Hannes Leicht

Incorporation of Vinyl Chloride in Insertion Polymerization

Catalytic polymerizations of ethylene and propylene are practiced industrially on a large scale. Vinyl chloride (VC) is the monomer produced on the largest scale after ethylene and propylene. However, the incorporation of VC into (co)polymers by insertion polymerization has remained elusive, and VC even inhibits polyethylene formation. The origin of this reactivity is believed to be very specific for VC. Unlike for other vinyl monomers, such as acrylates, acrylonitrile, and others, inhibition of polymerization by coordination of the functional groups of the free monomer and of repeat units formed from its incorporation into the polymer chain is not considered to be problematic for VC. Vinyl chloride is a weak k-Cl donor (comparable, for example, to methylene chloride). Extensive studies with earlyand late-transitionmetal catalysts have revealed that a different more fundamental problem prohibits the catalytic polymerization of vinyl chloride. Thermodynamically favorable b-chloride elimination occurs subsequently to the incorporation of VC into the growing chain to afford inactive metal chloride complexes, and thus irreversibly deactivates the catalyst. The ß-chloro substituted alkyl species that undergo this detrimental reaction are formed by a net 1,2-insertion of VC into the growing chain. For an insertion of VC into latetransition-metal carbon bonds, DFT calculations and experimental evidence point to an initial insertion in a 2,1fashion. However, the propensity of late-transition-metal polymerization catalysts for “chain-walking” results in a net 1,2-incorporation of VC (Scheme 1). This unfavorable outcome has been observed for a-diimine palladium complexes, bis(imino)pyridine iron and cobalt complexes, and for salicylaldiminato, and phosphine–enolato nickel complexes. Owing to the low propensity of neutral phosphine– sulfonato palladium complexes for chain walking, we decided to study their polymerization properties towards vinyl chloride. Most recently, related cationic phosphine– phosphine oxide palladium catalysts have been reported to exhibit reduced activity for ethylene polymerization in the presence of VC. However, an ethylene homopolymer that has no detectable chlorine content is formed by these catalysts. We now report, that among complexes L1Pd to L4Pd studied (Figure 1), complexes [L2PdCH3(dmso)], [{L2PdCH3}2], [L2PdH(PtBu3)], and [L4PdCH3(dmso)] catalyze the formation of chlorinated copolymers from ethylene and VC owing to a partial suppression of chain walking after monomer insertion.

Synergetic Effect of Monomer Functional Group Coordination in Catalytic Insertion Polymerization

PhS- and PhNH-functionalized dienes are copolymerized efficiently with butadiene to stereoregular copolymers by [(mesitylene)Ni(allyl)][BArF4] (Ni-1). Overall polymerization rates and comonomer incorporations depend strongly on the linker length between the diene moiety and functional group, in, e.g., PhS-(CH2)xC(═CH2)-CH═CH2 (PhS-x-BD, x = 3-7), in particular for certain linker lengths high comonomer reactivity ratios stand out. This effect is related to a favorable binding of the comonomer to the active site comprising coordination of its functional group, which significantly enhances comonomer incorporation in the growing polymer chain.

Allylboration as a versatile tool for the in situ post-polymerization functionalization of 1,4-cis-poly(butadiene)

Allylboration is a versatile tool for the post-polymerization functionalization of poly(butadiene-co-[4,4,5,5-tetramethyl-2-(3-methyl-1,3-butadienyl)-1,3,2-dioxaborolane]). Polar functionalized aldehydes HC(O)C6H4(CH2)R (R = Br, NR2, PPh3, P(O)(OEt)2) react readily with the allyl boronic acid ester groups in the copolymer without interfering with the reactive double bonds in the polymer backbone. This provides access to stereoregular poly(butadiene) functionalized with a broad range of polar groups. Functionalization proceeds under polymerization conditions and therefore does not require a prior polymer work-up.