Air-Stable Single-Component Pd-Catalysts for Vinyl-Addition Polymerization of Functionalized Norbornenes

Abstract

Well-defined single-component and tolerant Ru-, Mo-, and W-catalysts have been earlier developed for ring-opening metathesis polymerization of norbornene and its derivatives. Another way of norbornene polymerization�vinyl-addition polymerization (VAP)�is a more thermodynamically favorable process and gives polymers with saturated and rigid main chains. Single-component catalysts for VAP are required, which combine good activity, tolerance to functional groups, resistance to oxygen, and air moisture and are capable of catalyzing living VAP of norbornenes. Herein, we have shown that cationic Pd-complexes with N-heterocyclic carbene ligands (NHC ligands) and weak coordinating anion ligands ([(NHC)Pd(allyl)L] + A − , where allyl is an allyl group, L = CO, CH 3 CN, CH 2 Cl 2 , and A − = SbF 6 − or BARF − ) are highly efficient single-component catalysts for VAP of both norbornene and various substituted norbornenes. These complexes, being stable in the solid state and in solutions, have exhibited several important features as VAP catalysts. They are capable of catalyzing VAP of norbornenes in air and are tolerant to functional groups. The complexes have shown high catalytic activity and have allowed polymerizing monomers at very low catalyst loadings, as low as 2 ppm. The versatility of [(NHC)Pd(allyl)L] + A − complexes as catalysts of VAP has been demonstrated by successful polymerization of monomers with polar and bulky groups, containing alkyl, vinyl, alkylidene, ether, ester, imide, or organosilicon motifs, affording high-molecular-weight products in good and high yields. Tuning the nature of NHC and allyl ligands in [(NHC)Pd(allyl)L] + A − complexes let perform living VAP, yielding block copolymers.

Single‐Component Catalysts for the Vinyl‐Addition Polymerization of Norbornene and its Derivatives

“…Recently, we reported the catalytic behavior of cationic (NHC)Pd complexes with allylic ligands ( Pd69c−Pd79 , Figure 18) as single‐component catalysts for the VAP of various norbornenes [7b,81] . These complexes are stable in air and wet solvents and catalyze polymerization in air and water.…”

“…The incorporation of suitable ligands into the metal coordination sphere may improve the initiation efficiency and change the relative rates of the initiation and propagation steps, while chain termination and transfer reactions remain rare. For instance, the living/controlled VAP of norbornenes was realized for single‐ and multicomponent catalytic systems by adding a bulky phosphine ligand ( e. g ., t Bu 3 P, [16d,96] P i Pr 3 [97] ) or N ‐heterocyclic carbene ligand [16j,81] to Pd−Me, Pd‐allyl, or other moieties. However, the incorporation of such ligands is insufficient and requires metal−C bonds and labile ligands that provide free coordination sites at the metal center.…”

“…However, the incorporation of such ligands is insufficient and requires metal−C bonds and labile ligands that provide free coordination sites at the metal center. For example, the initiation efficiencies were high for (NHC)Pd complexes with labile MeCN and CH 2 Cl 2 ligands but were significantly lower for related complexes with more tightly bound CO ligands [81] . Haselwander and Heitz showed that replacing MeCN ligands in [Pd(MeCN) 4 ] 2+ (BF 4 − ) 2 with phosphino ligands suppressed norbornene polymerization and replacing them with ethylene accelerated initiation and resulted in narrower molecular weight distributions and controlled polymerization [59d] .…”

“…The nature of metal−C bonds is also essential. For instance, the nature of the allyl group on Pd dramatically affected initiation efficiency and polymer molecular weights [14a,78,81] . Therefore, although no clear guidelines exist for creating a single‐component catalyst with fast initiation and low propagation, systematic tuning of ligands in the metal coordination sphere results in optimal catalyst structural design.…”

Making accessible soluble silicon-containing polynorbornenes: hydrosilylation of vinyl-addition poly(5-vinyl-2-norbornene)

“…This result is unexpected and directly opposite to the previously observed relationship between the structure of related (NHC)Pd-complexes and their catalytic activity in VAP polymerization, where Pd-complexes with five-membered NHC-ligands turned out to be the most active ones. 35,37 Another feature of the effect of the NHC-ligand on the catalytic activity of Pt-complexes is that complexes with bulkier aryl groups in NHC-ligands exhibited higher catalytic activity (6DippPt(dvtms) vs. 6MesPt(dvtms) or 7DippPt(dvtms) vs. 7MesPt(dvtms), Fig. 4).…”