Miller & Macosko crosslink density The crosslink density υ of a classical, Bethe-lattice, An+An-1+…+A1+B2 step-growth polymer network can be calculated according to the equation υ = i − 2 2 X i

A chemical clock protocol that enables enhanced temporal control over the onset of two base-catalyzed ‘click’ reactions, the thiol-Michael addition reaction, and the thiol-isocyanate reaction, is described and used in polymerization reactions. Initiating protocols with predictable induction times for both click reactions are developed and characterized using a pair consisting of an electron deficient vinylic species and a nucleophile with an acid. The approach was successfully demonstrated such that the reaction onset is effectively and predictably delayed by up to 20 min, with rapid complete reaction following the controllable induction period. By implementing initiation systems with varying relative concentrations of the electron deficient vinyl, nucleophile, and acid, this approach to formulating a comprehensive initiator system affords a previously unavailable degree of temporal control that is extremely useful for designing and processing cross-linked polymers and other thiol-Michael and thiol-isocyanate polymerizations.

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The effect of temperature on the complex shear modulus (G*(ω)) of a model reversible covalent network formed by the Diels–Alder reaction was studied. The gel temperature of 119°C and the functional group conversion at this temperature were determined by the Winter–Chambon criterion. The complex modulus of the cross-linked network was measured from 110°C to 121°C, near the gel temperature, to determine the frequency ranges over which stress relaxation could occur. The crossover time was found to have a strong dependence on temperature (Ea ∼ 260 kJ mol–1); greater than would be expected from a typical thermally-activated retro-Diels–Alder process. Low frequency scaling of G*(ω) over the experimental frequency and temperature range was interpreted to be a result of the existence of a distribution of transient clusters in these thermoreversible covalent gels.

This work introduces a cross-linked resin network with controlled chemical composition, a clinically practical procedure to make it in situ, and appropriate analytical tools for chemical structure and kinetic studies.