Carbene-Based Polymerization

Fig. 6 /V-IIctcmcyclic carbene-based Ni(II) olefin polymerization catalysts...

Jensen, T. R. Breyfogle, L. E. HiUmyer, M. A. Tolman, W. B. Stereoselective polymerization of D,L-lactide using N-heterocyclic carbene based compounds. Chem. Common. 2004, 2504—2505. 

In the following sections, the polymerization of P-lactones will be discussed with regards to the nature of the active species, whether anionic, cationic, coordination-type or carbene-based. Finally, a brief overview will be provided of the enzymatic ROP of four-membered lactones. 

By contrast, much of the work performed using ruthenium-based catalysts has employed well-defined complexes. These have mostly been studied in the ATRP of MMA, and include complexes (158)-(165).400-405 Recent studies with (158) have shown the importance of amine additives which afford faster, more controlled polymerization.406 A fast polymerization has also been reported with a dimethylaminoindenyl analog of (161).407 The Grubbs-type metathesis initiator (165) polymerizes MMA without the need for an organic initiator, and may therefore be used to prepare block copolymers of MMA and 1,5-cyclooctadiene.405 Hydrogenation of this product yields PE-b-PMMA. N-heterocyclic carbene analogs of (164) have also been used to catalyze the free radical polymerization of both MMA and styrene.408... 

To date, the most frequently used ligand for combinatorial approaches to catalyst development have been imine-type ligands. From a synthetic point of view this is logical, since imines are readily accessible from the reaction of aldehydes with primary or secondary amines. Since there are large numbers of aldehydes and amines that are commercially available the synthesis of a variety of imine ligands with different electronic and steric properties is easily achieved. Additionally, catalysts based on imine ligands are useful in a number of different catalytic processes. Libraries of imine ligands have been used in catalysts of the Strecker reaction, the aza-Diels-Alder reaction, diethylzinc addition, epoxidation, carbene insertions, and alkene polymerizations. 

Scheme 8.13 Dynamic covalent polymers based on carbine dimerization, (a) Preparation of difnnctional carbene 58 and polymerization of 58 via carbene dimerization (b) Chain transfer reaction of 59 by the agency of monofnnctional carbene 60, and (c) Formation of the organometallic copolymer 62 by the insertion of PdCl [45],...

We note that there are NMR-based kinetic studies on zirconocene-catalyzed pro-pene polymerization [32], Rh-catalyzed asymmetric hydrogenation of olefins [33], titanocene-catalyzed hydroboration of alkenes and alkynes [34], Pd-catalyzed olefin polymerizations [35], ethylene and CO copolymerization [36] and phosphine dissociation from a Ru-carbene metathesis catalyst [37], just to mention a few. 

Tantalum carbene complexes such as 5 and Ta(=CHCMe3)(S—C6H2-i-Pr3-2,4,6)3 (py) are effective, provided the conditions are such as to allow the coordinated base (THF or py) to give way to monomer (M). In the first example the initially formed tantalacyclobutane complex has been isolated and shown to have a trigonal-bipyramidal structure, and to polymerize NBE at a rate that is independent of [M], In this case the rearrangement of the intermediate tantalacyclobutane complex, to form the tantalum carbene complex, controls the rate of polymerization. In contrast, in the second example the rate is first-order in monomer here the reaction of the tantalum carbene complex with the monomer is the slower step. In both cases the polymer, after termination by reaction with benzaldehyde, is nearly monodisperse93,94. 

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