A Dinuclear Mechanism Implicated in Controlled Carbene Polymerization

Abstract

Carbene polymerization provides polyolefins that cannot be readily prepared from olefin monomers; however, controlled and living carbene polymerization has been a long-standing challenge. Here we report a new class of initiators, (π-allyl)palladium carboxylate dimers, which polymerize ethyl diazoacetate, a carbene precursor in a controlled and quasi-living manner, with nearly quantitative yields, degrees of polymerization >100, molecular weight dispersities 1.2-1.4, and well-defined, diversifiable chain ends. This method also provides block copolycarbenes that undergo microphase segregation. Experimental and theoretical mechanistic analysis supports a new dinuclear mechanism for this process.

Cp(π-Allyl)Pd-Initiated Polymerization of Diazoacetates: Reaction Development, Kinetic Study, and Chain Transfer with Alcohols

“…Although Cp and the backbiting group at α- and ω-chain end polymer ( A ) have been identified as very minor signal in the MALDI-TOF-MS spectrum, the Cp transfer is not likely to be the major initiation process when a chain transfer agent is absent. This conjecture is based on the previous report that allyl-transfer initiation has not been observed in the polymerization of diazoacetates in (allyl)­PdCl-, (allyl)­PdCl/NaBPh 4 -, or (allyl)­Pd­(OAc)-initiated systems, , while Cp is more steady than allyl as a ligand on palladium. Unfortunately, in the absence of chain transfer agents, we failed to prepare polymers with low M n suitable for MALDI-TOF-MS analysis, apparently due to the slow initiation/fast chain propagation character of this polymerization system.…”

“…The rate constant was estimated to be 0.04 min –1 (Figure c). The slow initiation and fast propagation observed in the current Cp­(π-allyl)­Pd-initiated system suggest that the polymerization proceeded in an uncontrolled manner, in sharp contrast to the previous reports by Ihara and Toste, in which the M n of the polymer products could be controlled by the [M] 0 /[Pd­(II)] ratio. , …”

“…Unfortunately, in the absence of chain transfer agents, we failed to prepare polymers with low M n suitable for MALDI-TOF-MS analysis, apparently due to the slow initiation/fast chain propagation character of this polymerization system. It should be noted that the structure of the propagating Pd may be different from those reported previously for (allyl)­PdX, where low M n polymers could be obtained with low [M]/[Pd­(II)] ratios. , …”

“…Remarkably, the Rh-mediated polymerization of ethyl and benzyl diazoacetates reported by de Bruin and co-workers afforded polymers with high molecular weights and high stereoregularity. − On the other hand, extensive work by Ihara and co-workers identified a series of (π-allyl)­PdCl-based systems that could initiate the polymerization of various diazoacetates (Scheme ). Furthermore, they also carried out detailed studies on (π-allyl)­PdCl-based systems and other palladium initiators. − Recently, Wu and co-workers reported a living/controlled polymerization of various diazoacetates using (π-allyl)­PdCl ligated with bulky chiral bidentate phosphine ligands bearing a sulfinamide moiety. , Toste and co-workers studied the polymerization of diazoacetate with (π-allyl)palladium carboxylate dimers, demonstrating that the polymerization with such initiators occurred in a controlled quasi-living manner . While extensive studies have been devoted to C1 polymerization with diazoacetates, it is still highly desirable to search for new polymerization catalysts that can efficiently initialize the polymerization with diazoacetates in a controlled manner.…”

Pd-Initiated Polymerization of Dendron-Containing Diazoacetates to Afford Dendronized Poly(substituted methylene)s with Narrow Molecular Weight Distribution and Its Application to Synthesis of pH-Responsive Dendronized Polymers

“…As an initiator for the polymerization of alkyl diazoacetates, Rh- and Pd-based initiating systems have been reported to be effective in giving high- M n polymers. In particular, the Rh­(diene) complexes can afford high- M n products with high stereoregularity from ethyl diazoacetate ( EDA ), − whereas, even though the resulting polymers are basically atactic, the Pd-based initiating systems are capable of polymerizing diazoacetates with a variety of ester substituents to give polymers with relatively high M n . − …”