Mechanistic Considerations

Mechanistic aspects of the phosphinesulfonato palladium catalysts have been thoroughly investigated in both experimental and theoretical studies.⁴¹ For chain propagation, it is suggested that prior to insertion, a cis-trans isomerization takes place via a pseudo Berry-rotation, probably with participation of an additional donor,⁴² which results in the Pd-alkyl chain being now in trans position to the phosphorus (Atrans) (Scheme 1.7). Insertion from this less stable trans-complex was calculated to be energetically preferred in comparison to the insertion starting from the cis-product (Acis). This is attributed to the enhanced migratory ability of the polymer chain due to the strong trans-effect of the phosphorus in combination with a strong back-donation of the palladium to the coordinated olefin in trans-position to the sulfonato ligand which possesses weak σ-donor and weak π-acceptor properties.

General Introduction

Scheme 1.7 Mechanism of chain growth and β-hydride elimination in phosphinesulfonato Pd systems

A second important reaction pathway which occurs during polymerization is β-hydride elimination. Again, it was found that the transition state therefore is lower for the trans species (B_trans→C_cis) than for the cis species (B_cis→C_trans). Barriers for insertion of ethylene in A_trans, β-hydride elimination from Bcis to C_trans as well as all routes to B_trans from which insertion and elimination can occur are in a comparable energetic range. Under the consideration that ethylene is present in a large excess, this results in a preferential chain growth rather than elimination which prevails only at low ethylene concentration according to DFT studies.⁴¹

This simplified picture turns more complex in the presence of polar vinyl monomers (Scheme 1.8). κ-π Coordination of the polar vinyl monomer is a prerequisite for insertion. κ-X coordination becomes a competing coordination mode which renders the catalyst inactive.

General Introduction

The strength of κ-X coordination depends on (i) the electron density of the metal center and on (ii) the nature of the functional group. Electron poor metal centers tend to stronger κ-X coordination whereas electron rich metal centers prefer κ-π coordination. Nitriles coordinate more strongly than esters, thus κ-X coordination plays a more pronounced role for acrylonitrile than for methyl acrylate. The effect of different donors in polymerization reactions was thoroughly investigated in NMR and pressure reactor polymerization experiments.⁴⁷ Equilibrium constants of added donor and the coordinated dmso in [(P^O)Pd(Me)dmso] were determined and were set in relation to the inhibition of the polymerization catalyst. Functional groups with rather lower coordination ability vs dmso (Keq ~ 10^-2) such as methyl esters which represent κ-X coordination in ethylene/methyl acrylate copolymerizations only slightly affect the catalyst’s activity. E.g. turnover frequencies decrease from 12.4 × 10⁵ to 9.4 × 10⁵ (10 bar ethylene pressure, 80 °C polymerization temperature) in the presence of ethyl acetate (0.05 M in toluene). Stronger coordinating substrates, such as nitriles (Keq ~1), in contrast, inhibit the catalyst more pronouncedly. In this particular case, addition of acetonitrile (0.05 M in toluene) lowers activity to a turnover frequency of merely 0.3 × 10⁵ (Scheme 1.9).

Scheme 1.8 General mechanistic features of ethylene/polar vinyl monomer copolymerizations

General Introduction

Scheme 1.9 Inhibition of ethylene polymerization using dmso-substituted phosphinesulfonato Pd(II)Me catalyst by saturated compounds with various polar groups (10 bar ethylene pressure, 80 °C, 0.05 M of additive in toluene)

The regioselectivity of monomer insertion is affected by electronic and steric issues.

Acrylates, and more generally electron poor monomers prefer 2,1-insertion which is presumably due to interactions of the orbitals of the Pd-alkyl and the olefinic double bond.

Considering steric issues, a 2,1-insertion results in a crowded α-carbon adjacent to the metal center and affords steric interactions of the last inserted monomer with the ligand system.

This was shown for methyl methacrylate which is also electron poor, but sterically more demanding. Insertion of methacrylates was found to proceed preferentially in a 1,2-insertion fashion.³⁵

The 2,1-regioselectivity of acrylates can also be converted to an exclusive 1,2-insertion by sterically demanding ligand systems as shown for diazaphospholidene sulfonato palladium catalysts (Scheme 1.10). This is due to a destabilization of the transition state of 2,1-insertion.^43,44 Electron rich monomers such as vinyl acetate, in contrast, prefer 1,2-insertion due to inverted polarity of the olefinic double bond in comparison to electron poor monomers.

General Introduction

Scheme 1.10 Regioselective insertion of methyl acrylate with diazaphospholidene sulfonato- and phosphinesulfonato Pd-CH₃ complexes

The insertion of methyl acrylate has been studied intensively experimentally and theoretically.^32,45 Analogous to ethylene insertion, a preceding cis-trans isomerization occurs in which the Pd-alkyl is in trans position to the phosphorus (Scheme 1.11). The insertion initially affords a product which is stabilized by γ-agostic interactions and ends up in a 4-membered κ-O chelate. This chelate is rather weak, e.g. insertion of methyl acrylate in the dmso substituted (P^O)PdMe complex does not result in formation of this chelate but in a 2,1-inserted complex in which the fourth coordination site is occupied by dmso.³³ A unique feature of this catalyst is the ability for consecutive acrylate insertions. A second acrylate insertion into the 2,1-inserted complex again proceeds via a cis-trans isomerization and a γ-agostic stabilized intermediate which rearranges to the thermodynamically stable 6-membered chelate. This chelate exhibits two stereocenters and thus a meso- and a rac form exist in a 3:2 ratio according to ¹H NMR and x-ray diffraction analyses for the di(2-anisyl) substituted P^O system. This stable chelate is a key compound for any acrylate homooligomerization or ethylene/acrylate copolymerization as opening of this chelate by an incoming monomer is a prerequisite for further chain growth. Studies suggest that the opening of the chelate requires ca 100 kJ mol^-1.³² This is in the same range as the overall insertion barrier of olefin and thus agrees with the observed accessibility of copolymers with > 50 mol-% acrylate content or acrylate homooligomerization.

General Introduction

Scheme 1.11 Mechanism of consecutive acrylate insertions