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Title: Development of palladium catalysts with nitrogen-donor ligands for controlled copolymerization reactions
Authors: Meduri, Angelo
Supervisor/Tutor: Milani, Barbara
Issue Date: 23-Apr-2013
Publisher: Università degli studi di Trieste
Abstract: One of the great challenges of polymer chemistry is the introduction of polar moieties into polyolefin chains in order to obtain functionalised polyolefins that should show improved physical and chemical properties with respect to the polyolefins themselves. The most straightforward approach to reach this goal is represented by the direct, controlled homogeneously catalysed copolymerization of terminal alkenes with polar monomers. Indeed, in homogeneous catalysis thanks to the tuning of the electronic and steric properties of the ancillary ligands it is possible to tailor the chemical environment around the metal centre, and, in turn, it should be possible to exert a highly efficient control on the selectivity of the reaction, that in the field of polymerization reactions means to control the features of the final polymeric material. This PhD thesis is aimed to demonstrate the validity of this general principle with particular attention towards two copolymerisation reactions: the CO/vinyl arene and the ethylene/methyl acrylate copolymerisation. Both reactions share the coordination compounds applied as precatalysts, that are based on palladium(II) complexes with nitrogen-donor chelating ligands. The research work carried out in the framework of this thesis encompasses the typical steps of a project in homogeneous catalysis, that are: i. Synthesis and characterisation of the molecules used as ancillary ligands; ii. Synthesis and characterisation of the corresponding palladium complexes; iii. Study of the catalytic behaviour of the synthesised complexes in the two copolymerisation reactions, including the characterization of the catalytic products; iv. Mechanistic investigations performed through NMR studies of the reactivity of the precatalysts with the comonomers. It should be mentioned that the group of Dr. Milani has a longstanding experience in the CO/vinyl arene copolymerization, whereas this project represents the first work on ethylene/polar vinyl monomer copolymerisation and thus it has been necessary to set up and define the protocols of the catalytic experiments. Chapter 1 consists of a general introduction on both the fundamentals of catalysed polymerisation and the specific features of the two copolymerisation reactions, that are the topic of this thesis. An overview of the most relevant catalytic systems reported in the literature is given together with a critical discussion of the most prominent results achieved and highlighting the points where improvements are required. Chapter 2 is focused on the development of catalysts for the CO/vinyl arene copolymerisation. Terdentate nitrogen-donor ligands, belonging to the family of 2-(2′-oxazolinyl)-1,10-phenanthrolines, have been studied. When reacted with the palladium precursor they lead to dinuclear complexes, as demonstrated by their characterization both in solid state and in solution. In particular, PFG {1H – 15N} HMBC NMR experiments performed at the natural abundance of 15N have been of fundamental importance to substantiate the dimeric nature of these complexes in solution. When applied to the target copolymerisation they yield the corresponding CO/styrene oligoketones as major product with productivity values higher than those reported for the literature catalytic systems. In addition, traces of the corresponding polyketones are also obtained. The study of the stereochemistry of these copolymers evidences that fully syndiotactic polyketones have been produced for the first time. The characterization of the terminal groups of the oligoketones allowed to recognise that the control of the stereochemistry of the styrene insertion reaction is highly efficient since the insertion of the first two molecules of the vinyl arene comonomer. Chapter 3 represents a junction point between the two studied copolymerisations; indeed, it deals with the applicaton of the same precatalysts to both of them to point out their analogies and differences. For this purpose, Ar2-BIAN and Ar2-DAB ligands (Ar = 1-naphthyl or 2-naphthyl ring) have been synthesised and used to obtain the relevant palladium(II) complexes. A clear different coordinating behaviour is observed for the ligands depending on the naphthyl fragments, 1-naphthyl or 2-naphthyl, and regardless to the ligand skeleton, BIAN and DAB. In particular, complexes with the 1-naphthyl substituted ligands lead to syn and anti isomers in solution that interconvert each other at low rate on the NMR time scale. Both isomers are also found in the unit cell of [Pd(CH3)Cl(1-naphtBIAN)] in solid state, thus confirming what found in solution. When applied to the CO/styrene copolymerisation all the complexes generate active catalysts for the reaction, reaching a value of productivity of almost 5.0 (kg CP) (g Pd)-1 in the case of the 1-naphtBIAN-containing catalyst. The stereochemistry of the produced polyketones is also affected by the naphthyl substituents: atactic copolymers are obtained with catalysts having the 2-naphthyl-ligands, while copolymers with isotactic/atactic stereoblocks are the products of the catalysis with the 1-naphthyl-derivatives. The latter stereochemistry has been related to the syn/anti isomers present in solution and to their interconversion. When the complexes are applied to the ethylene/methyl acrylate copolymerisation, again active species are obtained reaching a productivity of 201 (g P) (g Pd)-1 in the case of the 1-naphtDAB-containing catalyst. The analysis of the overall catalytic results points out analogies and differences of the two copolymersations: • For the CO/styrene copolymerisation, ligands with the BIAN skeleton lead to catalysts remarkably more stable and more productive than those with the DAB skeleton; • For the ethylene/methyl acrylate copolymerisation, catalysts with DAB ligands are more stable and more productive than those with BIANs; • For both copolymerisations, catalysts with 2-naphythyl substituents, regardless to the skeleton of the ligand, show a catalytic behaviour similar to that of catalysts with meta-substituted α-diimines: they show similar productivities in the CO/styrene copolymerisation and are inactive in ethylene/methyl acrylate copolymerisation; • For both copolymerisations, catalysts with 1-naphythyl substituents, regardless to the skeleton of the ligand, show a catalytic behaviour similar to that of catalysts with ortho-substituted α-diimines: in the CO/styrene copolymerisation show low productivity, while are catalytically active in the ethylene/methyl acrylate copolymerisation. Chapters 4 and 5 are focused on the development of catalysts for the ethylene/methyl acrylate copolymerisation, that nowadays represents a highly challenging reaction in the fields of both polymer chemistry and homogeneous catalysis. In Chapter 4 the research has been addressed to the synthesis of a new nonsymmetric Ar,Ar′-BIAN bearing electron-donating ortho-substituents on one aryl ring and electron-withdrawing meta-substituents on the second aryl ring, with the aim of unbalancing the two nitrogen atoms from both the electronic and steric point of view. Its coordination chemistry to palladium is studied and for the first time the series of the monocationic complexes [Pd(CH3)(L)(Ar,Ar’-BIAN)][PF6] has been extended to the dimethyl sulfoxide derivatives. Despite the soft nature of palladium, the coordination of dmso through the sulphur or oxygen atoms is related to the Ar-BIAN bonded to the metal, and in the case of the nonsymmetric ligand Ar,Ar’-BIAN three isomers are present in solution, as demonstrated by detailed NMR investigation. The catalytic behaviour of the monocationic complex with Ar,Ar′-BIAN in the ethylene/methyl acrylate copolymerisation has been compared to that of the complexes with the related symmetrically substituted Ar2BIAN probing evidence that the catalyst with Ar,Ar′-BIAN is more productive and leads to ethylene/MA cooligomers with a higher content of polar monomer inserted than the catalysts with the symmetric ligands. In addition, it has been shown that the catalyst originated from the dmso derivative has a longer lifetime than that obtained from the acetonitrile counterpart. Kinetic investigations about the reactivity of the precatalysts with the polar monomer have been performed by NMR spectroscopy to gain information on both the relevant intermediates of the catalytic cycle and the differences in the rate constants. The promising results obtained with catalysts having the nonsymmetric Ar,Ar′-BIAN prompted us to study the symmetrically substituted Ar2BIANs featuring one group on the ortho position and one on the meta position of both aryl rings. This is the topic of Chapter 5. Ar2BIAN ligands already known from the literature as well as new molecules have been investigated. The study of their coordination chemistry to palladium points out that syn and anti isomers, depending on the relative position of the substituents on the aryl rings with respect to the square planar plane, are present in solution. The existence of these isomers has been also observed in solid state by X-ray analysis of single crystals of one exponent of this series of complexes. The monocationic palladium derivatives of all the ligands generated active catalysts for the ethylene/MA copolymerisation, but showing modest productivities. The low rate in the catalytic reaction makes them suitable candidates for detailed mechanistic investigations performed by NMR spectroscopy, that allows to correlate the rate of the migratory insertion of the polar monomer into the Pd-alkyl bond to the both electronic and steric effects of the substituents on the aryl rings. In addition, a relationship between the stability of the resting state of the catalytic cycle and the nature of the Ar2BIAN has been established. The synthetic methodology applied for the synthesis of the nonsymmetric Ar,Ar′-BIANs is not so trivial and Chapter 6 deals with the enlargement of this ligand library to a new component having one aryl ring substituted in meta position by the CF3 group and the other aryl substituted in position 2 by the CH3 and in 6 by the OCH3. Other new symmetric Ar2BIANs substituted on positions 2 and 6 with different groups have been also synthesised. The study of their coordination chemistry to palladium evidences again the presence of syn and anti isomers in solution. In addition, as a follow up of Chapter 4 these new ligands have been used to synthesise the corresponding monocationic complexes with dmso as labile ligand, the study of its coordination chemistry leading to interesting findings. Finally, the mechanistic NMR investigations have been extended to the Pd-dmso derivatives to unravel the nature of their better performing catalytic behaviour with respect to the acetonitrile derivatives. The future perspective of this chapter consists in the study of the catalytic behaviour of these new complexes in ethylene/methyl acrylate copolymerisation. In conclusion, the research work carried out in this PhD project has added important pieces to the α-diimine chemistry and has remarkably contributed to the development of the relationship between ancillary ligand properties and catalyst performances. The explorative research in the field of ethylene/methyl acrylate copolymerisation has been successfully accomplished achieving better results than the state-of-the-art.
PhD cycle: XXV Ciclo
Description: 2011/2012
Keywords: homogeneous catalysis
nitrogen-donor ligands
polar monomers
Main language of document: en
Type: Tesi di dottorato
Doctoral Thesis
Scientific-educational field: CHIM/03 CHIMICA GENERALE E INORGANICA
NBN: urn:nbn:it:units-10793
Appears in Collections:Scienze chimiche

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