High-performance adhesives require mechanical properties tuned to demands of the surroundings. A mismatch in stiffness between substrate and adhesive leads to stress concentrations and fracture when the bonding is subjected to mechanical load. Balancing material strength versus ductility, as well as considering the relationship between adhesive modulus and substrate modulus, creates stronger joints. However, a detailed understanding of how these properties interplay is lacking. Here, a biomimetic terpolymer is altered systematically to identify regions of optimal bonding. Mechanical properties of these terpolymers are tailored by controlling the amount of a methyl methacrylate stiff monomer versus a similar monomer containing flexible poly(ethylene glycol) chains. Dopamine methacrylamide, the cross-linking monomer, is a catechol moiety analogous to 3,4-dihydroxyphenylalanine, a key component in the adhesive proteins of marine mussels. Bulk adhesion of this family of terpolymers is tested on metal and plastic substrates. Incorporating higher amounts of poly(ethylene glycol) into the terpolymer introduces flexibility and ductility. By taking a systematic approach to polymer design, the region in which material strength and ductility are balanced in relation to the substrate modulus is found, thereby yielding the most robust joints.

Marine mussels clinging to rocks inspire the development of novel materials. Characterization of mussel adhesive plaques describes a matrix of proteins containing 3,4-dihydroxyphenylalanine (DOPA), which provides cross-linking chemistry that allows mussels to attach firmly. Several synthetic polymer systems have been developed based on this DOPA chemistry. High strength bonding has been achieved with poly[(3,4-dihydroxystyrene)-co-styrene], a simplified mimic of mussel proteins in which 3,4-dihydroxystyrene provides the cross-linking and adhesion of DOPA. The poly(styrene) host polymer stands in for a protein backbone. Prior efforts showed that a monomer ratio of 1:2 3,4-dihydroxystyrene:styrene within the statistical copolymer poly[(3,4-dihydroxystyrene)-co-styrene] yields the highest adhesion. To enhance adhesive performance of this biomimetic polymer, a systematic study is carried out in which a range of cross-linking agents, cure times, cure temperatures, polymer concentrations, and fillers are examined. Lap shear adhesion testing revealed substantial increases in bond strength from each study. Consensus conditions are then determined and bonding performance is assessed on several substrates. Adhesion of this system turns out to be one of the strongest of all biomimetic polymers. These studies show that DOPA chemistry may be able to stand alongside of cyanoacrylate (e.g., Super Glue) and epoxy when it comes to high strength bonding.

Carcinogens found in cooked foods, tobacco smoke, and vehicle exhaust undergo metabolic activation to pernicious alkylating toxins, yield damaged DNA, and promote cancerous growth. Vanadium has been shown to decrease the occurrence of cancers, possibly by intercepting such toxins before DNA damage can occur. According to recent results, nucleophilic oxido salts of vanadium can prevent this DNA alkylation. Although effective at detoxification and preventing DNA damage, vanadate salts equilibrate in solution to multiple coexisting species and can exhibit toxicity. Ligand-enforced coordination geometries may minimize such equilibrations, thereby decreasing toxicity and providing a means to control reactivity. As part of our efforts to detoxify alkylating agents, here we are studying reactions between oxidovanadium complexes and toxins. Alkylating agents such as diethyl sulfate were treated with a series of new oxidovanadium complexes of the salicylidenehydrazide ligand, [VO₂(salhyph(R)₂)]^–. These complexes consumed a collection of alkylating agents and brought about transformation to alcohols. Changing the ligand substituents (R = –OCH₃,–CH₃, –H, –NO₂) yielded a series of compounds with varied degrees of electron density. Kinetic experiments indicated that there may be a correlation between electron density and reactivity with alkylating toxins. The design and reactivity of these compounds indicate that we may be able to exert control over interactions between carcinogens and metal complexes. Such principles may be helpful in developing new compounds for the prevention of cancer. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

The cover picture shows oxidovanadium complexes transforming alkylating toxins into alcohols. Variation of the electron-donating ability of the ligand was found to permit a degree of control over the reactivity with alkylating agents. Kinetic studies revealed a small range of k_obsd. values for this series of K[VO₂(salhyph(R)₂)] complexes (R = –NO₂, –H, –CH₃, –OCH₃) examined. These experiments were carried out with an ultimate goal of developing compounds to consume toxins and prevent DNA alkylation damage. Details are discussed in the Short Communication by J. J. Wilker et al on p. 33 ff.