Synthesis of ultra-high molecular weight poly(ethylene)-co-(1-hexene) copolymers through high-throughput catalyst screening

A family of permethylindenyl titanium constrained geometry complexes, Me2SB(R′N,3-RI*)TiX2 ((3-R-η5-C9Me5)Me2Si(R′TiX2)), supported on solid polymethylaluminoxane (sMAO) are investigated as slurry-phase catalysts for ethylene/H2 homopolymerisation and ethylene/1-hexene copolymerisation by high-throughput catalyst screening. Me2SB(tBuN,I*)TiCl2 supported on sMAO [sMAO-Me2SB(tBuN,I*)TiCl2] is responsive to small quantities of H2 (<1.6%), maintaining high polymerisation activities (up to 4900 kgPE molTi−1 h−1 bar−1) and yielding polyethylenes with significantly decreased molecular weight (Mw) (from 2700 to 41 kDa with 1.6% H2). In slurry-phase ethylene/1-hexene copolymerisation studies, a decrease in polymerisation activity and polymer molecular weights compared to ethylene homopolymerisation is observed. Compared to many solid supported system, these complexes all display high 1-hexene incorporation levels up to a maximum incorporation of 14.2 mol% for sMAO-Me2SB(iPrN,I*)TiCl2). We observe a proportionate increase in 1-hexene incorporation with concentration, highlighting the ability of these catalysts to controllably tune the amount of 1-hexene incorporated into the polymer chain to produce linear low-density polyethylene (LLDPE) materials.


Introduction
The incorporation of longer chain a-olen monomers into polyethylene chains increases the degree of polymer branching, which lowers the melting point, crystallinity, and density of the polymers. 1 This can lead to signicant increases in polymer exibility, which gives the resultant polymers applications in packaging, foams, elastic bers, and adhesives. 2 Metallocene catalysts containing two h 5 -cyclopentadienyl (C 5 H 5 , Cp) ligands and two s-type ligands (Cp 2 MX 2 ) have similar reactivities with both ethylene and longer chain a-olens; 3 allowing them to incorporate much larger percentages of higher a-olens than traditional Ziegler-Natta catalysts. 4 Unlike the latter, copolymerisation using metallocene catalysts oen results in regular comonomer distributions and forms high strength, high clarity polymers. 5,6 Constrained geometry complexes (CGCs), bridged halfmetallocenes containing amide ligands, such as the Dow Chemical Co. complexes {(3-t Bu-h 5 -C 5 H 3 )Me 2 Si( tBu N)}TiMe 2 (Me 2 -SB( tBu N,Cp 3-tBu )TiMe 2 ), Me 2 SB( tBu N,Cp*)TiMe 2 , Me 2 SB( tBu N,I)TiMe 2 , and Me 2 SB( tBu N, 3-OMe I)TiMe 2 , 7 have been shown to be highly efficient ethylene/olen copolymerisation catalysts, with high levels of olen incorporated into the polymer chains. 5,8,9 For example, in the solution phase, a-olen incorporations of 25.3 mol% have been observed for ethylene/1-octene copolymerisation using Me 2 -SB( tBu N,Cp*)TiMe 2 /[HNMe(C 18 H 37 ) 2 ][B(C 6 F 5 ) 4 ] (20 bar ethylene and 300 g 1-octene), 7 and incorporations of 69.9% for ethylene/1-hexene copolymerisation using Me 2 SB( tBu N,Cp*)Ti(CH 2 Ph) 2 /MAO (1 bar ethylene and 44.5 mmol 1-hexene). 10,11 These CGCs are of industrial interest due to their enhanced ability to copolymerise ethylene and longer chain a-olens when compared to Cp 2 MX 2 metallocene catalysts. 7,8,11,12 This has been attributed to the less crowded coordination sphere, decreased tendency to undergo chain transfer reactions, and smaller bite angle (Cp cent -M-N angle) of CGCs compared to metallocenes (Cp cent -M-Cp cent ) (by approximately [25][26][27][28][29][30]. 13 CGCs are highly tuneable, and variation of the complex components can dramatically inuence polymerisation activities. 13 It has been found that for CGCs containing a substituted indenyl fragment, the addition of electron-donating substituents leads to both increased copolymerisation activity and polymer molecular weights. 14 One advantage of CGCs is their ability to produce polyethylenes with very ultra-high molecular weights, with M w oen in excess of 1000 kDa. 10,[15][16][17] The long polymer chains transfer pressure more effectively to the polymer backbone, resulting in very tough materials with the highest impact strength of any thermoplastic currently produced. 17 The extremely low moisture absorption, very low friction coefficient, biological inertness, and self-lubricating nature of UHMWPE have led to their use in shing lines, joint replacements, and impact-resistant materials in the military. [17][18][19] We recently reported the synthesis and characterisation of a new family of CGCs based on the permethylindenyl ligand (C 9 Me 7 , Ind*, I*): {(3-R-h 5 -C 9 Me 5 )Me 2 Si( R 0 N)}TiX 2 (Me 2 SB( R 0 N, 3-R I*)TiX 2 ; R ¼ H and Et; R 0 ¼ i Pr, t Bu, and n Bu; X ¼ Cl, Me, CH 2 Ph, and CH 2 SiMe 3 ) (Chart 1). 20,21 When immobilised on solid polymethylaluminoxane (sMAO), 22 an insoluble form of oligomeric MAO, the CGCs were found to be very active catalysts for slurry-phase ethylene polymerisation, ethylene/1-hexene copolymerisation, and ethylene/ styrene copolymerisation with activities up to 7048, 4248, and 2036 kg PE mol Ti À1 h À1 bar À1 respectively. 21 The catalysts showed low levels of 1-hexene and styrene incorporation (1.9-2.4 mol% and 1.6-2.5 mol% respectively) with 1-hexene incorporation levels found to increase with increasing copolymerisation temperature. 21 Herein, we report a systematic investigation of the polymerisation performance of sMAO supported permethylindenyl titanium constrained geometry complexes for ethylene and ethylene/1-hexene copolymerisation using a high-throughput catalyst screening methodology.

Results and discussion
The CGCs in Chart 1 were immobilised on solid polymethylaluminoxane (sMAO) with an initial aluminium to titanium catalyst loading ([Al sMAO ] 0 /[Ti] 0 ) of 200, using a procedure described in previous work. 20 The catalysts were studied under high-throughput conditions for ethylene homopolymerisation with or without dihydrogen (H 2 ), and ethylene/1-hexene copolymerisation. The high-throughput system allowed a large number of parallel experiments to be run simultaneously, enabling the screening of different conditions in a shorter time period. 23 Ethylene/H 2 homopolymerisation Polymerisation activities decreased with the addition of H 2 , however, the catalysts remained very active; activities of 6700, 5700, and 4800 kg PE mol Ti À1 h À1 bar À1 for 1 sMAO with 0, 0.8, and 1.6% H 2 respectively ( Fig. 1 and Table 1). The decrease in polymerisation activity with increasing H 2 pressure was found to be greater for the alkylated catalysts (2 sMAO , 3 sMAO , and 4 sMAO ) than the dichloride (1 sMAO and 6 sMAO ) and mono-chloride (5 sMAO ) catalysts; with 1.6% H 2 , activity decreased by 28, 30, and 42% for 1 sMAO , 5 sMAO , and 6 sMAO when compared to ethylene homopolymerisation, and by 43, 54 and 65% for 4 sMAO , 2 sMAO , and 3 sMAO . The differences in the relative changes in activities and the absolute activities of sMAO-Me 2 SB( tBu N,I*) TiX 2 (1 sMAO -4 sMAO ) catalysts also suggests that the initiator groups remain coordinated to the surface of the support and inuence the nature of the active species through a secondary coordination effect. 24 Chlorides initiating group could also block the active sites. Over the course of the polymerisation runs, the in situ ethylene uptake rate proles show lower uptake rates for ethylene polymerisation with H 2 compared to without H 2 ( Fig. 2 and S1-S3 †). The lower activities and ethylene uptake rates for ethylene/H 2 polymerisation are attributable to the formation of a metal hydride species from chain transfer to H 2 , which requires reactivation by propagation. [25][26][27] The lower polymerisation activities may also be due to the formation of dormant bimetallic resting states with a bridging hydride, as has been proposed in the solution phase, that require reactivation to form the cationic methyl species. 28,29 For 1 sMAO , ethylene polymerisation with 0.8% H 2 initially shows a higher ethylene uptake rate than for polymerisation without H 2 ; H 2 may activate an alternative site for a short period, 27 which then becomes deactivated as polymerisation progresses (Fig. 2).
Polymer molecular weights (M w ) decreased with increased addition of H 2 ; M w of $80 and $45 kDa with 0.8 and 1.6% H 2 respectively for all catalysts (Table 1, Fig. S9 and S11-S13 †). The narrowing of the molecular weight distributions with increased addition of H 2 , (M w /M n of 3.8 and 2.7 for 5 sMAO with 0 and 1.6% H 2 ) suggests increased control in the reaction. 30 Crystallisationelution fractionation (CEF) showed that the maximum elution temperature (T el,max ) of the polymers decreased slightly in the presence of H 2 (T el,max of 113.3, 112.1, and 111.8 C with 0, 0.8,   and 1.6% H 2 respectively for 2 sMAO ), indicating a slight decrease in melting point and crystallinity (Table S1 and Fig. S18-S20 †). The amorphous fraction (AF) increased in the presence of H 2 ; AF of 0.2, 0.5 and 0.7 with 0, 0.8, and 1.6% H 2 respectively for 2 sMAO .
Large reductions in activity were observed for ethylene/1hexene copolymerisation compared to ethylene homopolymerisation (6700 and 3600 kg PE mol Ti À1 h À1 bar À1 for 1 sMAO with 0 and 250 mL 1-hexene respectively), indicating that the negative comonomer effects outweigh the positive effects ( for 5 sMAO with 0 and 250 mL 1-hexene respectively. Many theories have been proposed for the positive comonomer effect, including fracturing of catalyst particles exposing new sites, the formation of new active species by coordination of a-olens, and activation of dormant active sites; however, many of these have been refuted for molecular catalyst systems. 33 Studies have also shown that the addition of 1-hexene to an alkane reaction mixture leads to a 7-10% increase in ethylene solubility between 70-90 C, 34 as well as improved diffusion of ethylene close to the catalytic site, which improves polymerisation activity. 35 The negative effects of comonomer addition are proposed to be due to competitive binding between ethylene and a-olens and, if the rate of migratory insertion of the a-olen is slower than that of ethylene, the rate of chain propagation will decrease leading to a decrease in polymerisation activity. 33 The negative effects of comonomers on ethylene polymerisation activity may also be due to slower rates of insertion; the increased steric bulk of a-olen comonomers in the polymer chain can lead to reduced rates of ethylene insertion. 36 Through monitoring changes in temperature during polymerisation, an exothermic temperature spike to approximately 85 C was observed at the start of the copolymerisation experiments. As the alkyl catalysts (2 sMAO , 3 sMAO , and 4 sMAO ) are much more sensitive to polymerisation temperature than the dichloride catalysts (1 sMAO , 6 sMAO , 7 sMAO , and 8 sMAO ), 21 this thermal spike caused more substantial decreases in polymerisation activities for these catalysts; activity decreases from 6700 to 3600 kg PE mol Ti À1 h À1 bar À1 for 1 sMAO and from 4400 to 280 kg PE mol Ti À1 h À1 bar À1 for 4 sMAO with 0 and 250 mL 1-hexene respectively. The decreases in polymerisation activity with increasing volumes of 1-hexene are highlighted by the in situ ethylene uptake rate proles, where sharp decreases in uptake rates with 125 and 250 mL 1-hexene are observed when compared to ethylene homopolymerisation ( Fig. 4 and S4-S7 †). Polymerisation activity was observed to increase with increasing electrondonating ability of the amido fragment ( t Bu > i Pr > n Bu; 1 sMAO > 7 sMAO > 8 sMAO ) (Fig. S8 †), as observed in previous work. 20 Kamigaito et al. and Nomura et al. have also observed similar effects when using Me 2 SB( R N,Cp*)TiCl 2 /MAO (R ¼ t Bu, Ph, and   Klosin et al. have previously reported the effects of variation of the indenyl moiety on ethylene/1-octene copolymerisation, nding that increased electron-donating ability led to higher activities and polymer molecular weights. 14 The opposite effect is observed for these systems, where 6 sMAO shows a lower ethylene polymerisation activity than 1 sMAO , attributed to its decreased thermal stability; activities of 1100 and 3600 kg PE mol Ti À1 h À1 bar À1 respectively with 250 mL 1-hexene. The lower polymerisation activity of 3-ethylpentamethylindenyl supported catalysts relative to the permethylindenyl analogs has been observed previously for ethylene polymerisation using 1 sMAO and 6 sMAO with 2 bar ethylene and 50 mL solvent at temperatures above 70 C, 21 and when using sMAO-Me 2 SB(2,7-tBu Flu, 3-R I*)ZrCl 2 catalysts. 40 6 sMAO also shows greater decreases in activities for ethylene/1-hexene copolymerisation compared to ethylene homopolymerisation (35 and 58% decreases for 125 and 250 mL 1-hexene respectively) than 1 sMAO (22 and 46% decreases respectively). Similar to alkylated catalysts (2 sMAO , 3 sMAO , and 4 sMAO ), this may be due to the exothermic temperature spike at the beginning of the copolymerisation experiment and the lower thermal stability of 6 sMAO compared to 1 sMAO . The catalysts produced polymers with very high levels of 1hexene incorporation for supported systems (up to 14.2 mol% for 7 sMAO ), conrming the production of ethylene/1-hexene copolymers. This is a trait commonly observed for CGCs that is attributed to the open metal centre resulting from the straininducing ansa-bridge (Table 2). 8,12,13 The incorporation levels observed for these catalysts are lower than for solution-phase ethylene/1-hexene copolymerisation using Me 2 SB( tBu N,Cp*)Ti(CH 2 Ph) 2 with an MAO cocatalyst (65-70% 1-hexene incorporation). 10 However, supported catalysts typically give lower incorporation levels than homogeneous catalysts due to mass transfer effects, where both the support and the propagating polymer chain cause diffusional resistance of the comonomer towards the active sites. 41,42 The active sites of supported catalysts may also become blocked with polymer more quickly than the same catalysts in solution and therefore become inaccessible. 42 It was found that 1 sMAO , 6 sMAO , and 7 sMAO produced polymers with similar incorporation levels with 125 mL 1-hexene (5.6-6.3 mol%). However, 7 sMAO produced polymers with much higher incorporation levels than 6 sMAO and 1 sMAO with 250 mL 1hexene (14.2, 8.4, and 6.6 mol% respectively). This suggests that higher levels of 1-hexene incorporation accompany reduced steric bulk in the amido substituent, likely due to easier coordination of 1-hexene to the metal centre. Catalysts containing at least one alkyl ligand (2 sMAO , 3 sMAO , 4 sMAO , and 5 sMAO ) produced polymers with similar incorporation levels; 3.1-3.6 and 3.3-7.4 mol% with 125 and 250 mL 1-hexene respectively.
A similar effect was also observed by Chen and Marks for solution-phase ethylene/1-hexene copolymerisation using Me 2 -SB( tBu N,Cp*)TiMe 2 /(BC 6 F 5 ) 3 and Me 2 SB( tBu N,Cp*)Ti(CH 2 Ph) 2 / MAO where both alkyl ligand containing catalysts produced polymers with $70% 1-hexene incorporation. 10 8 sMAO consistently produced polymers with lower incorporation levels (1.6 and 4.7 mol% with 125 and 250 mL 1-hexene respectively), which may be due to the reduced electron donating ability of n Bu. A proportionate increase in 1-hexene incorporation was observed for ethylene/1-hexene copolymerisation using 2 sMAO -7 sMAO (the amount of 1-hexene incorporated into the polyethylene chain approximately doubled when the amount of 1hexene in the system was doubled), which gives great potential for these catalysts to controllably tune the amount of 1-hexene incorporated into the polymer chain.
Gel permeation chromatography (GPC) showed that as the amount of 1-hexene added to the system increased, the molecular weights (M w ) of the polymers signicantly decreased; the polymers produced using 1 sMAO showed an eight-fold decrease in polymer molecular weights on the addition of 250 mL 1-hexene (M w of 2700 and 330 kDa with 0 and 250 mL 1hexene respectively) ( Table 2, Fig. S10 and S14-S17 †). The decrease in polymer molecular weights likely results from frequent chain termination following 1-hexene insertion and chain transfer to 1-hexene monomers, coupled with a decrease in the rate of chain propagation. 31,43 This effect has been observed and studied for ethylene/a-olen polymerisation using other CGC systems, such as Me 2 SB( tBu N,Cp*)TiMe 2 , Me 2 -SB( tBu N, 2-R I)TiMe 2 , and Me 2 SB( tBu N, 3-R I)TiMe 2 , with work having been undertaken in an attempt to negate the molecular weights decrease by adding heteroatom substituents in the 2and 3-positions on the indenyl moiety. 7,14 The catalysts produced polymers with relatively narrow molecular weight distributions (M w /M n ), which became narrower with increasing volumes of 1-hexene; M w /M n of 3.2, 3.0, and 2.7 for 1 sMAO with 0, 125, and 250 mL 1-hexeneThe polymers produced using 8 sMAO showed wider molecular weight distributions than the polymers produced using the other catalysts (M w /M n of 6.5, 4.4, and 4.0 with 0, 125, and 250 mL 1-hexene respectively), suggesting the potential for more than one active site (Fig. S17 †).
The decreases in T el,max are attributable to the weakening of intramolecular forces between the polymer chains with increasing incorporation of 1-hexene and decreasing molecular weights of the polymers. 44 The amorphous fraction (AF) also increased with increasing 1-hexene concentration; AF of 0.2, 0.7, and 27.2 wt% for 4 sMAO with 0, 125, and 250 mL 1-hexene respectively (Table S2 †). This corroborates with the high temperature 13 C{ 1 H} NMR spectra (Fig. S24-S27 †).