solvent at 30 ºC, utilizing TiBA/MAO as the activator system.The results are summarized in Table 4.As observed for titanium compounds, the use of low concentrations of Al determines the formation of highly active catalytic species.Polymerization runs varying the [Al]/[V] molar ratio from 50:1 to 3000:1 revealed that the system is active at [Al]/[V] molar ratio as low as 50:1 (6.6 x 10 3 kg of PE mol[V] -1 h -1 ).Maximum activity was attained using [Al]/[V] molar ratio of 2000:1 (14.6 x 10 3 kg of PE mol[M] -1 h -1 ).Increasing the [Al]/[V] molar ratio to 3000 resulted in lower activity (11.0 x 10 3 kg of PE mol[M] -1 h -1 ).According to these results and taking into account the high cost of the MAO, the optimum Al concentration required to obtain a good performance of the vanadium catalyst has been found using [Al]/[V] molar ratio of 100:1.
The viscosimetric analysis of the PE produced varying the [Al]/[V] molar ratio failed due to the insolubility of these polymer samples in decaline even using high temperature (165 ºC).From the DSC analysis, it was observed that the [Al]/[V] molar ratio does not seem to have a relevant effect on melting temperatures of the polyethylenes (T m = 138-140 °C).

Figure 1 .
Figure 1.Influence of polymerization temperature and solvent/ activator system on activity of 1.

Figure 3 .
Figure 3. Influence of polymerization temperature and solvent/ activator system on the viscosity average molecular weight using 2.

Figure 4 .
Figure 4. Influence of polymerization temperature and solvent/ activator system on activity of catalyst precursor 3.

Figure 5 . 6 .
Figure 5. Influence of polymerization temperature and solvent/ activator system on activity of catalyst precursor 4.

Table 2 .
Effect of the [Al]/[Ti] molar ratio on activity using [Tp Ms* TiCl 3 ] (1) a Figure 2. Influence of polymerization temperature and solvent/ activator system on activity of 2.