Kinetic Study of Coordinative Chain Transfer Polymerization of Ethylene by Neodymium Metallocene Catalysts

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

Rodolfo André da Costa Fialho Ribeiro

Thesis to obtain the Master of Science Degree in

Chemical Engineering

Supervisors

Prof.ª Dr.ª Maria do Rosário Gomes Ribeiro

Dr. Christophe Boisson

Examination Committee

Chairperson: Prof. Dr. Carlos Manuel Faria de Barros Henriques

Supervisor: Prof.ª Dr.ª Maria do Rosário Gomes Ribeiro

Member of the Committee: Prof. Dr. João Carlos Moura Bordado

November 2015

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Abstract

Ethylene polymerizations at 70-90ºC in toluene were performed using a combination of ∗ Cp2NdCl2Li(OEt2)2 (Cp* = C5Me5) or {Me2SiFlu2Nd(µ-BH4)[(µ-BH4)Li(THF)]}2 (Flu = C13H8) 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 for the determination of the propagation activation energy using the Arrhenius equation. A -1 ∗ value of 17.3 kcal.mol was obtained for the Cp2NdCl2Li(OEt2)2/n-butyl-n-octyl-magnesium system.

The {Me2SiFlu2Nd(µ-BH4)[(µ-BH4)Li(THF)]}2/n-butyl-n-octyl-magnesium system did not display a linear Arrhenius plot and consequently the activation energy was not determined for this catalyst.

Specific molar masses were targeted in order to evaluate the polymerization control efficiency. The co-solvent effect was also explored via addition of dibutyl ether (Bu2O). 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, polyethylene, metallocene, kinetics, mechanism, coordinative chain transfer polymerization

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Resumo

Polimerizações de etileno a 70-90ºC em tolueno foram realizados usando uma combinação de ∗ complexos de neodímio Cp2NdCl2Li(OEt2)2 (Cp* = C5Me5) ou {Me2SiFlu2Nd(µ-BH4)[(µ-BH4)Li(THF)]}2

(Flu = C13H8) com n-butil-n-octilmagnésio, sendo este usado tanto como agente alquilante e como agente de transferência de cadeia.

Estudos cinéticos foram realizados para os dois sistemas catalíticos variando a razão Mg/Nd, que é um parâmetro critíco para o controlo da polimerização. Ambos os sistemas permitiram realizar polimerizações controladas de etileno e mostraram as mesmas fases nos perfis cinéticos, reflectindo o mecanismo de polimerização de coordenação por transferência de cadeia. O efeito da temperatura na polimerização foi estudada de modo a determinar a energia de activação da propagação usando a -1 ∗ equação de Arrhenius. Um valor de 17.3 kcal.mol foi obtido para o sistema Cp2NdCl2Li(OEt2)2/n-butil- n-octilmagnésio. Para o sistema {Me2SiFlu2Nd(µ-BH4)[(µ-BH4)Li(THF)]}2/n-butil-n-octilmagnésio não foi obtido um ajuste linear do gráfico de Arrhenius, pelo que a energia de activação da propagação não foi determinada.

Polimerizações tendo como alvo massas molares específicas foram realizadas a fim de avaliar a eficiência do controlo da polimerização. O efeito do co-solvente foi também explorado pela adição de

éter dibutílico (Bu2O). Distribuições unimodais de massas molares foram obtidas e a velocidade de polimerização aumentou na presença do co-solvente sem afectar o controlo da polimerização. Por último, foi realizada uma reacção de polimerização com extracção de amostras ao longo do tempo de forma a observar o comportamento pseudo-vivo do mecanismo de polimerização de coordenação por transferência de cadeia.

Palavras-Chave:

Neodímio, polietileno, metaloceno, cinética, mecanismo, polimerização de coordenação por transferência de cadeia

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Acknowledgments

This thesis is the end of a life chapter. It would not be possible without the help, advice and collaboration of some great people, to whom I express a word of gratitude.

First of all, I would like to thank my supervisors at the C2P2 laboratory, Dr. Christophe Boisson and Dr. Frank D’Agosto, for their guidance, scientific advice and support during this internship and the final revision of this manuscript. Also, I would like to thank my IST supervisor, Professor Maria Rosário Ribeiro, for the opportunity to do this internship, fruitful discussions and suggestions/remarks given for the revision of this work.

I am very grateful to Islem Belaid for her advice, support and patience throughout my entire internship. I also would like to thank Dr. Sébastien Norsic for teaching me all the techniques and polymerization skills which were essential to the completion of this work. A word of appreciation to Manel Taam and Olivier Boyron for all the knowledge and guidance in the SEC analysis.

I would like to thank my colleagues from C2P2 for making my stay in Lyon a great experience and also my university colleagues for the friendship during the last five years.

A huge thanks to all my closest friends for their friendship and companionship during this last five years, and especially over the course of my life. We shared good and bad moments and I am sure we will be great friends for life.

I am deeply grateful to my entire family for their support, especially to my uncle Miguel and to my grandmothers Conceição and Albertina.

Last but not least, I would like to dedicate this thesis to my parents, António and Luísa, as a sign of respect and recognition for all their efforts and dedication throughout my entire life. Their love and unconditional support are essential for my happiness and without them I could not achieve my life objectives.

A todos, muito obrigado!

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Table of Contents

ABSTRACT ...... III

RESUMO ...... V

ACKNOWLEDGMENTS ...... VII

TABLE OF CONTENTS ...... IX

LIST OF FIGURES ...... XI

LIST OF TABLES ...... XIV

LIST OF ACRONYMS ...... XVI

LIST OF SYMBOLS ...... XVII

1. INTRODUCTION ...... 1

1.1. CONTEXT AND OBJECTIVE ...... 1 1.2. THESIS STRUCTURE ...... 2

2. LITERATURE REVIEW ...... 3

2.1. BRIEF POLYETHYLENE OVERVIEW ...... 3 2.2. POLYETHYLENE SYNTHESIS AND NEODYMIUM METALLOCENE CATALYSTS ...... 5 2.2.1. Free Radical Polymerization ...... 5 2.2.2. Coordination Polymerization ...... 6 2.2.2.1. Phillips Catalysts ...... 8 2.2.2.2. Ziegler-Natta Catalysts ...... 8 2.2.2.3. Late Transition Metals Catalysts ...... 8 2.2.2.4. Metallocenes ...... 8 2.2.3. Conclusion ...... 11

2.3. COORDINATIVE CHAIN TRANSFER POLYMERIZATION ...... 12 2.3.1. Concept ...... 12 2.3.2. Specialty Polymers ...... 15 Production of olefin multiblock copolymers via chain shuttling polymerization: [25] ...... 15 Production of Ethylene-Based Diblock Copolymers: [27] ...... 16 Synthesis of End-Functionalized Polyethylene: [24] [28] ...... 16 2.3.3. CCTP System Studied ...... 17

3. EXPERIMENTAL SECTION ...... 18

3.1. AIR-FREE TECHNIQUES ...... 18 3.1.1. Schlenk Line ...... 18 3.1.2. Glovebox ...... 19

3.2. MATERIALS AND CATALYSTS PRECURSORS ...... 20 3.2.1. Toluene ...... 20

3.2.2. Ethylene ...... 20 3.2.3. BOMAG – n-butyl-n-octyl-magnesium ...... 21 3.2.4. Dibuthyl Ether ...... 21 ∗ 3.2.5. 퐶푝2NdCl2Li(OEt2)2 ...... 21

3.2.6. {Me2SiFlu2Nd(µ-BH4)[(µ-BH4)Li(THF)]}2 ...... 21 3.3. POLYMERIZATION PROCEDURE AND APPARATUS ...... 22 3.4. POLYMER ANALYSIS ...... 25

4. RESULTS AND DISCUSSION ...... 26

4.1. KINETICS INTRODUCTION ...... 26 4.2. KINETIC STUDY: INFLUENCE OF MG/ND RATIO ...... 28 4.3. DETERMINATION OF THE ACTIVATION ENERGY ...... 32 4.4. MOLAR MASS TARGETING AND CO-SOLVENT EFFECT...... 34 4.5. SAMPLING EXPERIMENT ...... 38

5. CONCLUSIONS AND PERSPECTIVES ...... 42

5.1. CONCLUSIONS ...... 42 5.2. PERSPECTIVES ...... 43

6. REFERENCES ...... 44

7. APPENDIX ...... 47

7.1. DETERMINATION OF THE ETHYLENE CONSUMPTION ...... 47 7.2. INFLUENCE OF MG/ND RATIO: PRODUCTIVITY PROFILES ...... 48 7.3. DETERMINATION OF THE ACTIVATION ENERGY: ACTIVITY AND PRODUCTIVITY PROFILES ...... 50 7.4. DENSITY FUNCTIONAL THEORY (DFT) INVESTIGATION ...... 52 7.5. MOLAR MASS TARGETING: PRODUCTIVITY PROFILES ...... 53

List of Figures

Figure 1. World polymer demand in 2012...... 3

Figure 2. Polyethylene structure...... 3

Figure 3. Microstructural differences between polyethylene types...... 4

Figure 4. Coordination-insertion of the monomer to the transition metal of the active site...... 6

Figure 5. Polymer chain growth...... 6

Figure 6. β-H elimination mechanism...... 7

Figure 7. Activation process of a group 4 metallocene precursor with MAO giving rise to the cationic active species which can polymerize ethylene...... 9

Figure 8. In situ alkylation of a chloride/borohydrate metallocene precursor using MgR2 as alkylating agent. This results in a neutral active species which can polymerize ethylene...... 10

Figure 9. General view of polyethylene production and neodymium catalysts role...... 11

Figure 10. CCTP mechanism for olefin polymerization. [M] is the transition metal, L2 are the ligands, M’ is the main-group metal of the CTA, n is the number of equivalent alkyl substituents and PA and PB are polymers of polymerization degrees A and B, respectively...... 12

Figure 11. Chain growth by CCTP mechanism on the main-group metal alkyl...... 13

Figure 12. CCTP mechanism using neodymium catalysts in association with BOMAG as CTA and formation of Mg(PE)2...... 14

Figure 13. Chain shuttling polymerization mechanism...... 15

Figure 14. Chemical structures of the catalysts and CTA used...... 17

Figure 15. Schlenk lines...... 18

Figure 16. Glovebox...... 19

Figure 17. Solvent purification unit...... 20

Figure 18. Ethylene cylinder...... 21

Figure 19. Polymerization apparatus...... 22

Figure 20. The left image shows the reactor in the end of a polymerization and also the jacket detached from the reactor. The right image is the glass reactor itself...... 23

Figure 21. SPY FR® sensors and Sirius software for recording pressure and temperature...... 24

Figure 22. Left and top-right image shows the filtration and drying process, respectively. Bottom-right image is the final polyethylene obtained...... 24

Figure 23. Viscotek HT-GPC by Malvern®...... 25

* Figure 24. Kinetic profiles obtained for the catalytic system I. (T = 80ºC; 4 bar; [Cp2NdCl2Li(OEt2)2] = 50 µM; toluene = 400 mL)...... 28

Figure 25. Kinetic profiles obtained for the catalytic system II. (T = 80ºC; 4 bar; [{Me2SiFlu2Nd(µ-

BH4)[(µ-BH4)Li(THF)]}2] = 50 µM; toluene = 400 mL)...... 28

* Figure 26. Typical kinetic profile and its 3 phases. (T = 80ºC; 4 bar; [Cp2NdCl2Li(OEt2)2] = 50 µM; toluene = 400 mL; Mg/Nd = 80)...... 29

Figure 27. Heterobimetallic complex (dormant) in equilibrium with the alkyl neodymium complex and BOMAG...... 30

Figure 28. Example of the bimodal distribution obtained for system I with a Mg/Nd ratio of 60. (T = 80ºC; 4 bar; [Nd] = 50 µM)...... 31

app Figure 29. Arrhenius plot obtained by determining kp for the catalytic system I at 70, 75, 80 and 85ºC...... 33

app Figure 30. Arrhenius plot obtained by determining kp for the catalytic system II at 65, 70, 74, 80, 85 and 90ºC. ([Nd] = 50 µM; Mg/Nd = 80; toluene = 400 mL)...... 33

Figure 31. Example of the unimodal molar mass distribution obtained for Run 21. (T = 80ºC; [Nd] = 50 µM; Mg/Nd = 80; toluene = 400 mL)...... 36

Figure 32. Bu2O structure...... 36

Figure 33. Activity profiles obtained with and without Bu2O addition for the catalytic system II. (T = 80ºC; [Nd] = 50 µM; Mg/Nd = 80; toluene = 400 mL)...... 37

Figure 34. Productivity profiles obtained with and without Bu2O addition for the catalytic system II. (T = 80ºC; [Nd] = 50 µM; Mg/Nd = 80; toluene = 400 mL)...... 37

Figure 35. Kinetic profiles of the polymerizations with and without sampling for the catalytic system II. (T = 80ºC; 4 bar; [Nd] = 50 µM; Mg/Nd = 160; toluene = 200 mL)...... 38

Figure 36. Productivity profile obtained by sampling for the catalytic system II.. (T = 80ºC; 4 bar; [Nd] = 50 µM; Mg/Nd = 160; toluene = 200 mL)...... 40

Figure 37. Evolution of the experimental and theoretical molar masses...... 41

Figure 38. Evolution of molar mass distribution during the polymerization...... 41

Figure 39. Productivity profiles obtained for the catalytic system I. (T = 80ºC; 4 bar; * [Cp2NdCl2Li(OEt2)2] = 50 µM; toluene = 400 mL)...... 48

Figure 40. Productivity profiles obtained for the catalytic system II. (T = 80ºC; 4 bar; [{Me2SiFlu2Nd(µ-

BH4)[(µ-BH4)Li(THF)]}2] = 50 µM; toluene = 400 mL)...... 49

Figure 41. Activity profiles from the polymerizations experiments at different temperatures, catalytic system I. ([Nd] = 50 µM; Mg/Nd = 80; toluene = 400 mL)...... 50

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Figure 42. Productivity profiles from the polymerizations experiments at different temperatures, catalytic system I. ([Nd] = 50 µM; Mg/Nd = 80; toluene = 400 mL)...... 50

Figure 43. Activity profiles from the polymerizations experiments at different temperatures, catalytic system II. ([Nd] = 50 µM; Mg/Nd = 80; toluene = 400 mL)...... 51

Figure 44. Productivity profiles from the polymerizations experiments at different temperatures, catalytic system II. ([Nd] = 50 µM; Mg/Nd = 80; toluene = 400 mL)...... 51

Figure 45. Hetero bi- and tri-metallic species considered...... 52

Figure 46. Productivity profiles of the molar mass targeting study for the catalyst system II. (T = 80ºC; [Nd] = 50 µM; Mg/Nd = 80; toluene = 400 mL)...... 53

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List of Tables

Table 1. Ethylene specifications...... 20

app Table 2. Activity, kp and molar mass features from the polymerization trials for both catalytic systems. (T = 80ºC; 4 bar; [Nd] = 50 µM)...... 30

app Table 3. Determination of kp and Ea from the polymerizations experiments at different temperatures for both systems. ([Nd] = 50 µM; Mg/Nd = 80; toluene = 400 mL)...... 32

Table 4. Results of the molar mass targeting study for the catalyst system II. (T = 80ºC; [Nd] = 50 µM; Mg/Nd = 80; toluene = 400 mL)...... 35

Table 5. Results for the effect of Bu2O addition for the catalytic system II. (T = 80ºC; [Nd] = 50 µM; Mg/Nd = 80; toluene = 400 mL)...... 36

app Table 6. Activity and kp for the polymerizations with and without sampling for the catalytic system II. (T = 80ºC; 4 bar; [Nd] = 50 µM; Mg/Nd = 160; toluene = 200 mL)...... 39

Table 7. Productivity values from the sample extraction for the catalytic system II. (T = 80ºC; 4 bar; [Nd] = 50 µM; Mg/Nd = 160; toluene = 200 mL)...... 39

Table 8. Molar mass features from the SEC analysis of the samples...... 40

app Table 9. Data for the determination of kp from the productivity profiles, catalytic system I. (T = 80ºC; * 4 bar; [Cp2NdCl2Li(OEt2)2] = 50 µM; toluene = 400 mL)...... 48

app Table 10. Data for the determination of kp from the productivity profiles, catalytic system II. (T =

80ºC; 4 bar; [{Me2SiFlu2Nd(µ-BH4)[(µ-BH4)Li(THF)]}2] = 50 µM; toluene = 400 mL)...... 49

app Table 11. Data for the determination of kp from the productivity profiles at different temperatures, catalytic system I. ([Nd] = 50 µM; Mg/Nd = 80; toluene = 400 mL)...... 50

app Table 12. Data for the determination of kp from the productivity profiles at different temperatures, catalytic system II. ([Nd] = 50 µM; Mg/Nd = 80; toluene = 400 mL)...... 51

app Table 13. Data for the determination of kp from the productivity profiles of the molar mass targeting study, catalyst system II. (T = 80ºC; [Nd] = 50 µM; Mg/Nd = 80; toluene = 400 mL)...... 53

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List of Acronyms

BOMAG n-butyl-n-octyl-magnesium

Bu2O dibutyl ether

CCTP Coordinative Chain Transfer Polymerization

CSA chain shuttling agent

CTA chain transfer agent

HDPE high density polyethylene

LDPE low density polyethylene

LLDPE linear low density polyethylene

MAO methylaluminoxane

Mg(PE)2 di(polyethylenyl)magnesium

MgR2 dialkylmagnesium compound

PE polyethylene

SEC-HT high temperature size exclusion chromatography

VLDPE very-low density polyethylene

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List of Symbols

C* percentage of active sites

Ea propagation activation energy kex chain transfer rate coefficient kp propagation rate coefficient

app kp apparent propagation rate coeffcient

[M] concentration of metal centers

[M*] concentration of active sites

푴̅̅̅퐧̅ number average molar mass

weight average molar mass 푴̅̅̅̅퐰̅

푴̅̅̅̅퐰̅/̅푴̅̅퐧̅ dispersity

M0 monomer molar mass

MMtheo theoretical molar mass n number of ethylene units nchains/nMg number of polyethylene chains per magnesium

P pressure

R universal gas constant

Rp observable rate of polymerization t time

T temperature

Y yield

Cp* C5Me5

Flu C13H8

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1. Introduction

1.1. Context and Objective

Metallocene complexes have been widely investigated as catalysts for polymerization of various monomers, especially ethylene. The initial interest in metallocenes started with the discovery of the fact that the addition of MAO (methylaluminoxane) to a group 4 metallocene precursor increased + greatly the activity, due to the formation of active cationic species, L2M-R (L: ligand; M: Group IV transition metal; R: alkyl group). Interestingly, lanthanide metallocenes, such as neodymium metallocenes, don’t need a cocatalyst like MAO since a neutral active species of the type L2Ln-R (Ln: lanthanide) is formed by in-situ alkylation of the precursor, and have proved to be very active for ethylene polymerization.

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, which is very inefficient. Consequently, strategies to reduce the consumption of highly expensive transition metal catalysts, such as coordinative chain transfer polymerization (CCTP), have been developed. In this strategy, the growing polymer chain is able to transfer rapidly and reversibly from the catalyst to a chain transfer agent (CTA), such as AlR3 or

MgR2, leading to a controlled polymerization by adjusting the catalyst/CTA ratio. Moreover, If the chain transfer is highly efficient, the polymer seems to grow on the metal of the CTA and narrow molar distributions are obtained.

This work combines all of the aspects mentioned above by studying two independent CCTP systems to polymerize ethylene using two different neodymium metallocene catalysts. In both systems, it was used the same dialkylmagnesium compound (MgR2) - n-butyl-n-octyl-magnesium (BOMAG) as both alkylating agent and CTA. The main aim of this work is not only understand the CCTP mechanism involved, but also understand the influence of the ligands in the catalyst activity and chain transfer efficiency by comparing the two systems. In order to achieve this, kinetic studies were performed for the two CCTP systems by varying the Mg/Nd ratio (catalyst/CTA ratio), critical parameter for polymerization control, and polymerization temperature. It was also explored the co-solvent effect and a sampling experiment was performed to observe the pseudo-living behaviour of the CCTP mechanism.

1 1.2. Thesis structure

The structure of the thesis is the following:

Chapter 2 presents the state of the art of the main topics for total understanding of the concepts and purposes of this work.

Chapter 3 covers the experimental section of this work. It includes the air-free techniques employed, the materials and catalyst precursors used, the polymerization procedure and apparatus, and finally the polymer characterization by high temperature size exclusion chromatography (SEC- HT).

Chapter 4 includes all the experimental results and their discussion in what concerns: kinetic studies for the two CCTP systems by varying the Mg/Nd ratio and the polymerization temperature; molar mass targeting and the co-solvent effect. Finally, a sampling experiment is reported.

Chapter 5 reports the conclusions of this work and its future perspectives.

Chapter 6 contains the references used and Chapter 7 includes the appendices with all the supporting information for this work.

2 2. Literature Review

The literature review covers three main topics for total understanding of the principal concepts and purposes of this work. The first topic is a very brief polyethylene overview since it is the polymer produced in this study. It covers the definition, types and applications of polyethylene. Secondly, the polyethylene synthesis mechanisms are discussed, and more importantly the definition and role of neodymium metallocene catalysts on polyethylene synthesis is presented. The last topic refers the polymerization strategy followed in this study, namely coordinative chain transfer polymerization (CCTP), and it covers the concept, applications (specialty polymers) and the specific systems studied in this work. The understandings of these three topics are critical for the successful interpretation of the experimental results and final conclusions.

2.1. Brief Polyethylene Overview

Polyethylene is the most consumed polymer in the world (Figure 1) with an expected total global demand of 97 MTon in the year 2016. [1] It’s one of the most versatile and reliable material nowadays due to its great physical and chemical properties, and it has a wide variety of applications that ranges from pipes to plastic bags, being frequently present in our daily lives even without people realizing it.

Others PET 11% 9% PE 37% PVC 18%

PP 25%

Figure 1. World polymer demand in 2012. [1]

Polyethylene is a thermoplastic polymer and has the simplest basic structure of any polymer

(Figure 2) which consists of chains with the formula C2nH4n+2, where n is the number of ethylene units in the chain, normally referred as the degree of polymerization. Although it seems to have a simple structure, polyethylene can be branched to various degrees and contain small amounts of unsaturation. [2] As a consequence, several types of polyethylene exist differing mostly on its density and branching, exhibiting a specific set of properties depending on the crystallinity.

Figure 2. Polyethylene structure.

3 There are 3 main polyethylene types:

a) High-Density Polyethylene (HDPE): defined by a density in the range of approximately 0.945-0.97 g/cm3 [3]. This type of polyethylene is constituted primarily by linear chains with a low degree of branching and therefore presents a high degree of crystallinity, resulting in its higher density relatively to the other types of polyethylene. [2] HDPE is made by coordination- insertion polymerization and its main applications are blow molded articles (such as bottles), pipes and cable insulation. [4] b) Low-Density Polyethylene (LDPE): defined by a density in the range of approximately 0.915-0.94 g/cm3. [3] In contrast with HDPE, this type of polyethylene has a high degree of long and short chain branching which hinders the crystallization process resulting in relatively low densities [2]. LDPE is made only by free radical polymerization and its main applications are films and sheets for packaging. [5] c) Linear Low-Density Polyethylene (LLDPE): defined by a density in the range of approximately 0.915-0.94 g/cm3 like LDPE [3]. This type of polyethylene is produced by the copolymerization of ethylene with 1-alkenes (for example 1-butene, 1-hexene or 1-octene) which results in linear chains with significant numbers of short branches at random intervals. LLDPE is made by coordination polymerization as HDPE and its main applications are plastic bags and films for packaging. [6]

Figure 3 shows the differences between these types in a schematic manner based on their microstructural characteristics.

Figure 3. Microstructural differences between polyethylene types.

4 2.2. Polyethylene Synthesis and Neodymium Metallocene Catalysts

In this sub-chapter, the polyethylene synthesis mechanisms will be described with a major focus on coordination-insertion polymerization, more specifically on metallocene catalyst family which is one of the key components of this work. The objective is to understand the role and place of metallocene catalysts on polyethylene synthesis and lastly to give an insight into the unique and special type of catalysts based on neodymium metallocene complexes.

The synthesis of polyethylene can be done by two main mechanisms:

1) Free radical polymerization – The polymer forms by successive addition of monomer unit onto macroradical building blocks, requiring conditions of high temperature (100-300ºC) and pressure (500-1500 bar); [2] 2) Coordination-insertion polymerization – The polymer forms by monomer coordination- insertion into a polymer-metal bound, requiring generally milder conditions of temperature (30-100ºC) and pressure (1-20 bar). [2]

2.2.1. Free Radical Polymerization

Free radical polymerization proceeds via a chain-growth mechanism which requires a source of free radicals, monomer (ethylene) and conditions of high temperature and pressure. This process consists of four main types of reactions involving free radicals [7]:

1) Initiation – Decomposition of initiator molecules to generate free radicals which add a monomer. 2) Propagation – Addition of monomer to the chain-end of growing polyethylenyl radical brought into close proximity due to the high pressure. 3) Chain Transfer – when a growing polymer chain reacts in a way that the free radical transfers to a molecule (chain) on which further chain growth can occur. 4) Termination – Two radical species, at least one of which is at an active chain end, react together stopping the chain growth.

The nature of the produced polyethylene is controlled by the initiator concentration, temperature, ethylene pressure and the presence of chain transfer agents.

5 2.2.2. Coordination Polymerization

Coordination polymerization created a revolution in the industry by producing polyethylene under mild conditions of temperature and pressure. In addition the produced polyethylene has significant improvements in hardness and strength relatively to LDPE produced by the free radical process. [2] The addition of monomer units in coordination-insertion polymerization (Cossee mechanism) is based on the coordination of ethylene on a transition metal center followed by insertion of this molecule in a metal-carbon bond. [8] The repetition of these steps leads to the formation of a polyethylene chain. The chains are terminated by transfer or termination reactions. These reactions will be briefly discussed in the following.

1) Coordination-insertion step – The coordination is due to the π-electrons donation of the ethylene double bond to the empty d orbital on the metal. [8] The coordination is followed by insertion of ethylene in the metal-carbon bond of the catalyst. Figure 4 shows schematically the mechanism of coordination-insertion where L are the ligands – for example cyclopentadienyl groups for metallocenes, M is the transition metal atom and R is a alkyl group or a hydrogen atom. [9]

Figure 4. Coordination-insertion of the monomer to the transition metal of the active site.

2) Chain growth – Polyethylene chain growth proceeds by successive coordination-insertion of monomers as showed in Figure 5. [9]

Figure 5. Polymer chain growth.

6 3) Chain Transfer – One of the main chain transfer reaction is β-H elimination (Figure 6), which produces a polymer with a vinyl chain end. In any type of chain transfer the active center is regenerated and the propagation reaction continues. [9] The metal-polymer active site is transformed to a new metal-carbon (or H) active species generating a dead (or dormant) polymer chain.

Figure 6. β-H elimination mechanism.

4) Termination – Deactivation of the catalyst. This phenomenon is very complex and not very well known but one of the considered hypotheses is the homolytic cleavage of the transition metal- polymer bond via unimolecular or bimolecular reactions, causing the reduction of the active transition metal. [9] Deactivation can also occur via the poisoning of the catalyst by impurities present in the solvent or the monomer.

Knowing the general coordination-insertion polymerization mechanism, it is clear that the catalyst is a key component of this type of polymerization, explaining why there is a rich history of gradual improvements on catalyst design. We can distinguish four major coordination catalyst classes: classical Ziegler-Natta, Phillips, metallocene, and late transition metal catalysts. Note that the two latter catalytic systems can also be considered as new generation of Ziegler-Natta catalysts. The first two are multiple-site catalysts, since they have more than one type of active site, resulting in polymers with heterogeneous microstructures and broad molar mass distributions. [3] Metallocene and late transition metal catalysts are generally single-site catalysts meaning that they generate only one type of active sites, polymerizing the available monomers in an identical fashion, and allowing the production of polyethylene with more homogenous properties and Schulz-Flory molar mass distributions (expected dispersity of two). [10] Next it will be discussed each one of this four catalyst classes, giving more details on metallocene catalysts since it is the main focus of this work.

7 2.2.2.1. Phillips Catalysts The Phillips catalysts, discovered in the early 1950’s by Hogan and Banks at the Phillips Petroleum Company, is one of the most important industrial catalysts for ethylene production. [11] Phillips catalysts are heterogeneous catalysts based on metal oxide, generally chromium oxide (CrO3), supported on silica (SiO2) followed by calcination in dry air at high temperatures. It’s the most used catalyst for the production of HDPE having 40% of market share. [3]

2.2.2.2. Ziegler-Natta Catalysts The Ziegler-Natta catalysts were discovered in the laboratories of K. Ziegler in 1953 and G. Natta in 1954. [12] Its variety is enormous, there are a lot of articles, books and patents about this subject making this class of catalyst the most flexible and the most used on the industry. Due to its variety, the definition of Ziegler-Natta is very broad but generally consists of a transition metal salt from groups 4 to 8 in association with a metal alkyl from main groups (1, 2 or 13), known as the cocatalyst or activator. [2] This cocatalyst is required to form the active species by alkylation and possibly reduction [3] of the transition metal center. A classic example is the complex of triethyl aluminum (AlEt3) with titanium tetrachloride (TiCl4). These catalysts are generally supported on MgCl2.

2.2.2.3. Late Transition Metals Catalysts This catalyst class has been discovered in the early 1990s by researchers at DuPont. The unique feature of this type of catalyst is that they are much less sensitive to polar compounds than Ziegler- Natta, Phillips or metallocene catalysts, allowing the copolymerization of olefins and polar comonomers such as vinyl acetate and methyl metacrylate. [3] One example is the Ni-diimine catalyst which successfully copolymerized ethylene with acrylate comonomers. [13] Besides this high potential, these type of copolymers were only produced in an academic background and are not industrially available.

2.2.2.4. Metallocenes Metallocenes catalysts were discovered in the same time period as Ziegler-Natta catalysts (1950’s) but displayed very low activities at that time. [3] Only in 1975 Kaminsky and Sinn discovered that the addition of methylaluminoxane (MAO) increased greatly the activity (factor up to a 1 million!). [14] This important step allowed the use of metallocene systems in the industry to produce HDPE and LLDPE with more uniform molecular distribution.

In their most general form, metallocene catalysts are composed of group 4 metal (Ti, Zr, Hf) bound by two substituted or unsubstituted cyclopentadienyl (Cp), indenyl (Ind), or fluorenyl (Flu) ligand in a sandwich form. [15] These two ligands can be linked through bridges of different types modifying the ligand-metal-ligand angle, and are called ansa metallocenes. [3] By modifying these units or ligands and therefore the steric and electronic environment around the active sites, these catalysts can be tailored to control the polymer microstructure, that is, comonomer distribution, stereo-, and regioregularity of polyolefins.

8 In this work, the transition metal used is a neodymium metal atom from the lanthanide family (p.e Nd, Sm, Lu), whose main difference, relatively to group 4 metallocenes, relies on the activation step. [16] This distinction between these metallocene complexes and also the role of the cocatalyst in the case of group 4 metallocenes is now explained in more detail.

Group 4 metallocene and cocatalyst role:

Neutral group 4 metallocene complexes (Cp2MX2 with X = Cl or Me) that have a degree of oxidation of +4 are electron deficient but not active. As said before, the key to activate these complexes is the cocatalyst. Methylaluminoxane, synthesized by controlled hydrolysis of trimethyl aluminum, is the most common activator but its structure is very complex. [17] The main roles of the cocatalyst are: [18]

1) Alkylating agent; 2) Cl-, OR- or methyl anion abstractor; 3) Reactivation of inactive complexes formed by hydrogen transfer or bimolecular reactions;

4) Scavenger of the medium impurities (O2, H2O, etc.)

The reaction between the group 4 metallocene and the cocatalyst provides a cationic active species, as shown in Figure 7.

Figure 7. Activation process of a group 4 metallocene precursor with MAO giving rise to the cationic active species which can polymerize ethylene.

9 Lanthanide metallocene:

Lanthanide metallocene are neutral complexes that have the same structure as group 4 cationic metallocene [15], but have a degree of oxidation of +3. Consequently, lanthanide metallocene complexes do not require a cocatalyst to form an active species because there is already a free coordination vacancy for monomer coordination. However, these complexes can also bring practical problems because they are very sensitive to air, and require very pure solvents and monomers since there is no scavenger in the polymerization medium. [16] To overcome this drawback an in situ alkylation of a chloride/borohydrate metallocene precursor is performed using alkylating agents, i.e [16] dialkyl magnesium (MgR2), as shown on Figure 8. This can bring a great advantage since it can avoid the use of cocatalysts like MAO which can be very expensive and unstable.

Figure 8. In situ alkylation of a chloride/borohydrate metallocene precursor using MgR2 as alkylating agent. This results in a neutral active species which can polymerize ethylene.

10 2.2.3. Conclusion

The field of coordination catalysts is hugely rich and diverse; each catalyst family creates a great versatility in polymer design, allowing the production of polyethylene with different properties and characteristics that are suited for different applications. The objective of this sub-chapter was to understand the role of neodymium metallocene catalysts in the polyethylene synthesis. Figure 9 provides a schematic and simplified picture of the polyethylene synthesis ways. It also highlights the place that neodymium metallocene catalyst has in this industry. In conclusion, neodymium metallocene catalysts have the potential to produce polyethylene by coordination polymerization under mild conditions of temperature and pressure, with narrow molar mass distributions and great control of polymer microstructure by tailoring the ligands around the metal center, and also do not require an

expensive and unstable cocatalyst like MAO to be activated.

Neodymium Catalysts

Free Radical Polymerization Group III Lanthanides (Neutral) Metallocenes Group IV (Charged)

Late Transition Metals Coordination Polymerization Ziegler-Natta

PolyethyleneSynthesis Phillips

Figure 9. General view of polyethylene synthesis and neodymium catalysts role.

11 2.3. Coordinative Chain Transfer Polymerization

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 is formed per metal center. [19] Therefore the process turns out to be economically inefficient since the transition metal initiator can be highly expensive preventing any potential scale-up for the industry.

The first case of controlled coordination polymerization of ethylene was observed by Samsel et al. in 1993 through the reversible transfer of polyethylene chains between the catalyst and a chain transfer agent (CTA). [20] If this transfer is fast enough compared to the propagation and reversible, this type of system can be implemented to produce polyethylene in a controlled manner that exhibit narrowly distributed molar masses. Furthermore, it can overcome the economic drawback by allowing the growth of several macromolecular chains per catalyst molecule keeping the characteristics of a living polymerization. [21] This polymerization technique is called coordinative chain transfer polymerization (CCTP). [19] [21] This chapter explains in more detail the CCTP concept, its applications for specialty polymers production and finally the systems that were studied in this work.

2.3.1. Concept

CCTP involves the association of a transition metal based catalyst and a CTA, usually in the form of a main-group metal alkyl. [22] 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. [21] This mechanism is shown in Figure 10.

12 This mechanism is a dynamical equilibrium between a chain-growing state and a chain-transfer state [19] and is crucial for a successful CCTP process. If the chain transfer rate is much greater that the rate of propagation (kex>>kp), reversible, and if other chain termination pathways, such as β-H elimination, don’t occur (or occur in negligible way) everything happen as if the transition metal catalyzed the chain growth on the main-group metal (Figure 11). [21] A parallel can be seen between controlled radical polymerization mechanisms, such as RAFT process, and the CCTP concept. [23]

Figure 11. Chain growth by CCTP mechanism on the main-group metal alkyl.

In this case, CCTP displays several advantages:  High ratio between the CTA and the catalyst enables catalyst economy since each catalyst molecule generates several polymer chains; [21]  Narrow molar mass distributions since growth probability is the same for all chains; [21]  Polymer chains are end-capped with a reactive chains transfer metal enabling further functionalization; [24]  The preparation of olefin block copolymers is possible. [25] CCTP requires thus a suitable combination of single-site catalyst and a CTA to establish an efficient and reversible chain transfer and to limit or avoid other chain termination pathways, such as β-H elimination. [21] The electronic and steric environment of the ligands around the transition metal of the catalyst can be tailored to allow the reversible and fast chain transfer reaction at a higher rate than propagation and consequently ensuring good transfer efficiency to the CTA. [3] Also, the limitation in terms of molar masses is another challenge even though for many applications the molar masses obtained by CCTP could be high enough. Indeed, since CCTP reactions are normally carried out at temperatures lower than 100ºC, mainly to prevent β-H elimination reactions, the growing polymer chain on the CTA can precipitate which prevents the chain transfer reactions between the CTA and the catalyst due to the dramatic decrease of the chain mobility losing reaction control.

In this work, it was investigated a CCTP system based on neodymium metallocene catalysts used in combination with a dialkylmagnesium compound (MgR2) as a CTA. As explained before in chapter

2.2.2.4, lanthanide metallocenes do not require a cocatalyst, being MgR2 both the alkylating agent and the CTA. After the formation of the active catalyst species, L2NdR, 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, L2Nd(µ-R2)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 shown in Figure 12.

Figure 12. CCTP mechanism using neodymium catalysts in association with BOMAG as CTA and formation of Mg(PE)2.

The interesting features of this system are its pseudo-living behavior leading to the increase of molar masses with productivity and to the production of polyethylene with narrow molar mass distribution. It enables catalyst economy since two chains are produced per magnesium compounds which are used in large excess relatively to neodymium. As said earlier, the increase in molar mass during the polymerization has an upper limit depending of the polymerization temperature. When

Mg(PE)2 reach its limit of solubility at the polymerization temperature (80°C in this work) it precipitates, stopping the reversible chain transfer between the BOMAG and the neodymium centers due to the dramatic decrease of the chain mobility. The control of polymerization is consequently rapidly lost. [26]

14 2.3.2. Specialty Polymers

CCTP is a polymerization strategy that has the potential to finely control the polymer microstructure and architecture, resulting in new specialty polymers exhibiting original properties. There are several recent studies exploring this potential and here are shown three of those studies. The first two are about original macromolecular architectures: production of olefin multiblock and diblock copolymers. The last one is about the functionalization of polyethylene.

Production of olefin multiblock copolymers via chain shuttling polymerization: [25] In a very interesting study, Arriola and co-workers at Dow Chemicals managed to produce a new class of thermoplastic elastomers by copolymerization of ethylene with octene. This olefin multiblock copolymer displays alternating semicrystalline and amorphous segments. The crystallinity of each segment is related to the α-olefin content. This was achieved by using one CTA to transfer growing chains between two different catalysts with different octene response in a single polymerization reactor.

This process is called chain shuttling polymerization (Figure 13) by which one catalyst produces a segment of “hard” polymer with low comonomer content while a second catalyst grows a “soft” polymer segment with high comonomer content. In the presence of a CTA, named in this case chain shuttling agent (CSA), such as a main-group metal center, polymer chains are “shuttled” between the two catalysts and a multiblock copolymer is obtained with very interesting properties such as high melting temperatures, low glass transition temperatures and elastomeric properties at high temperatures.

Figure 13. Chain shuttling polymerization mechanism. [25]

Production of Ethylene-Based Diblock Copolymers: [27] Ethylene-based diblock copolymers synthesis from living polymerization is typically carried out in batch processes: one monomer is added and polymerization is carried out to complete conversion, and then this process is repeated with a second monomer. Adding the fact that these living polymerization processes only produce one polymer chain per catalyst molecule, these batch processes are not economically viable.

Dow Chemicals produced a diblock copolymer of high-density polyethylene (HDPE) and very low- density polyethylene (VLDPE) by CCTP using two continuous reactors. A pyridylamido catalyst that was capable of performing a CCTP mechanism and that has a high α–olefin response was used. Using diethyl zinc as CTA, a block of HDPE end capped with the CTA was produced in the first reactor and then went to a second reactor where a block of VLDPE is made forming a well-defined diblock copolymer containing no more than two blocks per chain, in contrast to the statistical multiblock composition produced via chain shuttling polymerization. This polymer synthesis is a powerful example of the use of CCTP for the production of olefin-based diblock copolymers in a continuous process while reducing the consumption of highly expensive transition metal based catalysts.

Synthesis of End-Functionalized Polyethylene: [24] [28] The modification of polyethylene has always been a challenge since functional or polar groups cannot be incorporated easily into these materials by catalytic polymerization. As explained before, in CCTP the chains grow on the catalyst and reversibly transfer to the metallic center of the CTA where the chains are stored. If the metallic center of the CTA is nucleophilic enough, the introduction of various functional groups at the end of a polyethylene chain can be explored.

Boisson, D’Agosto and co-workers used a di-alkylmagnesium (MgR2) as CTA and a neodymium metallocene catalyst to produce a dipolyethylenylmagnesium (Mg(PE)2) compound. Deactivation of the polymerization medium under adequate conditions was further performed and end-functionalized polyethylene, such as hydroxyl end polyethylene (PE-OH) or amino end polyethylene (PE-NH2) were isolated. These functional polyethylenes could open new ways to more complex structure architectures and multisegmented polymers.

16 2.3.3. CCTP System Studied

In this work, two different CCTP systems for ethylene polymerization were studied. They comprise the use of two neodymium metallocenes (lanthanide metallocenes) as the single-site catalysts, and a dialkylmagnesium compound (MgR2) as CTA for both systems. Toluene is the polymerization solvent for both systems. Although the formation of a neutral active species L2NdR and the chain transfer between the catalyst and CTA (chapter 2.3.1) is a well accepted mechanism, 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 pseudo-living 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.

The two CCTP systems studies were (catalyst precursor / CTA respectively):

∗ Catalyst I: Cp2NdCl2Li(OEt2)2 (1) / n-butyl-n-octyl-magnesium (BOMAG) (3)

Catalyst II: {Me2SiFlu2Nd(µ-BH4)[(µ-BH4)Li(THF)]}2 (2) / n-butyl-n-octyl-magnesium (BOMAG) (3)

The chemical structures of the catalysts precursors and CTA are shown in Figure 14. The main differences between the two neodymium catalysts are the ligands, with the presence of two cyclopentadienyl (Cp* = C5Me5) units on catalyst (I) and two fluorenyl (Flu = C13H8) units bridged by

Me2Si on catalyst (II).

Figure 14. Chemical structures of the catalysts and CTA used.

17 3. Experimental Section

The experimental section is a very important aspect of this work since the manipulation of reagents and catalysts are very sensitive to the air, therefore all the experimental steps are carried out under inert atmosphere. Consequently, an improper experimental protocol and careless manipulation can have a high impact in the results obtained, leading to inaccurate interpretations and conclusions. In this chapter the air-free techniques used will be described, the origin/synthesis of the materials and catalysts, the polymerization procedure and apparatus, and finally the polymer analysis by high temperature size exclusion chromatography (SEC-HT).

3.1. Air-Free Techniques

In order to ensure that all manipulation steps are performed under inert atmosphere, some techniques have to be used. These air-free techniques involve very often the use of Schlenk lines and a glovebox. Each one is used for specific tasks/techniques according to its characteristics.

One of the most important techniques to avoid air/water is the purge-and-refill technique which is performed in the glovebox and in any container (e.g flasks) by the use of Schlenk lines. This technique consists in the application of vacuum to remove air in the flask, and then refilled with inert gas, which is argon in this work. This process is repeated at least three times (vacuum is applied for a more period of time than refill) as each purge-and-refill further reduces the amount of air in the flask. Moreover, all the glassware used is pre-dried at 80ºC in an oven prior to use in order to ensure that there is no residual water on it. The Schlenk lines, the glovebox and the techniques used in each one are now described in more detail.

3.1.1. Schlenk Line The Schlenk line used consists in a single manifold with several ports, with rubber tubes attached, that are connected to a source of argon. This argon line is vented through an oil bubbler, filled with heptane, to prevent contamination by atmospheric oxygen and water. The Schlenk line used is showed in Figure 15.

Figure 15. Schlenk lines.

18 The glassware used is similar to standard glassware, but has additional side(s) arm(s) which can be connected to the Schlenk line. This enables the following techniques:

 Addition of solvents/reagent by counterflow. The flask is connected to the Schlenk line and the solvent/reagent is added against a flow of argon in order to avoid contact with air. One example in this work is the addition of toluene in a flask.  Transfer solutions between flasks using a cannula. Both flasks are under a flow of argon and a cannula, a thin plastic tube, is used to transfer the solution between flasks. One example in this work is the transfer of the solution of toluene and BOMAG into the polymerization reactor.

3.1.2. Glovebox

The glovebox is a sealed container with an argon atmosphere and built-in gloves in order to perform tasks inside the glovebox without air entering in the container. In contrast with the Schlenk line where the purge-and-refill is applied directly to the vessel, the glovebox applies the purge-and-refill, before entering the glovebox itself, to an airlock attached to the glovebox, commonly called the ante- chamber, which contains the glassware needed for manipulation. There is a big and small ante- chamber, according to the glassware size needed for manipulation.

In this work, the glovebox was used to safely store and weigh the precatalyst to a Schlenk flask. Only the small ante-chamber was used. Figure 16 shows the glovebox used and both ante-chambers.

Figure 16. Glovebox.

19 3.2. Materials and Catalysts Precursors

In this chapter, the origin/synthesis of the materials and the catalysts precursors used in this work are described. Obviously, all materials and catalyst precursors were handled using the air-free techniques previously explained.

3.2.1. Toluene

Toluene is the polymerization solvent used and it’s obtained from the solvent purification unit MBRAUN MB-SPS-800 (Figure 17). This unit operates by pushing the toluene through a series of drying columns resulting in moisture levels down to the ppm range.

Figure 17. Solvent purification unit. 3.2.2. Ethylene

Ethylene is the monomer used for this work. It was supplied by Air Liquide and stored in a gas cylinder (Figure 18). The ethylene specifications are showed in Table 1.

Table 1. Ethylene specifications.

Component CO2 H2 O2 N2 H2O S

Maximum 5 10 10 40 5 2 Impurity (PPM)

Figure 18. Ethylene cylinder. 3.2.3. BOMAG – n-butyl-n-octyl-magnesium

n-butyl-n-octyl-magnesium, BOMAG for abbreviation, is used as the CTA and it was supplied by Chemtura Manufacturing. The BOMAG used is a 20 wt% solution of n-butyl-n-octyl-magnesium in heptane.

3.2.4. Dibuthyl Ether

Dibuthyl Ether was supplied by Sigma-Aldrich, degassed by purging with argon and stored in a Schlenk flask with molecular sieves (3 Å).

∗ 3.2.5. 퐂퐩ퟐNdCl2Li(OEt2)2

∗ Cp2NdCl2Li(OEt2)2 is a catalyst precursor that was already synthetized prior to this work. However, the syntheses steps are now described. [29]

Lithium pentamethylcyclopentadienide was added to neodymium trichloride, and then tetrahydrofuran was added. The suspension was refluxed for 12 h. The tetrahydrofuran was removed by vacuum and the residue was extracted with diethyl ether. The red extracts were combined, ∗ concentrated and cooled at -10°C. The large blue prisms of Cp2NdCl2Li(OEt2)2 were collected and dried under vacuum. The catalyst precursor was stored in the glovebox afterwards.

3.2.6. {Me2SiFlu2Nd(µ-BH4)[(µ-BH4)Li(THF)]}2

{Me2SiFlu2Nd(µ-BH4)[(µ-BH4)Li(THF)]}2 is a catalyst precursor that was already synthetized prior to this work. However, the syntheses steps are now described. [30]

A diethyl ether solution containing [Me2Si(C13H8)2]Li2 was added to a diethyl ether solution of

Nd(BH4)3(THF)3. The mixture was stirred for 12 h at ambient temperature. The solution is filtrated and concentrated. Addition of pentane caused precipitation of red crystals of {Me2SiFlu2Nd(µ-BH4)[(µ-

BH4)Li(THF)]}2. The catalyst precursor was stored in the glovebox afterwards.

21 3.3. Polymerization Procedure and Apparatus

The apparatus for the polymerization trials is shown in Figure 19. The numbers in the figure represent the most crucial components and equipment in the set-up. These will be referred along the text and explained in more detail.

Figure 19. Polymerization apparatus.

Polymerizations were carried out in a 500 mL double-layer glass reactor (1) equipped with the IKA® EUROSTAR stainless steel blade stirrer (2) with a speed of 900 rpm in order to avoid diffusion problems. The reactor is replaced after each polymerization trial. The reactor is heated up to the desired temperature using water that circulates between the reactor and the jacket. Figure 20 shows the reactor itself and the jacket. To ensure a precise temperature control, the water is heated and pumped from the Julabo® F12 heating bath (3) with a temperature stability of ± 0,03ºC.

Figure 20. The left image shows the reactor in the end of a polymerization and also the jacket detached from the reactor. The right image is the glass reactor itself.

A desired volume of BOMAG, according to the desired concentration, was added to a 1000 mL round bottom Schlenk flask containing 400 mL of toluene from the solvent purification unit. Meanwhile, the catalyst precursor was weighted and transferred to a 50 mL round bottom Schlenk flask in the glovebox. A little portion of the BOMAG and toluene mixture (10-15 mL) was added to the catalyst precursor in order to solubilize it. Then, the mixture of BOMAG and toluene was introduced in the reactor and charged with ethylene pressure of 3,6-3,8 bar, via a reservoir connected to the reactor (4), in order to completely solubilize the ethylene in the polymerization solvent. This is critical to ensure that the ethylene consumption in the beginning of the polymerization correspond to the chain growth and not solubility effects. When ethylene was complete solubilized and the desired temperature was reached, the catalyst precursor solution was introduced in the reactor by the cartridge (5) on the top of it and pressure of ethylene was increased to 4 bar which was kept constant during the entire polymerization. For all these manipulations, air-free techniques were used including: purge-and-refill in the Schlenk flasks, reactor and glovebox; use of Schlenk lines to transfer solutions between flasks and to introduce in the reactor via cannula.

23 In order to measure activities and productivities, which are very important for the kinetic study, it was observed the pressure drop in the ethylene reservoir during the polymerization, and then converted to mass of ethylene consumed (Appendix 7.1). The pressure in the reactor was recorded by SPY RF® sensors (6) and sent wireless to a PC (Figure 21). These sensors also recorded the reactor temperature and pressure and the bath temperature.

Figure 21. SPY FR® sensors and Sirius software for recording pressure and temperature.

After the polymerization, the reaction was stopped by venting the gaseous ethylene through reactor exhaust and cooling the reactor to 20ºC. Methanol was added to the reactor contents and the suspension filtrated. The polyethylene recovered was dried under vacuum at 80ºC for 2 hours (Figure 22).

Figure 22. Left and top-right image shows the filtration and drying process, respectively. Bottom-right image is the final polyethylene obtained.

24 3.4. Polymer Analysis

All the polyethylene samples were characterized by size exclusion chromatography at high temperature (SEC-HT). This analytical technique is designed to determine several polymer properties: number and weight average molar masses (both 푀̅̅̅n̅ and 푀̅̅̅̅w̅), and the molar mass distribution.

SEC-HT works by separating the polymer chains in a column on the basis of their hydrodynamic volume. The separation is achieved by the differential exclusion of the macromolecules from the pores of the column packing material, as they pass through a bed of porous particles. Therefore, large polymer chains flow more quickly and are eluted first, whereas smaller chains take more time to elute since they penetrate deeply into the pores. This technique is done at high temperature for polyethylene to keep this polymer in solution. The result is in the form of spectrum signal (in volts) as a function of the elution volume.

The SEC-HT analysis was performed in the Viscotek HT-GPC by Malvern® (Figure 23). This system includes a high temperature refractive index, for use as a concentration detector, a light scattering detector for absolute molar mass determination and a viscometer for intrinsic viscosity determination and branching. It also includes one pre-column followed by three columns in series (PLgel Olexis 300 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 150°C. The mobile phase was stabilized with 2,6-di(tert-butyl)-4-methylphenol (200 mg.L-1). SEC-HT allows the determination of the number/weight average molar masses and the distribution of molar masses using a calibration curve based on narrow polyethylene standards (Mp: 170, 395, 750, 1110, 2155, 25000, 77500, 126000 g.mol-1) from Polymer Standard Service (Mainz). The OmniSEC® software was used for data acquisition and analysis.

Figure 23. Viscotek HT-GPC by Malvern®.

25 4. Results and Discussion

This chapter begins with some generalities on kinetics of ethylene polymerization, and then shows the resulting studies performed on the two catalytic systems mentioned earlier on chapter 2.3.3, namely:

∗  System I: Cp2NdCl2Li(OEt2)2 (1) / n-butyl-n-octyl-magnesium (BOMAG) (3)

 System II: {Me2SiFlu2Nd(µ-BH4)[(µ-BH4)Li(THF)]}2 (2) / n-butyl-n-octyl-magnesium (BOMAG) (3)

4.1. Kinetics Introduction

The complexity of the physical and chemical processes involved in ethylene polymerization, such as: the activation of the catalyst precursor, the highly sensititvity and reactivity of active species, the effect of reversible chain transfer, etc. make understanding the mechanistic aspects of catalytic polymerization a challenge. [31] Consequently, the kinetics of this type of polymerization is still a matter of debate making the commonly used rate expression (eq. (1)) not so trivial. [32]

푑[퐶 퐻 ] 푅 = − 2 4 = 푘 [푀∗][퐶 퐻 ]∝ (1) 푝 푑푡 푝 2 4

Rp is the observable rate of polymerization, kp is the propagation rate coefficient and [M*] is the concentration of active sites. This equation seems simple but raises some issues: although in most polymerizations α has a value of 1 (first-order reaction in monomer), it can range between 0 < α < 2. [32] But more important is the fact that the metal centers are not all active at the same time, therefore the concentration of active sites [M*] is given by the multiplication of the percentage of active sites (C*) with the concentration of metal centers [M]. In fact, some literature reported that C* values determined by stopped-flow techniques are only between 5% and 25%. [32] [33]

Consequently, the simple measurement of the rate of polymerization is not enough to determine the real propagation rate coefficient kp because the percentage of active sites C* is unknown, so It is not possible to separate these two variables and we only can determine an apparent propagation rate app coefficient kp (=kpC*). In order to get kp more complex investigations would be required, such as stopped-flow polymerizations or radio-tagging techniques.

푑[퐶 퐻 ] 푅 = − 2 4 = 푘 퐶∗[푀][퐶 퐻 ]∝ = 푘 [푀][퐶 퐻 ]∝ (2) 푝 푑푡 푝 2 4 푎푝푝 2 4

26 Assuming α = 1 and dividing the yield (Y), eq. (3), by the concentration of metal centers [M] (i.e the concentration of catalyst) the relationship between productivity, 푌/[푀], given in mol of ethylene consumed per mol of catalyst, and time, t, is given in eq. (4).

푌 = 푅푝푡 = 푘푎푝푝[푀][퐶2퐻4]푡 (3)

푌 = 푘 [퐶 퐻 ]푡 (4) [푀] 푎푝푝 2 4

The determination of kapp is obtained from the plot of the productivity vs time. The slope of this straight line is divided by the concentration of ethylene and kapp is obtained. The concentration of ethylene in the reactor (mol/dm3) is given by eq. (5) that relates it with the pressure of ethylene (bar, absolute), P, and the polymerization temperature (K), T.

2700 ( ) ;3 1,98푇 (5) [퐶2퐻4] = 푃 × 1,55 × 10 푒

27 4.2. Kinetic Study: Influence of Mg/Nd Ratio

In order to investigate the influence of Mg/Nd ratios on both systems, polymerization trials were done, at 80ºC and 4 bar of ethylene pressure, for the following Mg/Nd ratios: 20, 40, 60, 80 and 160 (this last ratio was only performed for system I). This was done by keeping constant the concentration of the respective neodymium catalyst at 50 µM and varying the concentration of BOMAG in the reactor. The kinetic profiles for these polymerization trials are given in Figure 24 and Figure 25 for system I and II respectively. Productivity profiles are found in Appendix 7.2

2500 Mg/Nd=20

Mg/Nd=40 )

1 2000

- Mg/Nd=60

1 - Mg/Nd=80 1500 Mg/Nd=160

1000 Activity Activity (kg.mol 500

0 0 50 100 150 200 250 300 t (min)

∗ Figure 24. Kinetic profiles obtained for the catalytic system I. (T = 80ºC; 4 bar; [퐂퐩ퟐNdCl2Li(OEt2)2] = 50 µM; toluene = 400 mL).

2500

Mg/Nd=20

) 2000 Mg/Nd=40

.h 1 - Mg/Nd=60 1500 Mg/Nd=80 1000

Activity Activity (kg.mol 500

0 0 50 100 150 200 250 300 350 400 t (min)

Figure 25. Kinetic profiles obtained for the catalytic system II. (T = 80ºC; 4 bar; [{Me2SiFlu2Nd(µ-BH4)[(µ- BH4)Li(THF)]}2] = 50 µM; toluene = 400 mL).

28 The kinetic profiles for the two catalytic systems shown are consistent with the CCTP mechanism. This pattern includes three sequential phases:

 Phase 1 – In the beginning of the polymerization, the active species is formed by in situ alkylation of the precursor with the BOMAG (chapter 2.2.2.4, Figure 8) resulting in the polymerization of ethylene. The initial activity dropped until a steady value is attained. This phenomenon is not yet fully understood but we assume a difference of rate exchange depending of the length of R group (butyl vs polyethylene chain).  Phase 2 – In this phase a steady activity is achieved due to the equilibrium between the

heterobimetallic complex and active catalyst species, L2NdR, 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 (see Figure 12.

 Phase 3 – The 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. Consequently, the number of active sites rises which leads to a peak of activity which is followed by a quick deactivation of the catalysts.

The only exception is for a Mg/Nd ratio of 160 for system I where only the first two phases are included because it took much more time for the precipitation to occur and the polymerization was stopped before it happened. Figure 26 shows an example of the typical kinetic profile (system I with a Mg/Nd ratio of 80) on which these three phases can be easily identified.

∗ Figure 26. Typical kinetic profile and its 3 phases. (T = 80ºC; 4 bar; [퐂퐩ퟐNdCl2Li(OEt2)2] = 50 µM; toluene = 400 mL; Mg/Nd = 80).

29 푎푝푝 Table 2 lists the main results for these polymerization trials: activity in the steady-state phase,푘푝 , number average molar mass (푀̅̅̅n̅) and dispersity (푀̅̅̅̅w̅/푀̅̅̅n̅) obtained by SEC analysis.

Table 2. Activity, and molar mass features from the polymerization trials for both catalytic systems. (T = 80ºC; 4 bar; [Nd] = 50 µM).

Steady-State 퐚퐩퐩 퐤 푴 [BOMAG] Activity 퐩 퐧 ̅̅̅̅̅ ̅̅̅̅ Run System Mg/Nd -1 -1 -1 푴퐰/푴퐧 (mM) (kg mol h ) (L.mol-1.s-1) (g.mol ) 1 1 20 982 45.6 8730 6.3

2 2 40 559 25.0 7690 5.3

3 I 3 60 451 20.6 9020 5.0

4 4 80 355 16.0 8310 4.3

5 8 160 226 10.1 2590 1.9

6 1 20 710 35.3 12000 5.8

7 2 40 320 15.6 11400 5.3 II 8 3 60 218 10.2 9360 5.9

9 4 80 153 7.3 12200 4.4

Both the results from Table 2 and the kinetic profiles from Figure 24 and Figure 25 reveal some important results:

 The increase in BOMAG (MgR2) concentration shifts the equilibrium towards the bimetallic complex. 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.

Figure 27. Heterobimetallic complex (dormant) in equilibrium with the alkyl neodymium complex and BOMAG.

30  The polymers were analyzed by SEC and showed a bimodal distribution of molar mass (Figure 28). This is expected since 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). The exception is for system I with a Mg/Nd ratio of 160 which did not reach precipitation resulting in a lower dispersity (푀̅̅̅̅w̅/푀̅̅̅n̅ = 1.9) but still relatively high considering the CCTP mechanism.

 Comparing the two systems, system I displayed approximately twice the activity of system II (with the exception of the experiment carried out with a Mg/Nd ratio of 20)1. A higher affinity between the active species of system II, on which ethylene insertion takes place, and the CTA can be a possible explanation for the lower activities observed on system II in comparison to system I. In terms of molar masses and respective dispersities both systems showed somewhat the same results and with similar orders of magnitude (4 < 푀̅̅̅̅w̅/푀̅̅̅n̅ < 7).

1,00

0,80

0,60

0,40

Normalized WF/dLogM 0,20

0,00 2 3 4 5 6 7 Log M n Figure 28. Example of the bimodal distribution obtained for system I with a Mg/Nd ratio of 60. (T = 80ºC; 4 bar; [Nd] = 50 µM).

1 The steady-state in the polymerization experiment performed for a Mg/Nd ratio of 20 in system II is very brief (15 minutes) making hard the determination of the actual average activity during this phase since phase I and II can overlap.

31 4.3. Determination of the activation energy

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 (eq. (6)). By app applying logarithms to both sides of the equation (eq. (7)) and plotting ln kp vs 1/T, which should be

linear, the energy of activation, Ea, is obtained multiplying the slope by -R, the universal gas constant (1,987 cal.mol-1.K-1).

퐸푎 푎푝푝 ; (6) 푘푝 = 퐴푒 푅푇

퐸 1 (7) ln 푘푎푝푝 = ln 퐴 − 푎 푝 푅 푇

For this study the polymerization is stopped just when precipitation was about to start, since the app value of kp in the steady-state phase is the targeted parameter. Also, since the concentration of ethylene in the reactor depends on the temperature, it was maintained constant for all trials by manipulating the pressure by eq. (5). Due to easier precipitation for lower temperatures than 80ºC, the choice of the Mg/Nd ratio is important for this investigation in order to get a steady-state phase that app lasts long enough to get reliable data for kp . Looking at the kinetic profiles, a Mg/Nd ratio of 80 was chosen for both systems. The results are listed in Table 3 for both systems and the Arrhenius plots are given in Figure 29 and Figure 30. Activity and productivity profiles are given in Appendix 7.3.

Table 3. Determination of and Ea from the polymerizations experiments at different temperatures for both systems. ([Nd] = 50 µM; Mg/Nd = 80; toluene = 400 mL).

Steady-State 퐚퐩퐩 퐤 Ea T Precipitation Activity 퐩 Run System -1 -1 퐥퐧 Slope -1 (ºC) Time (min) (kg mol h ) (L.mol-1.s-1) (kcal.mol ) 10 85 225 477 25.7 3.2

11 80 120 355 16.0 2.8 I -8716.6 17.3 12 75 70 285 12.4 2.5

13 70 65 207 8.6 2.2

14 90 a) 288 13.2 2.6

15 85 a) 277 12.7 2.5

16 80 305 153 7.3 2.0 II b) b) 17 74 105 183 8.6 2.2

18 70 30 219 9.9 2.3

19 65 40 202 9.7 2.3 a) Polymerization did not reach precipitation in a timely manner. b) The results do not allow performing any calculation.

32 4

y = -8716,6x + 27,536

) 3 R² = 0,986 app

p 2 ln (k ln 1

0 0,00275 0,0028 0,00285 0,0029 0,00295 -1 1/T (K ) 퐚퐩퐩 Figure 29. Arrhenius plot obtained by determining 퐤퐩 for the catalytic system I at 70, 75, 80 and 85ºC. ([Nd] = 50 µM; Mg/Nd = 80; toluene = 400 mL). 4

) 2

app p

1 ln (k ln

0 0,0027 0,00275 0,0028 0,00285 0,0029 0,00295 0,003 -1 1/T (K ) 퐚퐩퐩 Figure 30. Arrhenius plot obtained by determining 퐤퐩 for the catalytic system II at 65, 70, 74, 80, 85 and 90ºC. ([Nd] = 50 µM; Mg/Nd = 80; toluene = 400 mL). Analyzing the results for system I (Table 3), it can be seen that higher polymerization temperatures app lead to higher kp 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 app the reaction rate and consequently on the kp value. The linear regression of the Arrhenius plot (Figure 29) displays a very good fit, as seen by the coefficient of determination (R2) of almost 1. The activation energy value derived from it is 17.3 kcal/mol which is close to the value of 20.6 kcal.mol-1 determined by complex computational investigations (Appendix 7.4). Regarding the precipitation times, two opposed effects have to be considered. On the one hand, the chain growth rate increases app with temperature (as seen by the increase of kp ) 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.

app The results for system II are somewhat unexpected because kp 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 60°C and then raised the temperature to 80°C in order to compare the activity at the steady step. These unexpected results do not permit the calculation of the Ea for this catalyst.

33 4.4. Molar mass targeting and co-solvent effect

This study has two parts. Firstly, different polyethylene molar masses were targeted in order to evaluate the polymerization control efficiency by comparing the experimental and targeted molar mass. Secondly, the co-solvent effect was explored by the addition of dibutyl ether (Bu2O) to the polymerization medium. The objective is to perturb the equilibrium between the dormant bimetallic complex and the active species and observe its effects on activity and polymerization control. All polymerizations were stopped before precipitation of Mg(PE)2. Both investigations were done just for system II because it was already carried out successfully for system I in another work. [34]

In order to know when to stop the polymerization, it is crucial to determine how much ethylene has to be consumed to get the target molar mass, which is then converted into pressure drop of the ethylene reservoir, stopping the polymerization when this pressure drop is reached. The determination of the required ethylene consumption is based on the CCTP mechanism feature. In theory, two polyethylene chains are formed by magnesium center leading to Mg(PE)2. The number of polyethylene chains should be equal to two times the mole number of introduced BOMAG. Therefore the required ethylene consumption, meth, can be related to the target molar mass, MMtarget, and the number of moles of magnesium centres, nMg, by eq. (8). Since nMg is an experimental parameter and MMtarget is chosen previously, the targeted meth can be easily determined.

푚푒푡ℎ = 푀푀푡푎푟𝑔푒푡 × 2푛푀𝑔 (8)

Furthermore, with the experimental mass of polyethylene obtained , mPE, and with the number average molar mass measured by SEC, 푀̅̅̅n̅, it is possible to determine the experimental number of polyethylene chains per magnesium, nchains/nMg, by eq. (9) and also the theoretical molar mass, MMtheo, by eq. (10), assuming the CCTP involves two polyethylene chains per magnesium. These two parameters are very useful since they can provide a sort of “benchmark” for the CCTP process for the investigated system.

푚푃퐸⁄푀̅̅̅n̅ 푛푐ℎ푎𝑖푛푠⁄푛푀𝑔 = (9) 푛푀𝑔

푚푃퐸 (10) 푀푀푡ℎ푒표 = 2 × 푛푀𝑔

Polymerizations were carried out at 80ºC, with a BOMAG concentration of 4 mM and neodymium catalyst concentration of 50 µM, corresponding of a Mg/Nd ratio of 80. For run 20 a molar mass of 2000 g.mol-1 was targeted and for run 21 and 22 a molar mass of 1000 g.mol-1 was targeted. Results are listed in Table 4. The productivity profiles can be found in the Appendix 7.5.

Table 4. Results of the molar mass targeting study for the catalyst system II. (T = 80ºC; [Nd] = 50 µM; Mg/Nd = 80; toluene = 400 mL).

Activity 퐤퐚퐩퐩 푴̅̅̅̅̅ P 퐩 Time meth mPE MMtheo 푴̅̅̅퐧̅ 퐰 nChains Run (kg mol-1 h-1) -1 -1 -1 -1 ̅̅̅̅ (bar) (L.mol .s ) (min) (g) (g) (g.mol ) (g.mol ) /푴퐧 /nMg

20 232 10.5 60 6.27 4.97 1570 2370 2.1 1.3 4 21 304 15.0 26 3.26 2.67 839 2140 2.1 0.8

22 2 131 12.0 60 2.95 1.53 485 1240 1.6 0.8

From the results for run 20 and 21 several aspects can be discussed. Firstly, the activity and app consequently the kp values are higher than the ones seen on the kinetic profile for a Mg/Nd ratio of 80 (Table 2, 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 phase2. The same goes for the difference in app activity and kp between run 20 and 21. Secondly, a unimodal molar mass distribution was obtained (Figure 31) 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 4000 g.mol-1. [35] [36] -1 -1 Finally, the values of MMtheo are far from the ones analyzed by SEC (푀̅̅̅n̅), 1570 g.mol vs 2370 g.mol and 839 g.mol-1 vs 2140 g.mol-1 for run 20 and 21, respectively. Consequently, the number of chains per magnesium values are far from the expected value of two (1.3 and 0.8 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 20 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 1000 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 MMtheo was still far from the one obtained by SEC (485 g.mol-1 vs 1240 g.mol-1) and the number of chains per magnesium value of 0.8 was lower than the expected value of two. These results lead to the conclusion that the chain transfer efficiency for the catalytic system II is rather ineffective.

2 From the kinetic profiles for system II (Figure 25) the steady-state for a Mg/Nd ratio of 80 begin only at 50 minutes.

1,00

0,80

0,60

0,40

0,20 Normalized WF/dLogM

0,00 2 2,5 3 3,5 4 4,5 5 Log M n

Figure 31. Example of the unimodal molar mass distribution obtained for Run 21. (T = 80ºC; [Nd] = 50 µM; Mg/Nd = 80; toluene = 400 mL).

As said before, this study was already done for system I and the results obtained were positive with [34] values of MMtheo close to the experimental ones and values of nchains/nMg really close to two. This difference between the two systems leads to the conclusion that the ligands on the catalyst do influence remarkably the behaviour and effectiveness of the overall CCTP mechanism resulting in polyethylene chains with different molar mass characteristics.

As said earlier, another study was done regarding the effect of dibutyl ether (Bu2O) addition on the kinetic profiles and molar mass distribution of system II. The structure of Bu2O is showed in Figure 32

Figure 32. Bu2O structure.

In order to evaluate this, it was used the same conditions as Run 20: polymerization at 80ºC, 4 bar of pressure, Mg/Nd ratio of 80, but with addition of Bu2O with a Bu2O/Mg molar ratio of 10/1. The main goal is to compare the trials with and without Bu2O and therefore observe its effects. Results are listed in Table 5 and the activity and productivity profiles are shown in Figure 33 and Figure 34.

Table 5. Results for the effect of Bu2O addition for the catalytic system II. (T = 80ºC; [Nd] = 50 µM; Mg/Nd = 80; toluene = 400 mL).

Steady-State 퐚퐩퐩 퐤 Bu2O/ 퐩 Time meth mPE MMtheo 푴̅̅̅퐧̅ nChains Run Activity -1 -1 -1 ̅̅̅̅̅ ̅̅̅̅ -1 -1 (L.mol .s ) -1 푴퐰/푴퐧 Mg (kg mol h ) (min) (g) (g) (g.mol ) (g.mol ) /nMg 20 0 232 10.5 60 6.27 4.97 1570 2370 2.1 1.3

24 10 645 31.6 25 7.44 8.19 2570 3860 1.6 1.3

36 700

Bu2O/Mg = 10

) 600

.h 1 - 500 Bu2O/Mg = 0 400 300 200

Activity Activity (kg.mol 100 0 0 10 20 30 40 50 60 t (min)

Figure 33. Activity profiles obtained with and without Bu2O addition for the catalytic system II. (T = 80ºC; [Nd] = 50 µM; Mg/Nd = 80; toluene = 400 mL).

14000

12000

10000

8000

6000 Bu2O/Mg = 0

4000 Bu2O/Mg = 10 2000

Productivity Productivity (molEthylene/mol Nd) 0 0 10 20 30 40 50 60 t (min)

Figure 34. Productivity profiles obtained with and without Bu2O addition for the catalytic system II. (T = 80ºC; [Nd] = 50 µM; Mg/Nd = 80; toluene = 400 mL).

The addition of Bu2O highly increased the activity in a factor of almost three and also the kinetic profile obtained is more stable and steady than the previous ones. The reason for this behaviour may be the formation of a complex between Bu2O and BOMAG enabling a decrease in the free BOMAG concentration in the polymerization medium and therefore the equilibrium showed in Figure 27 is shifted towards the active species and consequently an increase in activity is seen. This activity increase happens as well for system I. [34]

Despite the lower dispersity value of 1.6 with the Bu2O addition, the number of chains per magnesium remains exactly the same as before at 1.3. Furthermore, the value of MMtheo is still far -1 -1 from the experimental one (푀̅̅̅n̅), 2570 g.mol vs 3860 g.mol . This shows that the addition of Bu2O 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.

37 4.5. Sampling Experiment

In order to investigate in more detail about the controlled behaviour of the polymerization mechanism, an experiment was done by taking several samples over the course of the polymerization reaction and therefore enabling the observation of the evolution of several important parameters, such as molar mass, dispersity or number of chains per magnesium. This investigation was done just for system II because it was already carried out successfully for system I in another work. [34]

Unfortunately, the reactor used for the previous results does not include a sample withdrawal system so another reactor had to be used for this study. It is very similar to the previous one with the difference being the volume which is 200 mL instead of 400 mL, and of course the sample withdrawal system on the top of it. However, this volume difference influences the kinetic profiles of the polymerization in a meaningful manner. One example of the relevant changes is the precipitation time: for a Mg/Nd ratio of 80 the sampling reactor reaches precipitation in 60 minutes as for the regular one is 300 minutes. In order to avoid this issue, a polymerization trial was carried out without sampling to obtain the kinetic profile before the sampling experiment.

The experimental conditions chosen for these trials were: temperature of 80ºC, pressure of 4 bar and Mg/Nd ratio of 160 (catalyst concentration of 50 µM and BOMAG concentration of 8 mM). The reason behind this choice is that precipitation is taking place under these conditions after 150 min, which leaves time to get all the desired samples while the polymerization medium is still homogeneous. For the sampling experiment, small samples were taken after 15, 30, 45, 60, 90 and 120 minutes from the polymerization medium. Figure 35 shows the kinetic profiles and Table 6 the results for both with and without sampling polymerization trials.

1800 Without

1600 Sampling

) 1400 1

- With Sampling .h

1 1200 -

1000

800

600

Activity Activity (kg.mol 400

200

0 0 50 100 150 200 250 300 t (min)

Figure 35. Kinetic profiles of the polymerizations with and without sampling for the catalytic system II. (T = 80ºC; 4 bar; [Nd] = 50 µM; Mg/Nd = 160; toluene = 200 mL).

Table 6. Activity and for the polymerizations with and without sampling for the catalytic system II. (T = 80ºC; 4 bar; [Nd] = 50 µM; Mg/Nd = 160; toluene = 200 mL).

퐚퐩퐩 [BOMAG] Steady-State Activity 퐤퐩 Run System Type Mg/Nd (kg.mol-1.h-1) -1 -1 (mM) (L.mol .s ) 25 Without Sampling 204 10.3 II 8 160 26 With Sampling 171 7.3

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 app kinetic profiles are similar with slight differences in the average activity and consequently on the kp . This difference may be caused by the consecutive extraction of samples which decreases the polymerization volume over the course of the polymerization (total of 91.7 mL extracted in a 200 mL of initial solution).

One important aspect to notice is that these results are based only on the ethylene consumption in the polymerization, but with sampling additional data can be obtained. Methanol is added to each withdrawn sample immediately after extraction to stop the polymerization. The samples are further dried. By weighting the samples obtained in each one of these steps, we are able to know 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 (catalyst concentration is 50 µM). From these data, the productivity can be obtained. In Table 7 the results from sample extraction are shown and the productivity profile can be found in Figure 36.

Table 7. Productivity values from the sample extraction for the catalytic system II. (T = 80ºC; 4 bar; [Nd] = 50 µM; Mg/Nd = 160; toluene = 200 mL).

Sample time V solvent mol m PE Productivity a) (min) (mL) catalyst (g) (mol Eth.mol Nd-1)

15 13.0 6.6E-07 0.15 8145

30 19.3 9.7E-07 0.22 8090

45 15.1 7.6E-07 0.26 12179

60 20.7 1.0E-06 0.37 12659

90 18.4 9.3E-07 0.35 13451

120 5.2 2.6E-07 0.23 31599

a) Mo = 28,05 g/mol

35000

)

1 - 30000

25000

20000

15000

10000

5000 Productivity Productivity (molEth.molNd 0 0 15 30 45 60 75 90 105 120 135 t (min) Figure 36. Productivity profile obtained by sampling for the catalytic system II.. (T = 80ºC; 4 bar; [Nd] = 50 µM; Mg/Nd = 160; toluene = 200 mL).

The productivity profile should be linear but it is very irregular instead. Between the 15 and 30 minutes the productivity decreases, 8144.7 to 8090.1 mol Eth.mol Nd-1, which is impossible to assign to any chemical/physical phenomenon. Also between the minute 90 and 120 minutes the productivity value is doubled from 13451 to 31599 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.

To see the evolution of molar mass in the course of the polymerization, the extracted samples of polyethylene were analyzed by SEC-HT and then the theoretical molar mass and number of chains per magnesium3 were determined by eq. (9) and eq. (10).

Table 8. Molar mass features from the SEC analysis of the samples.

Sample time MM theo 푴̅̅̅̅ (g/mol) 푴̅̅̅̅̅/̅푴̅̅̅ n /n (min) (g/mol) 퐧 퐰 퐧 Chains Mg

15 719 2130 2.4 0.7

30 714 2260 2.4 0.6

45 1075 2390 2.5 0.9

60 1117 2510 2.4 0.9

90 1187 2710 2.5 0.9

120 2788 2930 2.8 1.9

3 From the volume of the withdrawn solution and the fact that the concentration of BOMAG is 8 mM, the number of moles of magnesium can be determined

40 3500 3000

2500 2000 1500 n (g/mol) Experimental M 1000 Theoretical 500 0 0 20 40 60 80 100 120 t (min) Figure 37. Evolution of the experimental and theoretical molar masses.

The evolution of the samples molar mass is linear as shows Figure 37, which indicates that the polymerization is controlled in the time range of the sample extraction. However, the intercept with time axis is not 0. Earlier samples extractions would be needed to understand this peculiar behaviour at the beginning of the polymerization. The dispersity values are relatively high (2.4 < 푀̅̅̅̅w̅/푀̅̅̅n̅ < 2.8), the values of MMtheo are far from the experimental ones and also the number of chains per magnesium are far from the optimal value of two (0.7 < nChains/nMg < 0.9) which is consistent with the results from the molar mass targeting (Table 4). These results show again that the chain transfer efficiency for the catalytic system II is rather ineffective. The only exception is for the sample at 120 min having a number of chains per magnesium of 1.9 and a similar theoretical and experimental molar mass.

The molar mass distributions over the course of the polymerization are presented in Figure 38.

15 Min 30 Min 45 Min 60 Min 90 Min 120 Min

1,5 2 2,5 3 3,5 4 4,5 5 5,5 Log Mn

Figure 38. Evolution of molar mass distribution during the polymerization.

The smooth evolution of molar mass distribution with time shows clearly that the polymerization is controlled by the CCTP mechanism in the time range of sample extraction.

41 5. Conclusions and Perspectives

5.1. Conclusions

In the present work, the coordinative chain transfer polymerization of ethylene by neodymium metallocene complexes was studied. The influence of several parameters, such as CTA/catalyst ratio (Mg/Nd ratio) and polymerization temperature, was investigated for two different systems using different catalyst precursors and the same CTA. The main aim of this work was not only to obtain a deep knowledge on the CCTP mechanism by means of a kinetic study, but also to understand the influence of the ligands in the catalyst activity and chain transfer efficiency by comparing the two systems.

Both systems displayed the same pattern on the kinetic profiles that reflect the CCTP mechanism. After a higher initial activity, a steady-state phase is achieved in which the growing polyethylene chain is transferred, in a fast and reversible manner, from the active species to the magnesium center of the

CTA resulting in a period of well-controlled polymerization. After the precipitation of Mg(PE)2 the reversible chain transfer is stopped and the control is rapidly lost. Consequently, the polymers obtained showed a bimodal distribution of molar mass: a first population originated from the steady- state phase with low molar mass and a second population after the precipitation with higher molar mass.

From the Mg/Nd ratio influence on the kinetic profiles of both systems, it was concluded that higher app Mg/Nd ratios results in a decrease in kp value and on the activity in the steady-state phase and also in higher times of controlled polymerization. This behavior is due to the equilibrium shift towards the bimetallic complex, which is not active, by the increase in BOMAG (MgR2) concentration. Comparing ∗ the two systems, it can be seen that the Cp2NdCl2Li(OEt2)2/BOMAG system leads to higher activities than the {Me2SiFlu2Nd(µ-BH4)[(µ-BH4)Li(THF)]}2/BOMAG system.

∗ The activation energy determined from the Arrhenius plot for the Cp2NdCl2Li(OEt2)2/BOMAG system was 17.3 kcal.mol-1, with a good linear fit. The experimental value is close to the one determined by computational modeling (DFT investigation) for this system (20 kcal.mol-1). For the app {Me2SiFlu2Nd(µ-BH4)[(µ-BH4)Li(THF)]}2/BOMAG system, the kp values increase as the temperature decrease (80ºC, 74ºC and 70ºC trials) and consequently the linear fit is very poor. These unexpected results did not permit the calculation of the Ea for this catalyst. Further investigations are needed to confirm this value or explain the reason behind these results.

42 From the molar mass targeting study for the {Me2SiFlu2Nd(µ-BH4)[(µ-BH4)Li(THF)]}2/BOMAG system, it was concluded that the chain transfer efficiency to the magnesium is low for this system with values of MMtheo and nchains/nMg far from expected, even for lower pressures of ethylene. Furthermore, the targeted molar mass was significantly different from the experimental one analyzed by SEC. In ∗ contrast, the Cp2NdCl2Li(OEt2)2/BOMAG system has values of MMtheo close to the experimental ones [34] and nchains/nMg values of 2, demonstrating a good chain transfer efficiency.

The addition of Bu2O increased the activity without affecting the polymerization control. This behavior is due to the formation of a complex between the Bu2O and the BOMAG, shifting the equilibrium towards the active species. This result can be a useful tool to improve the efficiency of these systems.

The sampling experiment for the {Me2SiFlu2Nd(µ-BH4)[(µ-BH4)Li(THF)]}2/BOMAG system showed that the molar masses increase linearly in the time range of the samples extraction (15 min – 120 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.

In summary, 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. However, it’s clear that the electronic and steric environment of the ligands around the neodymium center influences in a great manner the behavior of the system, being critical for the CCTP mechanism ∗ to happen in its full potential. The Cp2NdCl2Li(OEt2)2/BOMAG system displayed higher activities and app kp values compared to the {Me2SiFlu2Nd(µ-BH4)[(µ-BH4)Li(THF)]}2/BOMAG system, but more importantly it showed a higher chain transfer efficiency, leading to narrower molar mass distributions and higher catalyst economy. An explanation for this behavior could be the higher interaction between the Me2SiFlu2NdR active species, on which the ethylene insertion takes place, and the CTA. This higher affinity can be related to the higher edge angle of the ligands in the {Me2SiFlu2Nd(µ-BH4)[(µ-

BH4)Li(THF)]}2 complex. 5.2. Perspectives

Some of the studies done in this work need further investigation: activation energy value of the

{Me2SiFlu2Nd(µ-BH4)[(µ-BH4)Li(THF)]}2/BOMAG system 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 CCTP mechanism for lanthanide metallocenes using MgR2 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.

43 6. References

[1] [Online] IHS. [Cited: August 17, 2015.] http://www.ptq.pemex.gob.mx/productosyservicios/eventosdescargas/Documents/Foro%20PEMEX%20Pe troqu%C3%ADmica/2012/PEMEX%20PE.pdf.

[2] Peacock, Andrew J. Handbook of Polyethylene - Structures, Properties, and Applications. s.l. : Marcel Dekker, Inc., 2000.

[3] McKenna, João B.P. Soares and Timothy F. L. Polyolefin Reaction Enginnering. s.l. : WILEY-VCH, 2012.

[4] Elizabeth Benham, Max McDaniel. Ethylene Polymers, HDPE. Encyclopedia Of Polymer Science and Technology. 2001, Vol. 2, pp. 382-407.

[5] Maraschin, N. Ethylene Polymers, LDPE. Encyclopedia Of Polymer Science and Technology. 2001, Vol. 2, pp. 412-438.

[6] Simpson, D. M. and Vaughan, G. A. Ethylene Polymers, LLDPE. Encyclopedia Of Polymer Science and Technology. 2001, Vol. 2, pp. 441-478.

[7] Davis, Krzysztof Matyjaszewski and Thomas P. Handbook of Radical Polymerization. s.l. : Wiley- Interscience, 2002.

[8] Cossee, P. Mechanism of polymerization of α-olefins with Ziegler-Natta catalysts. Journal of Catalysis. 1964, Vol. 3, 1, pp. 80-88.

[9] Osakada, Kohtaro. Organometallic Reactions and Polymerization. s.l. : Springer, 2014.

[10] Odian, George. Principles of Polymerization. s.l. : Wiley Interscience, 2004.

[11] Ruihua Cheng, Zhen Liu, Lei Zhong, Xuelian He, Pengyuan Qiu, Minoru Terano, Moris S. Eisen, Susannah L. Scott, Boping Liu. Adv Polym Sci. 257, 2013, pp. 135-202.

[12] John Boor, Jr. Ziegler-Natta Catalysts and Polymerizations. s.l. : Academic Press, 1979.

[13] Bertrand Heurtefeu, Henri Cramail, Florent Vaultier, Christophe Boisson, Reko Leino. Single-Site

44 Catalysts. [book auth.] Krzysztof Matyjaszewski. Encyclopedia of Polymer Science and Technology. s.l. : Wiley, 2012.

[14] 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 4404344 A 1983.

[15] Recent developments in organolanthanide polymerization catalysts. Zhaomin Hou, Yasuo Wakatsuki. 231, s.l. : Coordination Chemistry Reviews, 2002, pp. 1-22.

[16] C. Boisson, V. Monteil, D. Ribour, R. Spitz, F. Barbotin. Lanthanidocene Catalysts for the Homo- and Copolymerization of Ethylene with Butadiene. Macromol. Chem. Phys. 2003, pp. 1747-1754.

[17] Buchmeiser, Michael R. Makromol. Chem., Rapid Commun. 1983, Vol. 4, pp. 417-421.

[18] Walter Kaminsky, Michael Arndt. Metallocenes for Polymer Catalysis. Advances in Polymer Science. 2007, pp. 143-187.

[19] Kempe, Rhett. How to Polymerize Ethylene in a Highly Controlled Fashion? Chem. Eur. J. 2007, Vol. 13, pp. 2764-2773.

[20] David C. Eisenberg, Edward G. Samsel. Actinide catalyzed chain growth process. 0539867 Eur., 1993.

[21] Andreia Valente, André Mortreux, Marc Visseaux, Philippe Zinck. Coordinative Chain Transfer Polymerization. Chem. Rev. 2012.

[22] Sita, Lawrence R. Ex Uno Plures (“Out of One, Many”): New Paradigms. Angew. Chem. Int. 2009, pp. 2464-2472.

[23] F. D'Agosto, C. Boisson. Aust. J. Chem. 2010, Vol. 63, pp. 1155-1158.

[24] Jérôme Mazzolini, Edgar Espinosa, Franck D’Agosto, Christophe Boisson. Polym. Chem. 2010, Vol. 1, pp. 793-800.

[25] 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, 2006, pp. 714-718.

[26] J.F.Pelletier, A. Mortreux, X. Olonde, K.Bujadoux. Synthesis of New Dialkylmagnesium Compounds by Living Transfer Ethylene Oligo- and Polymerization with Lanthanocene Catalysts. Angew. Chem., Int. Ed.

45 Engl. 1996, Vol. 35.

[27] 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. 2007, Vol. 40, pp. 7062-7064.

[28] 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. 2008, Vol. 47, pp. 9311-9313.

[29] T. Don. Tilley, Richard A. Andersen. Pentamethylcyclopentadienyl derivatives of the trivalent lanthanide elements neodymium, samarium, and ytterbium. Inorganic Chemistry. 1981, Vol. 20, pp. 3267–3270.

[30] Julien Thuilliez, Louis Ricard, François Nief, Fernande Boisson and Christophe Boisson. ansa- Bis(fluorenyl)neodymium Catalysts for Cyclocopolymerization of Ethylene with Butadiene. Macromolecules. 2009, Vol. 42, pp. 3774-3779.

[31] Timothy F. L. McKenna, Estevan Tioni, Maria Maddalena Ranieri, Arash Alizadeh, Christophe Boisson, Vincent Montei. Catalytic Olefin Polymerisation at Short Times: Studies Using Specially Adapted Reactors. The Canadian Journal Of Chemical Engineering. 2013, Vol. 91, pp. 669-686.

[32] Vincenzo Busico, Roberta Cipullo, Valentina Esposito. Stopped-flow polymerizations of ethene and propene in the presence of the catalyst system rac-Me2Si(2-methyl-4-phenyl-1- indenyl)2ZrCl2/methylaluminoxane. Macromol. Rapid Commun. 20, 1999, pp. 116-121.

[33] Maria M. Ranieri, Jean-Pierre Broyer, Francesco Cutillo, Timothy F. L. McKenna, Christophe Boisson. Site count: is a high-pressure quenched-flow reactor suitable for kinetic studies of molecular catalysts in ethylene polymerization. Dalton Trans. 42, 2013, pp. 9049-9057.

[34] Ruivo, Rui. Investigation of the kinetics of ethylene polymerization for the bi-component catalyst Cp*2NdCl2Li(OEt2)2/MgR2. 2014.

[35] Kretschmer, W. P., et al. R. Chem. - Eur. J. 2006, Vol. 12, p. 8969.

[36] Rouholahnejad, F., Mathis, D. and Chen, P. Organometallics. 2010, Vol. 29, p. 294.

[37] Thomas Chena, Marc Visseaux. End-capped Oligomers of Ethylene, Olefins and Dienes, by means of Coordinative Chain Transfer Polymerization using Rare Earth Catalysts. Intech. 2014.

7. Appendix

7.1. Determination of the ethylene consumption

In order to measure activities and productivities, which are very important for the kinetic study, the mass of ethylene consumed has to be determined from the values of pressure drop in the ethylene reservoir.

The volumetric mass density of the ethylene per bar is calculated using the model of Michel-Florin (1977):

2 푥푒 = 푎0 + 푎1. 푃 + 푎2. 푃 (11)

Which has been validated for the following conditions: 1 bar < P < 10 bar, 20°C < T < 80°C. The three parameters a0, a1 and a2 depend on the temperature:

2 푎0 = 1.256 − 0.004505푇 + 0.0000109푇 (12)

2 푎1 = 0.0052 − 0.00001495푇 − 0.0000001244푇 (13)

2 푎2 = 0.0003252 − 0.00000732푇 + 0.00000004195푇 (14)

From xe, the mass of ethylene (meth) can be determined.

푚푒푡ℎ = 푥푒푉푟푒푠푒푟푣표𝑖푟 푃 (15)

Once the mass of ethylene of the reservoir at each instant n is known it is possible to determine the activity and productivity:

푚 − 푚 푛 푛:1⁄푛 퐴푐푡푖푣푖푡푦 = 푐푎푡푎푙푦푠푡 (16) 푡푛 − 푡푛:1

푚푛 − 푚0 푃푟표푑푢푐푡푖푣푖푡푦 = (17) 푛푐푎푡푎푙푦푠푡

47 7.2. Influence of Mg/Nd ratio: Productivity Profiles

Catalytic System I:

80000 Mg/Nd=20

Mg/Nd=40 Mg/Nd=60 60000 Mg/Nd=80 Mg/Nd=160

40000

20000 Productivity Productivity (molEth/mol Nd)

0 0 50 100 150 200 250 300 t (min)

∗ Figure 39. Productivity profiles obtained for the catalytic system I. (T = 80ºC; 4 bar; [퐂퐩ퟐNdCl2Li(OEt2)2] = 50 µM; toluene = 400 mL).

Table 9. Data for the determination of from the productivity profiles, catalytic system I. (T = 80ºC; 4 ∗ bar; [퐂퐩ퟐNdCl2Li(OEt2)2] = 50 µM; toluene = 400 mL).

[BOMAG] [Ethylene] Time range of the 퐚퐩퐩 Run System Mg/Nd Slope 퐤퐩 (mM) (M) steady-state phase (L.mol-1.s-1) 1 1 20 6 min - 20 min 9.96 45.6 2 2 40 20 min - 60 min 5.46 25.0 3 I 3 60 0.219 15 min - 80 min 4.51 20.6 4 4 80 20 min - 130 min 3.51 16.0 5 8 160 40 min - 220 min 2.21 10.1

Catalytic System II:

100000 Mg/Nd=20

Mg/Nd=40 80000 Mg/Nd=60 Mg/Nd=80 60000

40000

20000 Productivity Productivity (molEth/mol Nd)

0 0 50 100 150 200 250 300 350 t (min)

Figure 40. Productivity profiles obtained for the catalytic system II. (T = 80ºC; 4 bar; [{Me2SiFlu2Nd(µ- BH4)[(µ-BH4)Li(THF)]}2] = 50 µM; toluene = 400 mL).

Table 10. Data for the determination of from the productivity profiles, catalytic system II. (T = 80ºC; 4 bar; [{Me2SiFlu2Nd(µ-BH4)[(µ-BH4)Li(THF)]}2] = 50 µM; toluene = 400 mL).

[BOMAG] [Ethylene] Time range of the -1 -1 Run System Mg/Nd Slope (L.mol .s ) (mM) (M) steady-state phase

6 1 20 5 min - 20 min 7.71 35.3 7 2 40 20 min - 90 min 3.41 15.6 II 0.219 8 3 60 40 min - 160 min 2.23 10.2 9 4 80 50 min - 300 min 1.59 7.3

49 7.3. Determination of the activation energy: Activity and Productivity Profiles

Catalytic System I:

1000

85°C

)

1 80°C

- 800

.h 1

- 75°C 600 70°C

400

Activity Activity (kg.mol 200

0 0 50 100 150 200 250 t (min)

Figure 41. Activity profiles from the polymerizations experiments at different temperatures, catalytic system I. ([Nd] = 50 µM; Mg/Nd = 80; toluene = 400 mL).

80 000

60 000 85ºC 80ºC 40 000 75ºC 70ºC 20 000

Productivity Productivity (molEth/mol Nd) 0 0 50 100 150 200 250 t (min)

Figure 42. Productivity profiles from the polymerizations experiments at different temperatures, catalytic system I. ([Nd] = 50 µM; Mg/Nd = 80; toluene = 400 mL).

Table 11. Data for the determination of from the productivity profiles at different temperatures, catalytic system I. ([Nd] = 50 µM; Mg/Nd = 80; toluene = 400 mL).

[Ethylene] Time range of the 퐚퐩퐩 Run System T (ºC) Slope 퐤퐩 (M) steady-state phase (L.mol-1.s-1) 10 85 0.218 25 min - 220 min 5.58 25.7 11 80 0.219 20 min - 130 min 3.51 16.0 I 12 75 0.231 30 min - 60 min 2.87 12.4 13 70 0.245 35 min - 58 min 2.09 8.6

50 Catalytic System II

600 90°C

500 85°C

)

1 80°C -

.h 400 1

- 74°C 70°C 300 65°C 200

Activity Activity (kg.mol 100

0 0 50 100 150 200 250 300 350 t (min)

Figure 43. Activity profiles from the polymerizations experiments at different temperatures, catalytic system II. ([Nd] = 50 µM; Mg/Nd = 80; toluene = 400 mL).

70000,0

60000,0

50000,0 90ºC 40000,0 85ºC 80ºC 30000,0 74ºC 20000,0 70ºC 65ºC

10000,0 Productivity Productivity (molEth/mol Nd) 0,0 0 50 100 150 200 250 300 350 t (min)

Figure 44. Productivity profiles from the polymerizations experiments at different temperatures, catalytic system II. ([Nd] = 50 µM; Mg/Nd = 80; toluene = 400 mL).

Table 12. Data for the determination of from the productivity profiles at different temperatures, catalytic system II. ([Nd] = 50 µM; Mg/Nd = 80; toluene = 400 mL).

[Ethylene] Time range of the 퐚퐩퐩 Run System T (ºC) Slope 퐤퐩 (M) steady-state phase (L.mol-1.s-1) 14 90 0.221 40 min - Unknown 2.91 13.2 15 85 0.218 45 min - Unknown 2.76 12.7 16 80 0.219 50 min - 300 min 1.59 7.3 II 17 74 0.222 45 min - 105 min 1.91 8.6 18 70 0.220 10 min - 30 min 2.18 9.9 19 65 0.221 10 min - 40 min 2.14 9.7

51 7.4. Density Functional Theory (DFT) Investigation

Alongside with the experimental kinetic study, a computational mechanistic investigation, at the DFT level, was performed by Perrin et al. in order to determine the exact mechanism of this polymerization system.

Without entering in too much detail, the mechanism of ethylene insertion was studied4 considering the co-existence of heterotrimetallic (NdMg2) and heterobimetallic (NdMg) species (Figure 45) in ∗ equilibrium with a monometallic Cp2NdR complex. The main objective is to determine which metal- alkyl bonds leads to the lowest energy barrier for ethylene insertion.

By DFT study, it was found that the lowest insertion profile involves the dissociation of NdMg2 ∗ followed by insertion of ethylene in the Nd-alkyl bond of the Cp2NdR complex. The lowest overall energy barrier for ethylene insertion determined by this method is 20.6 kcal.mol-1. Considering the system complexity and accounting for experimental error, the value of 17.3 kcal.mol-1 determined by the kinetic study compares well with the value determined by the DFT study.

Figure 45. Hetero bi- and tri-metallic species considered.

4 Note that other mechanisms were studied as well, such as β–H elimination and chain transfer. For propagation activation energy purposes, only the ethylene insertion mechanism is considered.

52 7.5. Molar Mass Targeting: Productivity Profiles

12000

10000

Run 20 8000 Run 21 6000 Run 22

4000 molEth/mol Nd

2000

0 0 10 20 30 40 50 60 70 t (min)

Figure 46. Productivity profiles of the molar mass targeting study for the catalyst system II. (T = 80ºC; [Nd] = 50 µM; Mg/Nd = 80; toluene = 400 mL).

Table 13. Data for the determination of from the productivity profiles of the molar mass targeting study, catalyst system II. (T = 80ºC; [Nd] = 50 µM; Mg/Nd = 80; toluene = 400 mL).

[Ethylene] Time range of the steady- -1 -1 Run System P (bar) Slope (L.mol .s ) (M) state phase 20 4 0.219 45 min - 60 min 2.3 10.5 21 II 4 0.219 12 min - 26 min 3.27 15.0 22 2 0.109 25 min - 60 min 1.31 12.0