Methacrylate Polymerization using a Dinuclear Zirconocene Initiator: A New Approach for the Controlled Synthesis of Methacrylate Polymers

Abstract

There has been renewed interest in the controlled polymerization of acrylates and other susceptible monomers using Group 4 metallocene initiators during the past five years. These complexes have been applied to the synthesis of polyacrylates and polymethacrylates, where control over molecular weight (M W ), comonomer sequence distribution, and polymer tacticity is possible in some cases. [1] Over 10 years ago we reported that a two-component initiator system comprising [Cp 2 ZrMe 2 ] (Cp = C 5 H 5 ) and [Cp 2 ZrMe(L)][X], either preformed (X = BPh 4 , L = THF) [2a] or generated in situ (X = MeB(C 6 F 5 ) 3 , B(C 6 F 5 ) 4 , L = methyl methacrylate (MMA)) [2b, c] was competent for MMA polymerization. Mechanistic work revealed that propagation involved the rate-limiting, intermolecular Michael addition of zirconocene enolate 1 (formed in situ from complex 2 and [Cp 2 ZrMe 2 ]) to MMA, activated by complexation to 2 (Scheme 1). [3] We based this mechanism on the observed polymerization kinetics using preformed neutral enolates 1 in combination with 2 (independently first order in both and zero order in [MMA]) [2c, 3] and on the observation that 2 or even a discrete cationic zirconium enolate complex, [Cp 2 Zr{OC(OMe)= CMe 2 }(thf)][BPh 4 ][3] were much less competent initiators. It occurred to us that a covalently linked, dinuclear enolate initiator might function analogously to the twocomponent systems studied earlier and could represent a new strategy [4] for the controlled polymerization of methacrylates and other susceptible monomers.We concentrated on the use of dinuclear zirconocene complexes featuring the m-oxo linkage as this unit is robust. [5,6]

Dinuclear Silylium-enolate Bifunctional Active Species: Remarkable Activity and Stereoselectivity toward Polymerization of Methacrylate and Renewable Methylene Butyrolactone Monomers

“…There was no clear induction period observed for the polymerization at ambient temperature, and the first-order kinetics with respect to [M] held true for all the ratios investigated. This observation is in sharp contrast to the zero-order dependence on [M] observed for the polymerization systems based on the mono-SKA 9,10,24 and the μ-oxo-bridged dinuclear zirconocene, 42 pointing to a different propagation mechanism for the di-SKA polymerization system (vide infra).…”

Synthesis of biodegradable polymers from renewable resources

“…A catalytic cycle involving a Michael addition was proposed, similar to the mechanism described for polymerization of methacrylates with non-bridged group 4 metallocene catalysts. 75 MALDI-TOF mass spectrum analysis of an unpurified low molecular weight PMMBL ([MMBL]/[13c] ¼ 20) revealed that one of the two mass distributions corresponded to PMMBL with one chain-end group being the alkyl CH 2 SiMe 3 and the other a proton resulting from the acidic work-up procedure. Furthermore, the efficiency coefficient for the polymerization with these catalysts was found to be over 100%, suggesting the two alkyl species CH 2 SiMe 3 participated in chain initiation but only one chain grew at a time since a first order dependence to MMBL was observed.…”

Anionic polymerization of MMA and renewable methylene butyrolactones by resorbable potassium salts

“…A zero‐order dependence on monomer concentration in this activated‐monomer polymerization suggests that the rate determining step is the conjugate Michael addition of the monomer activated by the alane into the polymer chain (i.e., the CC bond forming step), whereas the recapture of the aluminum catalyst coordinated to the penultimate polymer chain end by the incoming monomer is relatively fast (Scheme ). This mechanism resembles that of the bimolecular, activated‐monomer anionic polymerization by the lithium enolaluminate/alane pair,36(a), 19, 38 or by nonbridged group 4 metallocene/metallocenium pair 39…”