Acidic Monomers in Insertion Copolymer–

ization

4.1. Introduction

Polyethylene with carboxylic, sulfonic or phosphonic functionalities are an important polymer class with wide-range applications. They do not only exhibit polar substituents, but can form inter- and intrachain interactions via hydrogen-bridging.⁸²

Well-known examples are methacrylic and acrylic acid copolymers with ethylene which provide enhanced adhesion, particularly in coextruded films or laminates.¹ The corresponding ionomers are produced by partial neutralization of acidic copolymers. The ionic salts and the unneutralized acid groups form strong interchain interactions producing thermally labile crosslinking which enhance tensile strength and melt viscosity.⁸² These polymers are used in a variety of applications like in orthotics and prosthetics or as a constituent in golf balls and bowling pins. They are generally produced in high-pressure free-radical processes which suffer from a lack of microstructure control.

Another class of polymers, in this regard, is sulfonated polyethylenes which are of interest due to enhanced adhesion and wettability and their application as membranes.⁸³ Synthesis of such polymers is performed polymer-analogous with gaseous SO₃ or fuming sulfuric acid.

Several side reactions occur during sulfonation including elimination of the sulfonates to form unsaturated sequences or formation of sulfates, ketones or sultones resulting in a rather ill-defined polymer structure.84,85,86,87,88,89

Phosphonated polyethylenes play a similarly important role. These polymers have been studied as membranes for ion transport, exchange or barriers, as well as biomaterials for dental cements, bone integration or cell adhesion.90,91,92,93,94,95,96,97,98

They are produced in free-radical post polymerization reactions with PCl3 and oxygen to phosphonyl

chloride-Acidic Monomers in Insertion Copolymerization with neutral Pd(II) phosphinesulfonato catalysts

modified polyethylene which is highly reactive towards hydrolysis to phosphonic acid or ester.^99,100

Since the syntheses of these types of polymers are based either on free-radical polymerization processes which suffer from a lack of microstructure control or free-radical polymer-analogous modifications which often result in a series of undesired side reactions and disadvantageously impact the initial polymer structure, other synthetic methods would be desirable.

Indeed, a catalytic pathway to ethylene-acrylic acid¹⁰¹ and ethylene-methacrylic acid¹⁰² copolymers was reported by either ADMET polymerization of a carboxylic acid functionalized α,ω-diene with post-polymerization hydrogenation to precisely-spaced copolymers or by ROMP of cyclooctene and functionalized cyclooctene, again followed by a hydrogenation step, to a randomly functionalized copolymer as reported for ethylene-acrylic acid copolymers (Scheme 4.1). Both polymerization methods, however, suffer from multistep syntheses of the monomers in which the functional group also has to be protected due to the sensitivity of the metathesis catalyst and from the polymer-analogous deprotection of the carboxylic acids and hydrogenation of the double bonds. The same polymerization pathway was successfully applied for sulfonated¹⁰³ and phosphonated¹⁰⁴ polyethylenes with similar restrictions.

Scheme 4.1 Synthesis of precise and random functionalized polyethylene via ADMET polymerization

Acidic Monomers in Insertion Copolymerization with neutral Pd(II) phosphinesulfonato catalysts

A more promising pathway would be the direct copolymerization of ethylene and the corresponding polar-substituted monomer since vinyl carboxylic-, sulfonic- and phosphonic monomers are bulk materials and are readily available in large amounts. Insertion copolymerization of ethylene and polar monomers re-experienced substantial progress since the seminal work of Drent and coworkers in that neutral κ²-(P,O)-phosphinesulfonato palladium catalysts were found to copolymerize ethylene with a variety of polar comonomers including acrylonitrile, vinyl acetate, vinyl chloride and vinyl amides.23,46,47,48,49,50,51,52,55,56,57,58

With the exception of vinyl halides and vinyl ethers which tend to perform β-X elimination (X: halide, ether) after insertion as found for the palladium diimine systems,^21,22 the challenge of these monomers is predominately the coordination of the functional group to the palladium center and thus blocking the coordination site for π-coordination of olefin prior to insertion (Introduction, Scheme 1.8). The presence of acidic protons of monomers like acrylic acid, phosphonic acid or sulfonic acid may open up new reaction pathways. These monomers cannot only coordinate to the metal center but may also harm the catalyst by conceivable protonation reactions.

This chapter provides in-depth analysis of the reactivity of these κ²-(P,O)-phosphinesulfonato palladium catalysts with acrylic, phosphonic and sulfonic acids and their respective esters by NMR insertion studies and copolymerization studies with ethylene and will demonstrate their beneficial properties for surfactant-free secondary dispersions and organic-inorganic hybrid nanoparticles.

Acidic Monomers in Insertion Copolymerization with neutral Pd(II) phosphinesulfonato catalysts

4.2. Acrylic Acid

4.2.1. Ethylene polymerization in the presence of carboxylic acid and insertion studies of acrylic acid

The presence of polar comonomers in copolymerizations with ethylene enables several new reaction channels such as reversible inhibition by κ-X coordination of an inserted polar repeat unit. Acidic monomers could be particularly prone to such reversible deactivation, as the corresponding anions (e.g. carboxylate) can coordinate relatively strongly. Also, irreversible protonation reaction of the bidentate ligand or the growing polymeryl chain could possible.

Notwithstanding this scheme, the effect of propionic acid on ethylene polymerization by the phosphinesulfonato Pd(II) complex 1-CH3-dmso was studied. Remarkably, polymerization by 1-CH₃-dmso occurred even in a 1 M propionic acid solution (Table 4.1). While polyethylene yields decrease with increasing propionic acid concentration (entries 1 to 4), a substantial activity of several 10³ turnovers per hour is observed even at the aforementioned acid concentration. A comparison of polymer yields at different reaction times (entries 3, 5 and 6) reveal that the catalyst retains its activity over the 30 min periods studied. The slight decrease in activity found is also observed in comparative polymerizations in the absence of propionic acid (entries 1, 7 and 8). Overall, this data indicates a reversible retardation of polymerization, likely by coordination of the carboxylic moieties, but no detrimental catalyst decomposition by carboxylic acid.

Acidic Monomers in Insertion Copolymerization with neutral Pd(II) phosphinesulfonato catalysts

Table 4.1 Polymerization of ethylene in presence of propionic acid^a

entry

For stoichiometric studies of the reactivity of acrylic acid towards the catalyst, 1-CH3-dmso and the chloride-complex 1-CH₃-Cl were employed. 1-CH₃-dmso allows for conclusions on coordination to the metal center as the ¹H NMR resonance of the coordinated dmso is significantly low-field shifted in comparison to free dmso. The chloride-complex 1-CH₃-Cl, in contrast, is used in conjunction with one equivalent of AgBF4 as a halide abstraction agent.

By comparison to 1-CH3-dmso, this route is advantageous for quantitative studies as it delivers the [(P^O)PdMe] (1) fragment without a relevant preequilibrium of dmso dissociation.

Scheme 4.2 Insertion studies of 1-CH₃-dmso and acrylic acid

Exposure of 1-CH3-dmso to an excess of acrylic acid (20 equivalents) in CD2Cl2 at room temperature results in formation of the 2,1-insertion product. Utilization of an excess of

Acidic Monomers in Insertion Copolymerization with neutral Pd(II) phosphinesulfonato catalysts

acrylic acid allows the extraction of a pseudo first-order rate constant of k_obs(30 mM Pd(II), 25 °C) = 5.5 × 10^-4 s^-1 which has to be considered as an overall rate constant for a prequilibrium of dmso vs comonomer binding followed by insertion (Scheme 4.2, Figure 4.2).

Characteristic ¹H NMR resonances for the 2,1-insertion product 1-AA-CH3-dmso are a triplet