Thermodynamic Control in the Catalytic Insertion Polymerization of Norbornenes as Rationale for the Lack of Reactivity of Endo-Substituted Norbornenes

Synthesis and Photochemical Uncaging of Alkene-Protected, Polymer-Bound Vicinal Frustrated Lewis Pairs

Polymeric materials bearing Frustrated Lewis Pair (FLP) functionality are promising candidates for use as heterogeneous catalysts and adaptive materials, but synthetic access to FLP-functional polymers remains limited due to the incompatibility of FLPs with standard polymerization chemistries. Herein, we describe a synthetic approach that "cages" highly reactive vicinal phosphine-borane FLPs as covalent alkene adducts, which are stable to Ni-mediated vinyl addition polymerization. We discovered that the caged FLP adducts can be photochemically activated to liberate vicinal FLPs, enabling spatiotemporally controlled release of FLPs from polymeric precursors.

3,5-Dihydroxybenzoic Acid-Based Selective Dopamine Detection via Subsititution-Enhanced Kinetics Differences

The selective recognition of dopamine (DA) over other neurotransmitter analogues is difficult due to the similar molecular structure and chemical reactivity. In this study, substitution-regulated chemical reactivity of the sensing substrate is utilized to explore a novel DA detection probe with satisfying selectivity. As a case study, 3,5-dihydroxybenzoic acid (DHBA, carboxy-substituted resorcinol)-based probes have been explored for selective and ratiometric DA sensing. The carboxy substitution benefits the stabilization of the carbanion intermediate and the azamonardine product, which enhances the reaction kinetics and thermodynamics and subsequently facilitates selective DA recognition over other analogues and interferents. By exploring DHBA emission as the internal reference, ratiometric fluorescence variation is realized, which contributes to sensitive DA analysis. With the combination of logic gate and fluorometric analysis, DA detection in both low and high concentrations can be readily achieved. In addition, the DA analysis in biological samples and the enzymatic transformation of DA analogues in cerebrospinal fluid samples are achieved by the proposed DHBA probe.

Ultra-High Tg Thermoset Fibers Obtained by Electrospinning of Functional Polynorbornenes

Insertion polynorbornenes (PBNEs) are rigid-rod polymers that have very high glass transition temperatures (Tg). In this study, two functional PNBEs were electrospun in the presence of a variety of cross-linkers, resulting in fibers with Tgs greater than 300 °C. The fibers are long (several mm), rigid, and with diameters that can be tuned in the range 300 nm-10 μm. The electrospinning process can be used to encapsulate dyes or graphene dots. Due to the high cross-linking density of the fiber, dye leaching is prevented. In contrast with other rigid-rod polymers, electrospinning of PNBE is facile and can be performed at injection rates as high as 1 mL/min.

Vinylic addition poly(norbornene-co-alkenylnorbornenes) synthesized with benzylic palladium catalysts: materials for manifold functionalization

Functional groups can be easily attached to robust vinylic addition polynorbornenes by the transformation of the pendant double bond of new efficiently synthesized copolymers.

A disubstituted-norbornene-based comonomer strategy to address polar monomer problem

The transition-metal-catalyzed copolymerization of olefins with polar comonomers is a direct strategy to access polar-functionalized polyolefins in an economical manner. Due to the intrinsic poisoning effect of polar groups towards Lewis acidic metal centers and the drastic reactivity differences of polar comonomers versus non-polar olefins, it is challenging to develop catalysts that provide the desired polymer molecular weight, comonomer incorporation, and activity. In this contribution, we tackle this issue from a comonomer perspective using 5,6-disubstituted norbornenes, which are highly versatile, easily accessible, inexpensive, and capable of introducing two functional groups in a single insertion. More importantly, they are only mildly poisoning due to the presence of long spacers between double bonds and polar groups, and are not prone to β-hydride elimination due to their cyclic structures. As strong π-donors, they can competitively bind to metal centers versus olefins. Indeed, phosphine-sulfonate palladium catalysts can catalyze the copolymerization of ethylene with 5,6-disubstituted norbornenes and simultaneously achieve a high polymerization activity, copolymer molecular weight, and comonomer incorporation. The practicality of this system was demonstrated by studying the properties of the resulting polymers, copolymerization in hydrocarbon solvents or in bulk, recovery/utilization of unreacted comonomer, molecular weight modulation, and large-scale synthesis.

Activation of Pd-precatalysts by organic compounds for vinyl-addition polymerization of a norbornene derivative

An approach to activating Pd-complexes without using additives such as Lewis acids has been developed for addition polymerization of norbornenes. Aryl iodides and aryl diazonium salts were efficiently applied as cocatalysts to Pd(0)- and Pd(2+)-complexes. The developed systems catalyzed polymerization of norbornenes containing bulky and polar functional groups both in an inert atmosphere and air resulting in soluble and high-molecular-weight saturated polymers.

Cocatalyst versus precatalyst impact on the vinyl-addition polymerization of norbornenes with polar groups: looking at the other side of the coin

Polymers derived from norbornenes with polar groups are of interest as modular templates and materials for membrane processes and microelectronic applications due to the attractive combination of their properties.

Highly efficient vinylic addition polymerization of 5-vinyl-2-norbornene using benzylic palladium complexes as precatalysts

A very small amount of a palladium benzylic complex as initiator brings about the vinylic addition polymerization of the reluctant 5-vinyl-2-norbornene.

Homo- and copolymerization of norbornene using tridentate IzQO palladium catalysts with dimethylaminoethyl as a side arm

Rigid IzQO–Pd catalysts were synthesized by Rh(iii)-catalyzed C–H/alkyne annulation and applied for the homo- and copolymerizations of norbornene with polar vinyl monomers.

Exo-selective, Reductive Heck Derived Polynorbornenes with Enhanced Molecular Weights, Yields, and Hydrocarbon Gas Transport Properties

Next-generation membranes use highly engineered polymeric structures with enhanced chain rigidity, yet difficulties in polymerization often limit molecular weights required for film formation. Addition-type polynorbornenes are promising materials for industrial gas separations, but suffer from these limitations owing to endo-exo monomeric mixtures that restrict polymerization sites. In this work, a synthetic approach employing the reductive Mizoroki-Heck reaction resulted in exo-selective products that polymerized up to >99% yields for ROMP and addition-type polymers, achieving molecular weights an order of magnitude higher than addition-type polymers from endo-exo mixtures and impressive side group stereoregularity. Due to this increased macromolecular control, these polynorbornenes demonstrate unique solubility-selective permeation with mixed gas selectivities that exceed commercially used PDMS. In addition to thermal and structural characterization, XRD and computational studies confirmed the results of pure and mixed-gas transport testing, which show highly rigid membranes with favorably disrupted chain packing.