Kinetic study of coordinative chain transfer polymerization of ethylene by neodymium metallocene catalysts

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1 Kinetic study of coordinative chain transfer polymerization of ethylene by neodymium metallocene catalysts Rodolfo Ribeiro, Christophe Boisson 1, Frank D Agosto 1, Maria do Rosário Ribeiro 2 1 Laboratory of Chemistry, Catalysis, Polymers and Processes (C2P2 UMR 5265), Université de Lyon 2 Institute for Biotechnology and Bioengineering (IBB), Instituto Superior Técnico (IST), Universidade de Lisboa November 215 Abstract. Ethylene polymerizations at 7-9ºC in toluene were performed using a combination of Cp 2 NdCl 2Li(OEt 2) 2 or {Me 2SiFlu 2Nd(µ-BH 4)[(µ-BH 4)Li(THF)]} 2 neodymium complexes with n-butyl-n-octyl-magnesium used as both alkylating and chain transfer agent. Kinetic studies were performed for the two catalytic systems by varying the Mg/Nd ratio, which is a critical parameter for polymerization control. Both systems were able to perform a controlled polymerization of ethylene and displayed the same phases on the kinetic profiles, reflecting the Coordinative Chain Transfer Polymerization mechanism. The effect of the polymerization temperature was studied which allowed the determination of the propagation activation energy using the Arrhenius equation. A value of 17.3 kcal.mol -1 was obtained for the Cp 2 NdCl 2Li(OEt 2) 2/n-butyl-n-octyl-magnesium system, while for the {Me 2SiFlu 2Nd(µ-BH 4)[(µ-BH 4)Li(THF)]} 2/n-butyl-noctyl-magnesium system the linear fit of the Arrhenius plot obtained was poor, and consequently the propagation activation energy was not determined. Specific molar masses were targeted in order to evaluate the polymerization control efficiency. The co-solvent effect was also explored by addition of dibutyl ether (Bu 2O). Unimodal molar mass distributions were obtained, and polymerization rates were enhanced in the presence of the co-solvent without affecting the polymerization control. Finally, a sampling experiment was performed to observe the pseudo-living behaviour of the Coordinative Chain Transfer Polymerization mechanism. Keywords. Neodymium, ethylene, metallocene, kinetics, mechanism, coordinative chain transfer polymerization Introduction Metallocene catalysts have been widely investigated as catalysts for polymerization of various monomers, including ethylene. They were discovered in the same time period as Ziegler- Natta catalysts (195 s) but had very low activities at that time. [1] In 1975, it was discovered that the addition of MAO (methylaluminoxane) to a group 4 metallocene precursor (degree of oxidation of +4) increased greatly the activity, due to the formation of active cationic species, L 2 M-R + (L: ligand; M: Group 4 transition metal; R: alkyl group). [2] However, lanthanide metallocenes (degree of oxidation of +3), such as neodymium metallocenes, don t need a cocatalyst like MAO since a neutral active species of the type L 2 Ln-R (Ln: lanthanide) is formed by in-situ alkylation of the precursor using alkylating agents, i.e dialkyl magnesium (MgR 2 ), as shown on Figure 1. [3] This neutral species has a free coordination vacancy for monomer coordination and have proved to be very active for ethylene polymerization. This can bring a great advantage since it can avoid the use of cocatalysts like MAO which can be very expensive and unstable. The control of polyethylene chain growth is a major challenge in the field of coordination polymerization. Living polymerization (no transfer, no termination) meets this requirement but only one polymer chain are formed per metal center [4], which is very inefficient since the transition metal initiator can be highly expensive, preventing any potential scale-up for the industry. Consequently, strategies to reduce the consumption of highly expensive transition metal catalysts, such as coordinative chain transfer polymerization

2 Figure 1. In situ alkylation of a chloride/borohydrate metallocene precursor using MgR 2 as alkylating agent. This results in a neutral active species which can polymerize ethylene. (CCTP), have been developed. This strategy involves the association of a transition metal based catalyst and a chain transfer agent (CTA), usually in the form of a main-group metal alkyl, such as AlR 3 and MgR 2. [5] The growing polymer chain is able to transfer between the transition metal catalyst, where propagation occurs (active species), to the metallic center of the CTA (dormant species) via a heterobimetallic intermediate. [6] This mechanism is a dynamical equilibrium between a chain-growing state and a chain-transfer state and is crucial for a successful CCTP process. [4] If the chain transfer rate is much greater that the rate of propagation, reversible, and if other chain termination pathways, such as β-h elimination, don t occur (or occur in negligible way) everything will happen as if the transition metal catalyzes the chain growth on the main-group metal alkyl. [6] In this case, CCTP enables catalyst economy since each catalyst molecule generates several polymer chains, and narrow molar mass distributions are obtained since growth probability is the same for all chains. [4], [6] It also can be implemented to make new specialty polymers, such as olefin block copolymers [7],[8] and end- [9], [1] functionalized polyethylene. This work combines all of the aspects mentioned above by studying two independent CCTP systems to polymerize ethylene using two different neodymium metallocene precursors: Cp 2 NdCl 2 Li(OEt 2 ) 2 (1) and {Me 2 SiFlu 2 Nd(µ-BH 4 )[(µ- BH 4 )Li(THF)]} 2 (2). In both systems, it was used the same dialkylmagnesium compound (MgR 2 ) - n-butyl-n-octyl-magnesium (BOMAG) as both alkylating agent and CTA. Toluene is the polymerization solvent for both systems. After the formation of the active catalyst species, L 2 NdR, by in situ alkylation, the CCTP mechanism takes place by which the growing polyethylene chain is able to transfer, in a fast and reversible manner, from the neodymium center (active species) to the magnesium center via the formation of a heterobimetallic complex, L 2 Nd(µ-R 2 )MgR considered as dormant species since they unlikely insert monomer. This CCTP process gives rise to the formation of di(polyethylenyl)magnesium compounds, Mg(PE) 2 (PE = polyethylene chain). The mechanism described is showed in Figure 2. Although the mechanism described is well accepted, few kinetic data is available for this catalyst class in order to confirm the aforementioned mechanism. Therefore, kinetic studies were performed for the two CCTP systems by varying the Mg/Nd ratio, which is a critical parameter for polymerization control, and the polymerization temperature. Certain molar masses were targeted in order to evaluate the polymerization control efficiency and the co-solvent effect was also explored. Finally, a sampling experiment was performed to observe the pseudoliving behaviour of the CCTP mechanism. The use of two neodymium complexes with different ligands allowed studying its influence in the catalyst activity and chain transfer efficiency by comparing the two catalytic systems.

3 Figure 2. CCTP mechanism using neodymium catalysts in association with BOMAG as CTA and formation of Mg(PE) 2. Experimental Section All the manipulations were carried out in an inert atmosphere (argon) using Schlenk techniques. Materials. Ethylene was supplied by Air Liquide with a 99.95% purity. Toluene was purified using a MBRAUN MB-SPS-8 purification unit. Dibuthyl Ether was supplied by Sigma-Aldrich with a 99,3% purity, degassed by argon passage and stored in a Schlenk flask with molecular sieves (3 Å). Butyl(octyl)magnesium (BOMAG) was supplied by Chemtura Manufacturing as a 2 wt% solution in heptane. The neodymium metallocene precursors (1) and (2) were already synthesized according to [11], [12] the literature. Polymerization procedure. Polymerizations were carried out in a 5 ml double-layer glass reactor equipped with a stainless steel blade stirrer. The temperature was controlled by circulating water between the reactor and the jacket. A solution of BOMAG and toluene (4 ml) was transferred to the reactor under argon atmosphere. The reactor was then charged with ethylene pressure of 3,6-3,8 bar in order to completely solubilize the ethylene in the polymerization solvent. When ethylene was complete solubilized and the desired temperature was reached, the ethylene pressure was increased to 4 bar and the catalyst precursor, solubilized in a toluene and BOMAG solution, was introduced in the reactor by a cartridge. In order to measure activities and productivities, which are very important for the kinetic study, it was monitored the pressure drop in the ethylene reservoir during the polymerization. After the polymerization, the reaction was stopped by venting the gaseous ethylene through reactor exhaust and cooling the reactor to 2ºC. Methanol was added to the reactor contents and the suspension filtrated. The polyethylene recovered was dried under vacuum at 8ºC for 2 hours. Size exclusion chromatography. All the polyethylene samples were characterized by size exclusion chromatography at high temperature (SEC-HT). It was performed using a Viscotek HT- GPC by Malvern equipped with one pre-column followed by three columns in series (PLgel Olexis 3 mm x 7 mm I. D. from Agilent Technologies). Sample solutions with concentration between 3 and 5 mg.ml -1 were eluted in 1,2,4-trichlorobenzene using a flow rate of 1 ml.min -1 at 15 C. The mobile phase was stabilized with 2,6-di(tert-butyl)-4- methylphenol (2 mg.l- 1 ). The OmniSEC software was used for data acquisition and analysis.

4 Activity (kg.mol -1.h -1 ) Activity (kg.mol -1.h -1 ) Results and discussion Several studies were performed on the two catalyst systems: (1)/BOMAG and (2)/BOMAG: Influence of Mg/Nd ratio. Figure 3 and Figure 4 shows the kinetic profiles for different Mg/Nd ratios for both systems. All these profiles show a pattern consistent with the CCTP mechanism which includes there phases: after a short initial period during which the activity dropped (phase 1), a steady activity is achieved due to the equilibrium between the heterobimetallic complex and active catalyst species, L 2 NdR, enabling the CCTP mechanism to take place, by which the growing polyethylene chain is able to transfer, in a fast and reversible manner, from the active species to the magnesium center (phase 2). This steady-state phase is stopped when Mg(PE) 2 reaches its solubility limit, at the polymerization temperature, and precipitates, losing mobility and stopping the reversible chain transfer Mg/Nd=2 Mg/Nd=4 Mg/Nd=6 Mg/Nd=8 Mg/Nd= t (min) Figure 3. Kinetic profiles obtained for the catalytic system (1)/BOMAG. (8ºC; 4 bar; [Nd]=5 µm; toluene=4 ml) Mg/Nd=2 Mg/Nd=6 Mg/Nd=4 Mg/Nd=8 Consequently, the number of active sites rises which leads to a peak of activity which is followed by a quick deactivation of the catalyst (phase 3). The only exception is for a Mg/Nd ratio of 16 for system (1)/BOMAG, where only the first two phases are included because it took too long for the precipitation to occur and the polymerization was stopped before it happened. Table 1 lists the main results for these polymerization trials. This investigation shows that: The increase in BOMAG (MgR 2 ) concentration shifts the equilibrium towards the bimetallic complex (Figure 2). The rate of chain growth decreases resulting in lower activities in the kinetic profile and longer times required for the precipitation to be observed since it takes more time to reach the solubility limit of the Mg(PE) 2. This result confirms that this species is dormant which is in agreement with the proposed CCTP mechanism. The polymers analyzed by SEC showed a bimodal distribution of molar mass. This is expected because during the steady-state phase, reversible chain transfer takes place between the magnesium and the neodymium center, resulting in a population of low molar masses. After precipitation, the chain transfer is inhibited, the polymerization control is lost and the activity increases strongly leading to the formation of a second population of higher molar masses. Consequently, the resulting polymer displays a broad molar mass distribution (here bimodal). Comparing the two systems, system (1)/BOMAG displayed approximately twice the activity of system (2)/BOMAG. In terms of molar masses and respective dispersities both systems showed somewhat the same results and with similar orders of magnitude (4 < < 7) t (min) Figure 4. Kinetic profiles obtained for the catalytic system (2)/BOMAG. (8ºC; 4 bar; [Nd]=5 µm; toluene=4 ml).

5 ln (k p app ) ln (k p app ) Table 1. Activity, [Nd] = 5 µm). Run System Mg/Nd 1 and molar mass features from the polymerization trials for both catalytic systems. (T = 8ºC; 4 bar; Steady-State Activity (kg mol -1 h -1 ) (L.mol -1.s -1 ) (g.mol -1 ) (1) / BOMAG (2) / BOMAG Influence of temperature. Polymerizations trials were done for both systems at different temperatures in order to determine the activation energy for this polymerization process using the Arrhenius equation. The results are listed in Table 2 for both systems and the Arrhenius plots are given in Figure 5 and Figure 6. Analyzing the results for system (1)/BOMAG, it can be seen that higher polymerization temperatures lead to higher values, as expected. At higher temperatures, molecule collisions are more frequent and the proportion of reactant molecules with sufficient energy to react is higher, leading to increase in the reaction rate and consequently on the value ,275,28,285,29,295 1/T (K -1 ) Figure 5. Arrhenius plot for system (1)/BOMAG. ([Nd] = 5 µm; Mg/Nd = 8; toluene = 4 ml) ,27,28,29,3 1/T (K -1 ) Figure 6. Arrhenius plot for system (2)/BOMAG. ([Nd] = 5 µm; Mg/Nd = 8; toluene = 4 ml). The linear regression of the Arrhenius plot (Figure 5) displays a very good fit. The activation energy value derived from it is 17.3 kcal/mol. Regarding the precipitation times, two opposed effects have to be considered. On the one hand, the chain growth rate increases with temperature (as seen by the increase of ) and therefore it would reach its molar mass limit in less time, but on the other hand, the solubility limit of Mg(PE) 2 increases for higher polymerization temperatures which would lead to higher precipitation times. According to the experimental results, the solubility increase effect prevails and therefore higher precipitation times are observed for higher polymerization temperatures. The results for system (2)/BOMAG are somewhat unexpected because values increased as the temperature decreased (run 16, 17 and 18). The reason why this does not happen in this system is unknown, and only hypothesis can be assumed. One possibility could be a more efficient alkylation of the neodymium complex at lower temperature leading to higher percentages of active species. It would be interesting to start the polymerization at 6 C and then raised the temperature to 8 C in order to compare the activity at the steady step. These unexpected results do not permit the calculation of the E a for this catalyst.

6 Table 2. Determination of and E a from the polymerizations experiments at different temperatures for both systems. ([Nd] = 5 µm; Mg/Nd = 8; toluene = 4 ml). Run System T (ºC) Precipitation Time (min) 1 (L.mol -1.s -1 ) (1) / BOMAG a) a) (2) / BOMAG a) Polymerization did not reach precipitation in a timely manner. b) The results do not allow performing any calculation. E a Slope (kcal.mol -1 ) b) b) Molar mass targeting and co-solvent effect. Different polyethylene molar masses were targeted in order to evaluate the polymerization control efficiency by comparing the experimental and targeted molar mass. Moreover, the co-solvent effect was explored by the addition of dibutyl ether (Bu 2 O) to the polymerization medium. Both investigations were done just for system (2)/BOMAG because it was already carried out successfully for system (1)/BOMAG in another work. [13] Results are listed in Table 4. From the results for run 2 and 21 several aspects can be discussed. Firstly, the activity and consequently the values are higher than the ones seen on the kinetic profile for a Mg/Nd ratio of 8 (Table 1, Run 9). This may be caused by the short polymerization times (65 and 26 min) which does not let the polymerization reach the steady-state phase. The same goes for the difference in activity and between run 2 and 21. Secondly, a unimodal molar mass distribution was obtained with dispersity values of 2.1 that are relatively high in comparison with others CCTP systems that provide dispersity values lower than 1.3 and with molar masses up to 4 g.mol -1 [14], [15]. Finally, the values of MM theo are far from the ones analyzed by SEC ( ), 157 g.mol -1 vs 237 g.mol -1 and 839 g.mol -1 vs 214 g.mol -1 for run 2 and 21, respectively. Consequently, the number of chains per magnesium values are far from the expected value of two (1.3 and.8 respectively). All these aspects reveal that the chain transfer is not fast enough comparing to propagation and the chain transfer efficiency to the magnesium is rather low. Moreover, the first instants of polymerization may be not very controlled since run 21, which was the shortest trial, compared to run 2 showed a lower number of chains per magnesium and a higher difference between the experimental and theoretical molar masses. In order to decrease the rate of propagation of ethylene while maintaining the chain transfer rate, the ethylene pressure was decreased to 2 bar and a molar mass of 1 g.mol -1 was targeted (run 22). A unimodal molar mass distribution was obtained with a dispersity value of 1.6 that is lower than the other runs, but still relatively high. The value of MM theo was still far from the one obtained by SEC (485 g.mol -1 vs 124 g.mol -1 ) and the number of chains per magnesium value of.8 was lower than the expected value of two. These results lead to the conclusion that the chain transfer efficiency for the catalytic system (2)/BOMAG is rather ineffective.

7 Activity (kg.mol -1.h -1 ) Table 3. Results of the molar mass targeting study and effect of Bu 2O addition for the catalyst system (2)/BOMAG. (T = 8ºC; [Nd] = 5 µm; Mg/Nd = 8; toluene = 4 ml). Run P (bar) Bu 2O /Mg Activity (kg mol -1 h -1 ) (L.mol -1.s -1 ) Time (min) m eth (g) m PE (g) MM theo (g.mol -1 ) (g.mol -1 ) n Chains/n Mg As said before, this study was already done for system (1)/BOMAG and the results obtained were positive with values of MM theo close to the experimental ones and values of n chains /n Mg really close to two. [13] From the run 24, it can be seen that the addition of Bu 2 O highly increased the activity in a factor of almost three. The reason for this behaviour may be the formation of a complex between Bu 2 O and BOMAG enabling a decrease in the free BOMAG concentration in the polymerization medium and therefore the equilibrium is shifted towards the active species. Consequently, an increase in activity is seen. This activity increase happens as well for system (1)/BOMAG. [13] Despite the lower dispersity value of 1.6 with the Bu 2 O addition, the number of chains per magnesium remains exactly the same as before at 1.3. Furthermore, the value of MM theo is still far from the experimental one ( ), 257 g.mol -1 vs 386 g.mol -1. This shows that the addition of Bu 2 O does not affect, to a great degree, the chain transfer efficiency between the catalyst and the CTA. Its main advantage is thus the increase of activity that can be a useful tool to improve the efficiency of these systems. Sampling experiment. In order to investigate, in more detail, the controlled behaviour of the polymerization mechanism, an experiment was done by taking several samples over the course of the polymerization reaction, enabling the observation of the evolution of several important parameters, such as molar mass, dispersity or number of chains per magnesium. This investigation was only carried out for the catalytic system (2)/BOMAG. For this purpose, another reactor with a sample withdrawal system was used. The experimental conditions chosen were: temperature of 8ºC, pressure of 4 bar and Mg/Nd ratio of 16. Samples were taken after 15, 3, 45, 6, 9 and 12 minutes. Figure 7 shows the kinetic profiles for both with and without sampling polymerization trials. The kinetic profile of the sampling polymerization has several spikes in the activity due to sample withdrawal which momentarily increases the feed of ethylene in the reactor. With this in mind, the two kinetic profiles are similar with slight differences in the average activity (24 kg.mol -1.h -1 without sampling and 171 kg.mol - 1.h -1 with sampling). This difference may be caused by the consecutive extraction of samples which decreases the polymerization medium volume over the course of the polymerization (total of 91.7 ml extracted in a 2 ml of initial solution) Without Sampling With Sampling 1 t (min) 2 3 Figure 7. Kinetic profiles of the polymerizations with and without sampling for the catalytic system (2)/BOMAG. (T = 8ºC; 4 bar; [Nd]=5 µm; Mg/Nd=16; toluene=2 ml).

8 Productivity (moleth.molnd -1 ) Mn (g/mol) Knowing the mass of polyethylene (PE) produced on the time frame of each sample and also the mol of catalyst from the volume of the withdrawn solution, the productivity can be obtained. In Table 4 the results from sample extraction are shown and the productivity profile can be found in Figure 8. The productivity profile should be linear but it is very irregular instead. Between the 15 and 3 minutes the productivity decreases, to 89.1 mol Eth.mol Nd -1, which is impossible to assign to any chemical/physical phenomenon. Also between the minute 9 and 12 minutes the productivity value is doubled from to mol Eth.mol Nd -1. These values are clearly not very reliable being a possible reason for this the error propagation that happens with the several experimental measures that had to be made. Table 4. Productivity values from the sample extraction for the catalytic system (2)/BOMAG. (T = 8ºC; 4 bar; [Nd] = 5 µm; Mg/Nd = 16; toluene = 2 ml). Time (min) V solvent (ml) mol catalyst m PE (g) Productivity a) (mol Eth.mol Nd -1 ) E E E E E E a) M o = 28,5 g/mol Figure 8. Productivity profile obtained by sampling for the catalytic system (2)/BOMAG. (T = 8ºC; 4 bar; [Nd] = 5 µm; Mg/Nd = 16; toluene = 2 ml). To see the evolution of molar mass in the course of the polymerization, the extracted samples of polyethylene were analyzed by SEC-HT. The theoretical molar mass and number of chains per magnesium were also determined. The evolution of the samples molar mass is linear as shows Figure 9, which indicates that the polymerization is controlled in the time range of the sample extraction. However, the intercept with time axis is not. Earlier samples extractions would be needed to understand this peculiar behaviour at the beginning of the polymerization. From Table 5, it can be seen that the dispersity values are relatively high (2.4 < < 2.8). The values of MM theo are far from the experimental ones and also the number of chains per magnesium are far from the optimal value of two (.7 < n Chains /n Mg <.9) which is consistent with the results from the molar mass targeting (Table 3). These results show again that the chain transfer efficiency for the catalytic system (2)/BOMAG is rather ineffective. The only exception is for the sample at 12 min having a number of chains per magnesium of 1.9 and a similar theoretical and experimental molar mass. Table 5. Molar mass features from the SEC-HT analysis of the samples. Sample time (min) t (min) MM theo (g/mol) (g/mol) n Chains/n Mg t (min) Figure 9. Evolution of the molar masses obtained by SEC- HT analysis.

9 Conclusions The kinetic investigation showed that both systems were able to perform a controlled polymerization of ethylene by CCTP mechanism associated with polyethylene chain growth on the magnesium center of the BOMAG. It was concluded that higher Mg/Nd ratios results in decrease of activity. This behavior is in agreement with the proposed CCTP mechanism, since the equilibrium shifts towards the bimetallic complex, which does not insert ethylene, by the increase in BOMAG (MgR 2 ) concentration. Furthermore, the catalytic system (1)/BOMAG displayed higher activities and values compared to the catalytic system (2)/BOMAG. An explanation for this behavior could be the higher interaction between the Me 2 SiFlu 2 NdR active species, on which the ethylene insertion takes place, and the CTA; large interaction disfavor chain transfer. An activation energy of 17.3 kcal.mol -1 was obtained for the system (1)/BOMAG, while for the system (2)/BOMAG the results did not allow performing any calculation. From the target molar mass study, it was concluded that the system (2)/BOMAG has low chain transfer efficiency to the magnesium. It was shown that the addition of Bu 2 O as a co-solvent increased the activity without affecting the polymerization control. Finally, the sampling experiment for the system (2)/BOMAG showed that the molar masses increase linearly in the time range of the samples extraction (15 min 12 min) indicating that the polymerization is controlled by the CCTP mechanism.. However, it s not linear from the origin indicating that the polymerization isn t very well controlled in the beginning. Perspectives. Some of the studies done in this work need further investigation: activation energy value of the system (2)/BOMAG has to be confirmed, and what is happening in the beginning of the polymerization needs to be addressed and understood. The experimental data of this work will be used as an input for a computational model that is currently under development, in order to confirm and/or deciphering the exact CCTP mechanism for lanthanide metallocenes using MgR 2 compounds as CTA, which would be a great achievement and the ultimate goal of this project. The influence of the CTA in the CCTP mechanism is another important component that can be explored in further investigations. References. [1] McKenna, João B.P. Soares and Timothy F. L. Polyolefin Reaction Enginnering. s.l. : WILEY-VCH, 212. [2] Hansjorg W. Sinn, Walter O. Kaminsky, Hans- Jurgen C. Vollmer, Rudiger O. H. H. Woldt. Preparing ethylene polymers using Ziegler catalyst comprising cyclodienyl compound of zirconium. US A [3] C. Boisson, V. Monteil, D. Ribour, R. Spitz, F. Barbotin. Lanthanidocene Catalysts for the Homoand Copolymerization of Ethylene with Butadiene. Macromol. Chem. Phys. 23, pp [4] Kempe, Rhett. How to Polymerize Ethylene in a Highly Controlled Fashion? Chem. Eur. J. 27, Vol. 13, pp [5] Sita, Lawrence R. Ex Uno Plures ( Out of One, Many ): New Paradigms. Angew. Chem. Int. 29, pp [6] Andreia alente Andr Mortreux Marc isseaux Philippe Zinck. Coordinative Chain Transfer Polymerization. Chem. Rev [7] Daniel J. Arriola, Edmund M. Carnahan, Phillip D. Hustad, Roger L. Kuhlman, Timothy T. Wenzel. Catalytic Production of Olefin Block Copolymers via Chain Shuttling Polymerization. Science. 312, 26, pp [8] Phillip D. Hustad, Roger L. Kuhlman, Daniel J. Arriola, Edmund M. Carnahan, Timothy T. Wenzel. Continuous Production of Ethylene-Based Diblock Copolymers Using Coordinative Chain Transfer Polymerization. Macromolecules. 27, Vol. 4, pp

10 [9] Franck D'Agosto, Christophe Boisson, Roger Spitz. Polyethylene Building Blocks by Catalyzed Chain Growth and Efficient End Functionalization Strategies, Including Click Chemistry. Angew. Chem. Int. 28, Vol. 47, pp [1] Jérôme Mazzolini, Edgar Espinosa, Franck D Agosto Christophe Boisson. Polym. Chem. 21, Vol. 1, pp [11] T. Don. Tilley, Richard A. Andersen. Pentamethylcyclopentadienyl derivatives of the trivalent lanthanide elements neodymium, samarium, and ytterbium. Inorganic Chemistry. 1981, Vol. 2, pp [12] Julien Thuilliez, Louis Ricard, François Nief, Fernande Boisson and Christophe Boisson. ansa- Bis(fluorenyl)neodymium Catalysts for Cyclocopolymerization of Ethylene with Butadiene. Macromolecules. 29, Vol. 42, pp [13] Ruivo, Rui. Investigation of the kinetics of ethylene polymerization for the bi-component catalyst Cp*2NdCl2Li(OEt2)2/MgR [14] Kretschmer, W. P., et al. R. Chem. - Eur. J. 26, Vol. 12, p [15] Rouholahnejad, F., Mathis, D. e Chen, P. Organometallics. 21, Vol. 29, p. 294.