Elemental Reactions in Copolymerization of Î±-Olefins by Bis

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Elemental Reactions in Copolymerization of r-Olefins by Bis(cyclopentadienyl) Zirconocene and Hafnocene: Effects of the Metal as a Function of the Monomer and the Chain End Anniina Laine,† Mikko Linnolahti,*,† Tapani A. Pakkanen,*,† John R. Severn,‡ Esa Kokko,‡ and Anneli Pakkanen‡ † ‡

Department of Chemistry, University of Eastern Finland, P.O. Box 111, FI-80101, Joensuu, Finland R&D, Borealis Polymers Oy, P.O. Box 330 FI-06101, Porvoo, Finland

bS Supporting Information ABSTRACT: The effect of the metal on elementary chain propagation and termination reactions in hafnocene- and zirconocene-catalyzed olefin copolymerization processes has been systematically studied by quantum chemical methods. Two consecutive monomer insertions, parallel with the competing chain termination reactions, were studied for copolymerization of ethene with propene, 1-butene, and 1-hexene. For the purpose of a comparative study, analogous species along the reaction pathway were studied for each metallocene/monomer combination. Effects due to incorporation of a comonomer were analyzed as a function of both the central metal and the comonomer size. In accordance with the general experimental observations of zirconocenes producing lower molecular weight polymer than the hafnocenes, differences in activation energies for chain propagation and termination are smaller for the zirconocene. For the zirconocene, the activation energy is particularly low for β-hydrogen elimination after secondary comonomer insertion.

1. INTRODUCTION Group 4 metallocene catalysts, hafnocenes and zirconocenes in particular, are well-known for their capability of producing a variety of synthetic polymers.1 In addition to homopolymerization of various R-olefins, the range of the produced polymers can be further expanded to a diverse group of elastomers by copolymerization of olefin monomers.1 In the homopolymerization process, the properties of the produced material ultimately depend on the microstructure of the polymer, which can be effectively controlled by tailoring the reaction conditions in combination with structural modification of the catalyst.2,3 In the case of copolymerization, the properties of the product are further modified by the ratio of the two monomers, alongside with their distribution, in the polymer.4 Copolymerization of ethene with a higher R-olefin typically yields linear low-density polyethylene (LLDPE), insertion of a higher olefin resulting in short branching of the chain.5 As a consequence, the ability of the catalyst to incorporate the comonomer has a decisive role in determining the properties of the produced polymer. Comonomer incorporation has been observed to induce a “comonomer effect”, altering the rates of chain growth and termination with respect to ethene homopolymerization.1 In homopolymerization, the rate of the polymerization decreases as r 2011 American Chemical Society

a function of the size of the monomer, whereas upon addition of a small amount of comonomer, an increase in the reaction rate is observed. The overall changes in polymerization rates have been attributed to numerous factors, such as electronic and steric changes after comonomer insertion causing larger separation between cation and anion, increase in the number of active sites, and varying crystallinity of the polymer in the reaction medium.6 While the effects are typically attributed to the ultimate monomer, there are data available suggesting that the effects of the penultimate comonomer may also play a role.7 In addition to experiments, computational studies on copolymerization have been reported, with the primary focus on the catalyst ligand structure-polymerization property relationship, comonomer incorporation, and effects of the inserted comonomer.8 In addition to the ligand structure,2,4 the central metal of the hafnocene and zirconocene catalysts has a distinct influence on the polymerization process. Whereas the effects of the ligand structure can often be interpreted in terms of a combination of steric and electronic effects, the effect of the central metal is less straightforward to understand. Notwithstanding the structural Received: August 5, 2010 Published: February 21, 2011 1350

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and electronic similarities of hafnocenes and zirconocenes,9 their polymerization behaviors, while being strongly dependent on the reaction conditions, sometimes show significant differences. It is often the case that methylaluminoxane (MAO)-activated zirconocenes are more active and that hafnocenes produce higher molecular weight polymers.2b,10 The difference has been proposed to arise from the stronger M-C bonds in the hafnocenes11 and has been suggested to affect not only the feasibility of the polymerization reactions but also the efficiency of the activation step.12 Comparative computational studies on the effects of the metal have also revealed different reactivities of the two metallocenes.13 Moreover, experimental data on copolymerization suggests that hafnocenes are able to incorporate higher amounts of comonomer,6e,14even having a tendency of forming blocks of comonomers, whereas zirconocenes tend to produce more random composition copolymers.15 The motivation of the theoretical study reported herein is on the different polymerization behaviors of zirconocenes and hafnocenes. Due to the complexity of the whole catalytic system, many factors contribute to the differences between the catalysts. Nevertheless, a relevant question is if there are fundamental differences between hafnium and zirconium in the elementary copolymerization reactions between olefin monomers. To address the question, a comparative quantum chemical study on the copolymerization of ethene with higher olefins by bis(cyclopentadienyl) zirconocene and hafnocene is reported.

2. COMPUTATIONAL DETAILS All calculations were performed by the hybrid density functional B3LYP method.16 A standard 6-31G* basis was utilized for hydrogen and carbon atoms. The LanL2DZ basis set17 with the corresponding ECP was applied for hafnium and zirconium. The described methodology has been previously demonstrated to apply to both zirconocenes18 and hafnocenes.19 Geometries were fully optimized without any constraints, and all catalytic intermediates were confirmed either as a minimum or a transition state on the potential energy surface by calculation of vibrational frequencies. The calculations were carried out with the Gaussian03 program package.20 All energies are reported in terms of Gibbs free energy (T = 298.15 K, p = 101.325 kPa).

3. RESULTS AND DISCUSSION Analogous studies were performed for ethene-propene, ethene-butene, and ethene-hexene copolymerization pathways, all data for which are reported in the Supporting Information. For clarity, the results, illustrated in Figures 4-12, are based on ethene-butene copolymerization pathways. 3.1. Polymerization Pathways. The elementary copolymerization reactions under study were selected by taking into account all structural cross-propagation combinations for the first two monomer insertions and the corresponding termination reactions. Concerning the insertion steps, ethene insertion has no regio- or stereostructure. Instead, a longer R-olefin adopts either a primary (1,2)-configuration, as in regular homopolymer chain growth, or a secondary (2,1)-configuration generating a structural misprint into the chain.1b This leads to three structural alternatives for the first insertion step. It follows that the number of monomer combinations for the second step is four. These are illustrated in Figure 1 together with two consecutive ethene insertions (A) and two consecutive primary olefin insertions (E), A and E being included as reference homopolymerization reactions. The termination reactions under study are also illustrated

Figure 1. Studied combinations of consecutive reactions with olefin monomers.

in Figure 1 (reactions G-L). The three chain ends resulting from the first insertion can alternatively undergo β-hydrogen transfer to monomer. In addition, β-hydrogen elimination can take place after any of the insertion combinations (reactions A-F). With olefins beyond ethene, alternative stereochemical structures become an issue. However, when another counterpart is an ethene monomer, lacking stereochemistry, the only choice to be made concerns the orientation of the chain. Throughout the study, chain orientations with the minimum steric requirements were considered. The insertion reactions were considered to proceed according to the modified Cossee-Arlman mechanism (Figure 2).21 The starting point for the polymerization process is the generation of the cationic monomethyl form of the metallocene (1 in Figure 2), and thus generation of a vacant coordination site for the incoming 1351

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Figure 2. Schematic presentation of the energetics of the chain propagation pathways (A-F in Figure 1). The reaction intermediates (1-7) are illustrated for ethene homopolymerization. Et = ethene; co(12) = primary comonomer; co(21) = secondary comonomer.

Figure 3. Schematic presentation of the energetics of the chain transfer reactions together with the corresponding insertion reaction. The reaction intermediates (1-6) are illustrated for ethene chain termination routes. 1352


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Figure 4. Energetics of the first insertion step for ethene, primary butene, and secondary butene: (left) hafnocene; (right) zirconocene; Reactant = 1 in Figure 2 þ monomer; TS = 3 in Figure 2; product = 4 in Figure 2. The energy of the reactants is set to 0.

monomer (2). The reaction proceeds through an R-agostic transition state (3), resulting in polymer chain growth (4) and simultaneous generation of a new vacant site for coordination of the next monomer (5). After that, the reactions can continue by either chain propagation (6 and 7) or by the chain termination reactions illustrated in Figure 3. The chain transfer reactions involving only olefins, namely β-hydrogen transfer to metal and β-hydrogen transfer to monomer, were included in the study. The bimolecular β-hydrogen transfer to monomer reaction proceeds through monomer coordination to the β-agostic resting state (1 in Figure 3), resulting in an alkylated metallocene cation coordinated to the polymer chain (3). The β-hydrogen transfer to monomer reaction is an alternative reaction pathway to monomer insertion and hence is energetically directly comparable. β-Hydrogen transfer to metal is a unimolecular reaction, initiating from a β-agostic resting state (4 in Figure 3), proceeding through a transition state (5) to form a complex between metallocene hydride and an unsaturated polymer chain (6). The chain termination finally takes place after release of the chain from the complex. 3.2. Chain Propagation. First Insertion Step. The copolymerization process is a competition between two monomers. The energetics of the first insertion step is illustrated in Figure 4. This shows insertion of ethene both thermodynamically and kinetically favored over insertion of comonomer, as has been reported previously both computationally8 and experimentally.5 This in turn suggests that monomer insertion in general is more likely preceded by insertion of ethene. Second Insertion Step. In addition to the effect of the metal, the study of the second insertion step enables us to examine the fundamental differences between the chain ends in discriminating between the olefin monomers and thereby to single out the direct effects of both the metal and the chain end in the absence of other contributing factors of the complex catalytic system. The results are analyzed on the basis of activation and reaction energies of the total reaction from reactants (product of the first insertion step þ monomer) to reaction products. The effects of the metal and the chain end are illustrated in the three graphs of Figure 5, which show the energetics of the second insertion step for the homopropagation and cross-propagation combinations. The transition-state structures for the studied insertion combinations (A-F in Figure 1) are illustrated in Figure 6. Figure 5a is in connection with comonomer incorporation ability

Figure 5. Reaction energetics for (a) competing homopropagation and cross-propagation reactions after ethene insertion (A vs B vs C in Figure 1) (b) competing homopropagation and cross-propagation reactions after 1-butene insertion (D vs E in Figure 1), and (c) ethene insertion to ethene or 1-butene chain end (A vs D in Figure 1): (left) hafnocene; (right) zirconocene. Reactant = 4 in Figure 2 þ monomer; TS = 6 in Figure 2; product = 7 in Figure 2. The energy of the reactants is set to 0.

of the catalyst, showing the energetics of the insertion of either an ethene or a comonomer after ethene insertion (A vs B in Figure 6). Analogous to homopolymerization,8 ethene insertion is preferred to comonomer insertion, and the comonomer prefers regular primary insertion to secondary insertion. Concerning the metal, the activation energy for ethene insertion is practically equal for both the hafnocene and the zirconocene. However, the difference in activation barriers for ethene-comonomer and ethene-ethene insertions is lower for the hafnocene, suggesting easier comonomer insertion. 1353


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Figure 6. Transition states for insertions of the second monomer (see A-F in Figure 1): (A) ethene homopolymerization; (B) 1,2-butene insertion to ethene chain end; (C) 2,1-butene insertion to ethene chain end; (D) ethene insertion to 1,2-butene end; (E) 1-butene homopolymerization; (F) ethene insertion to 2,1-butene end. The comonomer is illustrated in yellow.

Figure 7. Energetics of the β-hydrogen elimination after two monomer insertions (A-F in Figure 1) for ethene/1-butene copolymerization.

After insertion of a comonomer, the chain growth has two alternatives: cross-propagation by ethene monomer insertion or olefin block formation by takeup of another comonomer (structures D and E in Figure 6), the energetics of which is illustrated in Figure 5b. Comparison between homopropagation and cross-propagation steps shows that following primary comonomer insertion, ethene insertion is favored both kinetically and thermodynamically. Also, the difference in activation barriers for comonomer-comonomer and comonomer-ethene insertions is lower for the hafnocene. Figure 5c illustrates the effect of the chain end by comparing ethene insertion to either the ethene

or 1-butene chain end (A vs D in Figure 6). For both metals, insertion to the ethene chain end is slightly favored over insertion to the comonomer chain end. 3.3. Termination. β-Hydrogen Transfer to Metal. In β-hydrogen elimination, a hydrogen from the β-position of the polymer chain is transferred to the metal, thereby resulting in formation of a complex between the metal hydride and a polymer chain, eventually leading to termination of the chain growth after release of the chain. The release of the chain was not included in the study, as it would lead to an energetically unfavorable naked hydride cation. The effects of the metal and the chain end 1354


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Figure 8. Illustration of transition states for β-hydrogen elimination after two monomer insertions (A-F in Figure 1) for ethene/1-butene copolymerization.

Figure 9. Transition states for β-hydrogen elimination after secondary (2,1) 1-butene insertion for hafnocene (left) and zirconocene (right). The comonomer is illustrated in yellow.

for β-hydrogen elimination are illustrated in Figure 7, the reaction taking place after insertion of the second monomer. Reactions A, D, and F in Figure 7 illustrate the β-hydrogen elimination taking place after ethene insertion; the penultimate monomer (ethene, primary comonomer, or secondary comonomer) has a marginal influence on reaction energetics. β-Hydrogen elimination reaction is endothermic for the ethene chain end. Replacing the ethene chain end by 1,2-comonomer facilitates the reaction both kinetically and thermodynamically. This is due to the change of the β-hydrogen from secondary to tertiary carbon, hence affecting its reactivity. Overall, the activation energies are practically independent of the penultimate monomer but are notably affected by the last inserted monomer. The transition-state structures are illustrated in Figure 8. The β-hydrogen of the ethene chain end is bound to secondary

carbon (A, D, and F in Figure 8) but in the 1,2-comonomer to tertiary carbon (B and E in Figure 8). Regioirregular 2,1-insertion of comonomer generates a β-hydrogen bound to secondary carbon (C in Figure 8), similar to the case of ethene, and hence one could expect the reaction to be less feasible than the reaction initiating from the 1,2-comonomer chain end. This is exactly the case for the hafnocene. However, for the zirconocene the reaction is more feasible after insertion of the 2,1-comonomer chain, the effect increasing as a function of comonomer size. The origin of the effect being in the structure of the chain end, it also applies for homopolymerization.13a The reason is a structural difference of the transition states: in the case of the zirconocene, a hydrogen bound to the alkyl side chain provides an additional agostic interaction to the metal (Figure 9). This apparently originates from the decrease of the strengths of M-H agostic interactions down a transition-metal triad.22 β-Hydrogen Transfer to Monomer. Alternatively to the βhydrogen elimination, a hydrogen from the β-position of the polymer chain can transfer to a coordinated monomer, either ethene or comonomer, thereby resulting in formation of an alkylmetal cation and ultimately in termination of the chain growth. The effects of the metal, the chain end, and the coordinated monomer for β-hydrogen transfer to monomer are illustrated in Figure 10 and the structures of the transition states (for the case of ethene) in Figure 11. The coordinated monomer has a strong influence on the energetics of the reaction, chain transfer to ethene being favored over chain transfer to comonomer in terms of both kinetics and thermodynamics and the ethene chain end having the clearest 1355


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Figure 10. Reaction energetics for (a) β-hydrogen transfer to ethene monomer and (b) β-hydrogen transfer to comonomer. Reactant = 1 in Figure 3; TS = 2 in Figure 3; product = 3 in Figure 3.

Figure 11. Illustration of transition states for β-hydrogen transfer to ethene (see G, I, and K in Figure 1) from (G) ethene chain end, (I) 1,2butene chain end, and (K) 2,1-butene chain end. The comonomer is illustrated in yellow.

preference for chain transfer to ethene. The metal has a distinct influence, the activation barriers being systematically lower for the zirconocene. The role of the chain end is less straightforward. Similar to the β-hydrogen elimination, the effect of the chain end is dependent on the chemical environment of the β-hydrogen. With regard to chain transfer to ethene, the 1,2-chain end of the comonomer (I in Figure 11) has the highest activation barrier, apparently due to the β-hydrogen being bound to tertiary carbon, thereby making the reaction site more crowded. 3.4. Monomer Insertion vs Chain Termination. The molecular weight of the produced polymer is ultimately determined by the ratio in the rate of olefin insertion and chain termination. Therefore, it is of interest to compare the corresponding insertion and termination reactions discussed above. Comparison of the reactions was based on chain transfer to ethene, since, among the monomers, chain transfer to ethene is preferred (see section 3.3). A comparison between monomer insertion and chain termination by hydrogen transfer to monomer is shown in Figure 12, as the two form a pair of competing reactions between the metal and the olefin. Comparison of chain propagation and chain termination shows propagation a more viable route, a prerequisite of the polymerization process. The metal shows a systematic effect: the differences between the activation energies for chain propagation and chain termination are smaller for the zirconocene, suggesting termination more likely to take place with the zirconocene catalyst. Concerning the effect of the chain end, changing ethene to 1,2comonomer chain end has no practical influence on the ratio of insertion and termination reactions. Instead, 2,1-comonomer

chain end decreases the relative ratios of the insertion and termination, suggesting the termination more likely to take place after monomer misinsertion.The effect is the strongest for 1-butene. 3.5. Comparison to Experiments. The model system has been selected from the point of view of studying the fundamental copolymerization reactions rather than all complex features of the whole catalytic system. Nonetheless, it is of interest to make comparisons to general experimentally observed trends about the relative catalytic performances of zirconocenes and hafnocenes. The absolute activation energies for monomer insertion are roughly the same for the hafnocene and the zirconocene. One could assume the latter should be lower, solely on the basis of the generally higher polymerization activities of zirconocenes.2b,10 Apparently, though, polymerization activities are significantly affected by the catalyst activation step. The alternative insertion steps represent the ongoing competition between an ethene and a comonomer. There is a systematic preference for ethene rather than comonomer insertion, with differences in activation energies ranging from 14 to 22 kJ/mol (see Figure 5a,b). This suggests that the comonomer insertions are less frequent than ethene insertions, a typical characteristic of the copolymer.1 Regarding particularly the cyclopentadienyl metallocenes, experiments show the hafnocene to incorporate more comonomer than zirconocene. Calculations are in line with experiments, the differences in activation energies, in favor of ethene insertion, being 14-18 kJ/mol for hafnocenes and 17-22 kJ/mol for zirconocenes. The comonomer chain end is even more discriminating between ethene and comonomer insertions, the 1356


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chain transfer to monomer show the activation energy to be lower for the insertion, a prerequisite of the polymerization process. The difference between the two is systematically less for the zirconocene, by ca. 10 kJ/mol (see Figure 12), independent of the last inserted monomer, and is lowered by a a 2,1misinsertion. These are possibly connected to the general experimental observations of zirconocenes producing lower molecular weight polymer than hafnocenes, while being more prone to the effects of occasional misinsertions.10

4. CONCLUSIONS A comparative quantum chemical study on the elementary copolymerization reactions between olefin monomers, namely ethene with propene, 1-butene, and 1-hexene, and metallocenes, namely bis(cyclopentadienyl)hafnocene and zirconocene, was reported. The study focused on the effects of the metal on aspects relevant for olefin copolymerization: i.e., comonomer incorporation and the effect of the last inserted monomer. Chain propagation was modeled in terms of the first two monomer insertions, taking into account alternative homopropagation and crosspropagation reactions. The polymer chain growth was terminated by either β-hydrogen transfer to metal or by β-hydrogen transfer to monomer, thereby enabling also direct comparisons between chain growth and chain termination processes. Analogous polymerization pathways were calculated for each metallocene/monomer combination to single out the effects of the metal as a function of the monomer and chain end. The hafnocene shows somewhat better comonomer incorporation. The size of the comonomer has no practical influence on the behavior of the two metallocenes. The two most notable differences between the metallocenes are (1) a decrease in the β-hydrogen elimination barrier for the zirconcene after secondary comonomer insertion, which is a major effect of the chain end, and (2) kinetically more competitive termination reactions for the zirconocene in comparison to propagation reactions. Both of the dissimilarities are in accordance with the general experimental observations of zirconocenes producing polymer with lower molecular weight than for the hafnocenes. ’ ASSOCIATED CONTENT

bS Figure 12. Comparison of reaction energetics for competing monomer insertion and β-hydrogen transfer to monomer for (a) ethene homopolymerization, (b) reaction of ethene to 1,2-butene chain end, and (c) reaction of ethene to 2,1-butene chain end: (left) hafnocene; (right) zirconocene.

hafnocene having a minor preference for consecutive comonomer insertions. Hence, regarding ethene vs comonomer insertion, the calculations are in line with the general experimental observations of hafnocenes being able to incorporate higher amounts of comonomer than is the case for zirconocenes.14,15 Concerning termination reactions, calculations suggest chain transfer to ethene monomer to be more feasible for the zirconocene, the activation energies being 7-10 kJ/mol lower than for the hafnocene. This, as such, suggests the zirconocene produces lower molecular weight, and that is, indeed, what is usually observed.10 Ultimately, though, the molecular weight of the resulting polymer is determined by the ratio of insertion and termination rates. Comparisons between monomer insertion and

Supporting Information. Tables giving Cartesian coordinates and absolute energies of all reported molecular structures. This material is available free of charge via the Internet at http://pubs.acs.org.

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