Oxovanadium(IV) complexes with [ONNO]-chelating ligands as catalysts for ethylene homo- and copolymerization

Prior to their use in the polymerization reaction, oxovanadium complexes with [ONNO]-ligands have to be activated by an alkylaluminium compound to form the M-alkyl bond and the coordination site for an olefin. The activator can react by attacking on the V = O bond, as it was suggested for oxovanadium(V) complexes with the strongly coordinated tetradentate calix[4]arene ligand [19] or, as it was postulated for vanadium bis(phenoxyimine) complexes, by replacing the ligand with an alkyl-anion [20].

The catalytically active vanadium sites are known for their low stability in olefin polymerization due to their reduction to low-valence species. To overcome that problem, olefin polymerization processes are often run in the presence of a promoter, halogenated hydrocarbons or halocarbon esters like Cl₃CCOOEt [3, 12]. Another method is based on the use of an activator with a lower reduction potential. Earlier reports [18, 22] indicate that salen complexes of vanadium dichloride and vanadium complexes with diamino-bis(phenolate) ligands give higher polymerization activities in conjunction with chloroalkylaluminium compounds, especially with EtAlCl₂. It was also shown that active species in the catalysts which contain oxovanadium complexes are formed only after their activation with a chlorine atom-containing alkylaluminium compound and vanadium is in an oxidation state of +4 in them [19, 20]. Therefore, ethylene polymerization processes in the presence of oxovanadium complexes were conducted in toluene with EtAlCl₂ as an activator.

All complexes upon activation proved to be active in ethylene polymerization. The typical results as summarized in Table 1 indicate that the studied complexes display different catalytic activities under the same conditions. The activity at the Al/V molar ratio equal to 129 (1.8 mmol of activator in the polymerization feed) increase for the following sequence of the catalysts: [VO(salen)] < [VO(acetph)] < [VO(aceten)] < [VO(acacen)]. The catalyst [VO(acacen)]/EtAlCl₂ shows the highest activity also at Al/V = 385 mol/mol (5.4 mmol, entry 23). All the catalysts produce linear polyethylene (which was confirmed by their FTIR spectra) with the melting temperature in the rage 135–140 °C and with high molecular weight.

Not only the ligand structures but also the Al/V molar ratio considerably influences the polymerization behaviour of the catalytic complexes. The activity of [VO(aceten)] (Table 1, entry 28 and 12) and of [VO(acacen)] (Table 1, entry 25, 21 and 23) decrease with the increase of the Al/V molar ratio from 129 to 386. The inverse relation can be observed for [VO(salen)], i.e. increasing Al/V molar ratio up to 386 results in better catalytic activity (entry 32 and 6). However, any further increase in the ratio (entry 9) reduces the catalyst productivity. The temperature effect on the catalytic activity was evaluated for the same complex at the Al/V molar ratio equal to 514 (Table 1, entry 6 and 7). As can be seen, the catalytic system based on the oxovanadium complex bearing a salen ligand did not offer a good thermal stability, just alike salen complexes of vanadium(IV) dichloride reported by us earlier [22].

The polymerization conditions, the Al/V molar ratio and polymerization temperature, also significantly influenced the polymer properties. The molecular weights were decreased for the increasing Al/V molar ratio. The increase of Al/V from 129 to 386 and to 514 changed the molecular weight of polyethylene produced by [VO(salen)] from 539,000 g/mol to 458,000 g/mol and to 250,000 g/mol, respectively. The same effect was observed for polymers synthesized by [VO(acacen)]/EtAlCl₂ (Table 1, entry 25 and 23). These results indicate that the chain transfer to the alkylaluminium compound was an important chain-transfer pathway under adopted conditions. The chain transfer reaction was enhanced at higher temperatures and polyethylene with lower molecular weight was formed at 50 °C then at 30 °C (Table 1, entry 6 and 7). It is a result of higher activation energies for the chain transfer reactions then for the monomer insertions which affects the relative rate of chain transfer as compared to the rate of propagation [34]. In addition, the molecular weight distributions increase with the temperature increase.

The SEM micrographs of polyethylenes obtained with different oxovanadium(IV) complexes (Fig. 1) show that morphology of the polymer produced by [VO(acetph)] is different from those of other polyethylenes: the polymer in question had the form or flakes while other polyethylenes were obtained as irregular granules with diversified sizes.

One of the most important grade of polyethylene, linear low-density polyethylene, is obtained by copolymerization of ethylene with α-olefin and this process allows to control density and crystallinity of polyethylene. The presence of short-chain branches decreases the melting temperature and crystallinity of polymer because the comonomer units interfere with crystallization and reduce the size of crystallites [18]. The length of side chains is dependent on the type of α-olefin and the number of those chains is dependent on the comonomer concentration in the feed and on the ability of the catalytic system to incorporate the comonomer. The ethylene/1-octene copolymerization tests with [V^IVOL]/EtAlCl₂ systems were run under the same conditions as described for ethylene homopolymerization, i.e. 30 °C, 5 atm ethylene, 30 min, and with different cocatalyst-to-complex ratios. The standard comonomer concentration was equal to 0.40 mol/dm³ thought some experiments with other comonomer concentrations were also performed. The results can be seen in Table 2.

All the catalysts lost some activity after introduction of 1-octene. The increase in the 1-octene concentration reduced that activity even further. The activity of vanadium catalysts, when used in copolymerization at Al/V = 129 mol/mol and at the comonomer concentration of 0.40 mol/dm³ (Table 2, entry 33, 31 and 26), was dependent on the ligand structure and that relation was the same as for ethylene homopolymerization ([VO(salen)] < [VO(aceten)] < [VO(acacen)]). However, the amount of polymer obtained from copolymerization was each time considerably lower than from homopolymerization, in most cases a few times lower. That high drop in the reaction yield may be indicative for a very high incorporation of 1-octene. Indeed, in spite of a relatively low comonomer concentration in the polymerization feed, 1-octene incorporation to the polyethylene chain was very high and it amounted to 6.25, 5.40 and 5.26 mol% for copolymers obtained with [VO(salen)], [VO(aceten)] and [VO(acacen)], respectively. Those results suggest that the methyl substituent at the carbon atom in the imine group affects the comonomer incorporation. The catalyst bearing the salen ligand showed the highest comonomer incorporation ability, probably due to lower steric hindrance caused by the hydrogen atom. The increase in comonomer concentration results also in higher comonomer incorporation (Table 2, entry 11 and 10, 29 and 31, 27 and 26).

The effect of the Al/V molar ratio on the copolymerization reaction was tested for the catalytic systems: [VO(salen)]/EtAlCl₂ (Table 2, entry 33, 10) and [VO(acacen)]/EtAlCl₂ (Table 2, entry 26, 22). It is seen that the comonomer incorporation is evidently lower at a higher cocatalyst/complex molar ratio than at a lower Al/V ratio. Moreover, the results confirmed a better comonomer incorporation ability of the salen complex.

It can be seen in Table 2 and in Fig. 2 that the molecular weight of polyethylene obtained with [VO(acacen)]/EtAlCl₂ is much higher than that for copolymers, and that it decreases with the increase of comonomer concentration in the feed. Such effects were observed repeatedly for copolymerizations catalyzed by different catalytic systems [21, 35‐38]. Lower molecular weights of polymers for increasing 1-octene concentrations may result from the chain transfer to comonomer and/or more favourable β-hydride elimination after comonomer incorporation. Besides, the presence of a comonomer and its increasing concentration lead to a broader distribution of molecular weights. And even an additional higher molecular weight fraction can be is observed on the MWD curve of copolymer with higher comonomer content (Fig. 2, curve 3) although its share is low. It is interesting that the difference between the molecular weights of polyethylene and copolymers synthesized with [VO(salen)] was slight only, and copolymers had even higher MW values than the homopolymer (Table 2, Fig. 3). It is not a typical situation, although a much higher molecular weight of copolymer than that of homopolymer was observed earlier for propylene/1-octene copolymers and polypropylene synthesized with the metallocene catalyst [39].

The information on the polymer chain transfer reactions can be obtained from the data on the nature of double bounds in copolymers. The FTIR spectra show the same end groups in copolymers obtained with both catalysts. For copolymers 26 and 33, synthesized with [VO(acacen)]/EtAlCl₂ and [VO(salen)]/EtAlCl₂, respectively, the intensive absorption bands at 1,370 and 1,380 cm⁻¹ _, which are specific for the groups CH₃ and CH₂, are accompanied by very low intensity bands which represent vibrations of the vinyl group (about 910 cm⁻¹) and the bands for the vinylidene and trans-vinylene groups, at about 890 cm⁻¹ and 967 cm⁻¹, respectively (Fig. 4). The vinyl and vinylidene groups may be formed by β-hydrogen elimination or β-hydrogen transfer to the monomers with ethylene or with 1-octene (after 1,2-insertion) as a last inserted monomer. The trans-vinylidene terminal group is formed when a chain with 1-octene after 2,1-insertion is involved in the reaction as the last inserted monomer [40‐42]. Hence, different effects of comonomer on molecular weights of copolymers produced over different catalytic systems do not result from different chain termination reactions but rather from different relations between the rates of chain propagation reactions and chain termination reactions.

The effect of incorporation of 1-octene on melting behaviour of polymers produced by [VO(salen)]/EtAlCl₂ and [VO(acacen)]/EtAlCl₂ is shown in Figs. 5 and 6, respectively. The incorporation of comonomers shifts melting peaks of copolymers in relation to homopolymers towards lower temperatures. Very wide melting peaks can be observed in some thermograms, while other thermograms of 26 and 27 (Fig. 6) reveal not only principal melting peaks at 111 °C and 126 °C, respectively, but also additional small peaks at 125 °C and 138 °C, correspondingly. Those observations are indicative of heterogeneous comonomer distribution. One of the techniques used for characterization of short chain branching distribution in copolymers is the SSA technique (SSA – self-nucleation annealing) which is based on the sequential application of self-nucleation and annealing steps to polymer samples [26, 43]. We apply six self-nucleation and annealing steps with isothermal treatment at 125, 119, 114, 111, 107 and 103 °C. After the use of that procedure, the fractionated copolymers show a series of melting peaks which are consistent with the melting of different mean lamellar thickness crystallites formed and annealed at every temperature applied [43]. Figure 7 shows that four fractions were produced for 31 and 33 polymer samples at temperatures ranging from 98 to 114 °C and that their distribution was unimodal. The shapes of the DSC endotherms obtained for both copolymers are very similar to each other. The copolymer 26 obtained with [VO(acacen)] exhibited a higher number of melting peaks (six) with endotherm minimums ranging from 98 to 129 °C and the distribution of melting peaks for this copolymer being bimodal. Though, the area under two high temperature peaks (with the minima at about 122 °C and 129 °C), corresponding to higher lamellae thickness (and lower 1-octene content), is small.

The microstructures of two selected poly(ethylene-co-1-octene)s synthesized with [VO(salen)] and [VO(aceten)] were analyzed using ¹³C NMR. The spectra of both copolymers were similar to each other, hence only the spectrum of the sample produced by [VO(salen)]/EtAlCl₂ is presented in Fig. 8. As can be seen, no resonances ranging from 40 to 42 ppm for the αα-carbon in the block type sequence are present. This indicates that the octene units are isolated by ethylene units in the polymer chains. The resonance at about 25 ppm for the ββ-carbon in the alternative sequence is not present, either [35].

All oxovanadium complexes, irrespective of the ligand structures, turned out effective in the ethylene/1-octene copolymerization process. Copolymers with a very high content of 1-olefin and essentially different properties in relation to ethylene homopolymer could be produced, even at low comonomer concentrations in the reaction medium. The possibility of using a lower amount of a costly comonomer to obtain the planned change(s) in the polyethylene properties makes that catalyst attractive from the viewpoint of commercial applications. Such complexes, however, have a disadvantage – very low activity. Improved activity of various generations of organometallic catalysts becomes available in many cases after anchoring such catalysts on a support. In our study, [VO(acacen)] was chosen for immobilization. It was immobilized on the complex of magnesium chloride with THF and on the complex of magnesium chloride with EtOH. Both supports were modified with Et₂AlCl [23, 29]. All (co)polymerization reactions with the use of such catalytic systems were carried out in hexane for 30 min, under 5 atm. of ethylene. About 0.07 g of a supported catalyst and 8 mmol of an activator were used.

The activity of previously investigated salen complexes of vanadium(IV) dichloride spectacularly increased after immobilization on the magnesium support [23]. Unfortunately, the activity of supported oxovanadium(IV) complexes was not much improved, although some changes in their catalytic behaviour in comparison with unsupported counterparts could be observed. Both the supported catalysts were found more active in conjunction with MAO then after activation by common alkylaluminium compound, and from among them trialkilaluminium compounds are better activators then Et₂AlCl (Table 3). Their activity increased at higher polymerization temperatures. This is indicative for good thermal stability of the supported systems. Moreover, the supported systems similar to the unsupported one give linear polyethylene with a high melting temperature, in the range of 135.4–137.9 °C, and with high molecular weight. On the other hand, immobilization of a complex on a support affects the molecular weight distribution in the PE product. It is clearly lower in PE synthesized with the use of supported systems (1.4–2.3) than with the use of non-immobilized complexes (4.4–4.6), irrespective of the activator type employed (MAO or Me₃Al). It can be said that single site catalyst is formed after complex immobilization. Immobilization of a complex also influenced the morphology of the polymer product, with some effect also observed for the type of the activator used (Fig. 9). The polymer granules produced by Cat-S2/MAO show porous surfaces and irregular shapes, alike for the polymer obtained from an unsupported system (Fig. 9a), although they are somewhat bigger. The same catalyst activated with Me₃Al yields a polymer with definitely bigger grains of more regular shapes. The polymer obtained after the use of Et₂AlCl as an activator, in turn, has the form of flat flakes.

The use of a higher 1-olefin in the reaction medium depresses the activity of both supported catalysts in relation to their performance in ethylene homopolymerization; that decline is more prominent for higher concentrations of 1-olefin. Immobilization was disadvantageous for a complex in efficient incorporation of a comonomer to the PE chain. At the same comonomer concentration of 0.40 mol/dm³, the copolymer produced by [VO(acacen)] complex contained 5.26 mol % (Table 2, run 26) whereas in the presence of supported catalysts the copolymers having less than 1 mol % of 1-octene units were obtained (Table 4, runs S45, S56, S63). The obtained copolymers had different properties than homopolymers produced by the same catalytic systems. The principal differences include lower melting points, lower contents of crystalline phase and reduced molecular weights in relation to homopolymers.

Higher temperatures improve the catalyst activity, reduce molecular weights and melting points of copolymers (S49, S50 and S51). Increasing temperature also gives a nearly linear growth in 1-octene incorporation to the polyethylene chain which may result from the fact that ethylene solubility in the reaction medium declines at higher reaction temperatures.