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Copolymerization of Ethene and Functionalized Comonomers with Cationic α-Diimine Palladium Catalysts

(2008) Li, Weidong

Polyolefins form a highly important class of materials, with a wide range of applications, which are produced industrially on a huge scale (> 85 Mton per year). By far the largest volume of these polyolefins is polyethene (HDPE) and its copolymers with 1-alkenes (LLDPE), and isotactic polypropene. These polymers are all prepared by the catalytic (co-)polymerization of olefins using transition-metal catalysts. As polyethenes and polypropenes consist essentially of very long alkane molecules, these materials are very apolar and lipophilic. It can be desirable to modify this apolar nature (with retention of the other favorable properties), e.g. to facilitate dyeing of the material, or to improve its adhesive properties to more polar materials. The most straightforward method to accomplish this would be the incorporation into the polymer of monomers with polar functional groups. Nevertheless, this is made difficult by the highly reactive nature of the catalysts used to polymerize ethene and propene: their reactions with the polar functionalities often lead to significant catalyst inhibition or even deactivation. In recent years, considerable effort has been directed to finding strategies for incorporation functional comonomers into polyolefins by catalytic copolymerization. For further development in this area, an increased understanding of the strength and nature of the interactions between these functional comonomers and various catalyst types is desirable. The work described in this thesis seeks to contribute to this understanding.

A highly promising catalyst family, that shows considerable tolerance to polar functionalities, is formed by cationic alkyl complexes of the group 10 metals Ni and Pd with bidentate Lewis based ancillary ligands. In this thesis, the -diimine palladium catalyst system [(N^N)PdMe(OEt2)][BAF] (2, N^N = ArN=CMe-CMe=NAr with Ar = 2,6-diisopropylphenyl; BAF = B[3,5-(CF3)2C6H3]4) is used to study the reactivity and (co-)polymerization characteristics of a series of olefins bearing heteroatom-containing functional groups.

Chapter 1 gives an overview of the issues regarding the incorporation of functionalities into polyolefin materials, and the various approaches taken to this end.

In Chapter 2, the synthesis of the cationic α-diimine palladium catalyst 2 is revisited. A new, convenient synthesis of the catalyst precursor (N^N)PdMe2 (1) is presented. It turns out that the details of the reaction conditions are important in the generation of the ionic complex 2. A side product in the reaction was identified as the -methyl,-methylene complex [(N^N)Pd(-CH3)(-CH2)Pd(N^N)][BAF] (3). The compound is formed by reaction of 2 with the starting material 1. The molecular and electronic structure of 3 and some of its reactivity features were explored.

Chapter 3 describes the reaction of the cationic Pd-catalyst 2 with olefins bearing oxygen-containing functionalities: acrolein dimethyl acetal (ADMA), allyl ethyl ether (AEE) and 2-vinyl-1,3-dioxolane (VDO). AEE and ADMA smoothly give 1,2-insertion into the Pd-Me bond of 2 to give the 5-membered chelate complexes [(N^N)Pd(CH2CHMeCH2OEt)][BAF] (9) and {(N^N)Pd[CH2CHMeCH(OMe)2]}[BAF] (11) respectively. Both are able to catalyze the homopolymerization of ethene. Attempted ethene/AEE copolymerization resulted in rapid catalyst deactivation, forming the allyl complex [(N^N)Pd(3-1-CH2CHCH2)][BAF] (10) and ethanol. Nevertheless, the ethene/ADMA copolymerization successfully yielded a branched polyethene copolymer bearing acetal functionalities. Here too, gradual deactivation takes place through formation of an allylic complex, [(N^N)Pd(3-1-CH2CHCHOMe)][BAF] (12), and methanol. This deactivation could be retarded by the addition of aliquots of methanol to the reaction mixture. In contrast, VDO very readily ring-opens in the presence of 2, and could not be copolymerized.

Chapter 4 describes the reaction of the cationic Pd-catalyst 2 with olefins bearing nitrogen-containing functionalities: allyl dimethyl amine (ADA), N-allyl carbazole (NAC) and 5-pentenyl carbazole (NPC). ADA reacts with 2 via smooth 1,2-insertion to give the 5-membered chelate complex [(N^N)Pd(CH2CHMeCH2NMe2)][BAF] (14). In contrast to the ether chelates from chapter 3, this compound does not catalyze the homopolymerization of ethene, due to the stronger Lewis basicity of the amine. NAC and NPC react with 2 to form unusual 3-membered chelate complexes after insertion into the Pd-Me bond and “chain-walking”. The product from NPC, {(N^N)Pd[Me2CHC2H4CHN(C6H4)2]}[BAF] (15), was structurally characterized. These 3-membered chelates readily react with ethene, and NPC was successfully copolymerized with ethene to give branched polyethene copolymers bearing carbazole functionalities. The fluorescence of the carbazole group in these copolymers is highly dependent on the comonomer content. In contrast to NPC, NAC is not incorporated upon attempted copolymerization, probably due to the steric hindrance imparted by the carbazole group close to the olefinic moiety.

Chapter 5 describes the reaction of the cationic Pd-catalyst 2 with olefins bearing sulfur-containing functionalities: allyl methyl thioether (AMT), allyl tert-butyl thioether (ABT), 2-allyl-1,3-dithiane (ADT) and 2-pentenyl-2-methyl-1,3-dithiane (PMDT). AMT reacts with 2 to give the stable thioether adduct [(N^N)PdMe(1-MeSCH2CH=CH2)][BAF] (17): the soft Lewis basic thioether effectively competes with the olefinic moiety for coordination with the metal, blocking insertion into the Pd-Me bond. The substrates ABT and PMDT, where the S-atoms are more screened by steric hindrance, do insert into the Pd-Me bond to give 5-membered chelates, which were structurally characterized. Nevertheless, due to the strong intramolecular Pd-S interactions in these complexes, reactivity with ethene (and thus the possibility for copolymerization) is impeded.