Catalytic living ring-opening metathesis polymerization

In living ring-opening metathesis polymerization (ROMP), a transition-metal–carbene complex polymerizes ring-strained olefins with very good control of the molecular weight of the resulting polymers. Because one molecule of the initiator is required for each polymer chain, however, this type of polymerization is expensive for widespread use. We have now designed a chain-transfer agent (CTA) capable of reducing the required amount of metal complex while still maintaining full control over the living polymerization process. This new method introduces a degenerative transfer process to ROMP. We demonstrate that substituted cyclohexene rings are good CTAs, and thereby preserve the ‘living’ character of the polymerization using catalytic quantities of the metal complex. The resulting polymers show characteristics of a living polymerization, namely narrow molecular-weight distribution, controlled molecular weights and block copolymer formation. This new technique provides access to well- defined polymers for industrial, biomedical and academic use at a fraction of the current costs and significantly reduced levels of residual ruthenium catalyst

Accelerated FoxP2 Evolution in Echolocating Bats

FOXP2 is a transcription factor implicated in the development and neural control of orofacial coordination, particularly with respect to vocalisation. Observations that orthologues show almost no variation across vertebrates yet differ by two amino acids between humans and chimpanzees have led to speculation that recent evolutionary changes might relate to the emergence of language. Echolocating bats face especially challenging sensorimotor demands, using vocal signals for orientation and often for prey capture. To determine whether mutations in the FoxP2 gene could be associated with echolocation, we sequenced FoxP2 from echolocating and non-echolocating bats as well as a range of other mammal species. We found that contrary to previous reports, FoxP2 is not highly conserved across all nonhuman mammals but is extremely diverse in echolocating bats. We detected divergent selection (a change in selective pressure) at FoxP2 between bats with contrasting sonar systems, suggesting the intriguing possibility of a role for FoxP2 in the evolution and development of echolocation. We speculate that observed accelerated evolution of FoxP2 in bats supports a previously proposed function in sensorimotor coordination

Increasing the initiation efficiency of ruthenium-based ring-opening metathesis initiators: Effect of excess phosphine

Ring-opening metathesis polymerization (ROMP) has evolved into a valuable tool for the polymer chemist. The polymerization is generally mild and well controlled, and a large pool of readily available cyclic olefins (monomers) can be polymerized to nearly any size or shape. Employing the Ru-based initiator 1 (or its more active derivative 2) permits incorporation of high degrees of functionality and affords polymers with novel mechanical, electronic, and more recently biological properties. However, the polydispersity indices (PDIs) of the polymers obtained from initiator 1 are generally broad (between 1.3 and 1.5), which arises from an unfavorable rate of initiation (k_i) relative to propagation (k_p) as well as considerable secondary metathesis (“backbiting”). This creates difficulties when attempting to accurately predict polymer molecular weight a priori or when preparing well-defined block copolymers (where complete initiation is necessary). A recent disclosure from Gibson and co-workers revealed that the initiation efficiency of 1 was enhanced when the PCy_3 ligands were substituted with Cy_2PCH_2Si(CH_3)_3. When the resulting complex (3) was used to initiate the ROMP of norbornene derivatives (similar to 4), the k_i/k_p was found to be 4.35 (up from 0.06 when initiated with 1), and the resulting polymers were nearly monodispersed (PDIs ∼ 1.1). The enhanced initiation was attributed to a combination of the lower basicity and smaller size of Cy_2PCH_2Si(CH_3)_3 (relative to PCy_3) which respectively helped facilitate phosphine dissociation (a key step in Ru-based ROMP, see below) and increase monomer accessibility

Dynamic covalent polymers based upon carbene dimerization

Thermally-reversible covalent polymers featuring dynamic carbon - carbon double bonds and tunable molecular weights were prepared from difunctional carbenes; addition of transition metal complexes to these materials afforded the respective main-chain organometallic polymers

Triazene formation via reaction of imidazol-2-ylidenes with azides

Treatment of N-heterocyclic carbenes (as their free carbenes or generated in situ) with alkyl, aryl, acyl or tosyl azides afforded the respective substituted triazenes in excellent yields

An "endless" route to cyclic polymers

A new synthetic route to cyclic polymers has been developed in which the ends of growing polymer chains remain attached to a metal complex throughout the entire polymerization process. The approach eliminates the need for linear polymeric precursors and high dilution, drawbacks of traditional macrocyclization strategies, and it effectively removes the barrier to producing large quantities of pure cyclic material. Ultimately, the strategy offers facile access to a unique macromolecular scaffold that may be used to meet the increasing demand of new applications for commercial polymers. As a demonstration of its potential utility, cyclic polyethylenes were prepared and found to exhibit a variety of physical properties that were distinguishable from their linear analogs

Tandem catalysis: Three mechanistically distinct reactions from a single ruthenium complex

Organometallic catalysts are traditionally designed and optimized to mediate a single reaction. As the number of applications that require combinatorial and other high-speed synthetic protocols increases, it will become desirable for catalysts to mediate multiple, mechanistically distinct transformations directly or upon simple modification. As an example of such a system, we demonstrate the ability of a single component precatalyst to mediate three different reactions to form well-defined block copolymers

Highly efficient syntheses of acetoxy- and hydroxy-terminated telechelic poly(butadiene)s using ruthenium catalysts containing N-heterocyclic ligands

Bis(acetoxy)-tenninated telechelic poly(butadiene) (PBD) with molecular weights controllable up to 3.0 X 10(4) have been prepared via the ring-opening metathesis polymerization (ROMP) of cyclooctadiene when 1,4-bis(acetoxy)-2-butene was included as a chain transfer agent (CTA). The polymerizations were catalyzed by a highly active ruthenium catalyst 1,3-bis(2,4,6-trimethylphenyl)imidazol-2- ylidene)(PCp3)(Cl2Ru = CHCHC(CH3)(2) (Cp = cyclopentyl) (6) with monomer/catalyst ratios as high as 9.8 X 10(4). Removal of the acetoxy groups with sodium hydroxide afforded hydroxy end-terminated telechelic PBD (HTPBD). Examination of the telechelic PBDs revealed an exclusive 1,4-PBD microstructure with a predominately trans geometry (up to 90%). The high activity and stability of 6 permitted a one-step synthesis of HTPBD using the unprotected free alcohol, 2-butene-1,4-diol, as the CTA