Equilibrium relationships for non-equilibrium chemical dependencies

In contrast to common opinion, it is shown that equilibrium constants determine the time-dependent behavior of particular ratios of concentrations for any system of reversible first-order reactions. Indeed, some special ratios actually coincide with the equilibrium constant at any moment in time. This is established for batch reactors, and similar relations hold for steady-state plug-flow reactors, replacing astronomic time by residence time. Such relationships can be termed time invariants of chemical kinetics

A Rotating Fluidized Bed in a Static Geometry: Experimental Proof of Concept

The new concept of a rotating fluidized bed in a static geometry (RFB-SG) is presented (1). The rotating motion of the particle bed and the tangential fluidization of the solids are obtained by the tangential injection of the fluidization gas via multiple gas inlet slots in the outer cylindrical wall of the fluidization chamber. The new fluidization concept is experimentally investigated and proven using either large diameter, low density polymer particles or small diameter, higher density Alumina particles

A detailed characterization and design of copolymerization

Many industrial polymerizations are copolymerizations in which two or more comonomers are copolymerized together to obtain a final product with a wide variety of properties originating from the related homopolymers. Crucial is the identification of the correct comonomer types and the reaction conditions so that the suited connectivity of monomer units is ensured in the copolymer chains. In view of this challenge a detailed characterization tool is indispensable. A sole focus on experimental tools is insufficient as they only allow the assessment of copolymer properties through relative properties and/or are limited to average properties [1-4]. The latter implies the lack of validation of intermolecular homogeneities, inhibiting process control on the polymer property level. To solve this issue and thanks to the advance in recent computer technologies, simulation tools have been developed which allow a characterization of copolymerization processes at the molecular level (see Figure 1; [3]). Monomer sequences of individual chains can be visualized allowing an unambiguous product qualification. In this contribution, the potential of these simulation tools is highlighted through several case studies. Focus in on both bulk/solution radical and cationic polymerizations and the interplay of chemistry and diffusional limitations [5-7]. Please click Additional Files below to see the full abstrac

Designing controlled radical polymerization: A selection of a terminal or penultimate model for the intrinsic reactivities

In the past decades many efforts have been devoted to understand and design controlled radical polymerization (CRP) techniques such as atom transfer radical polymerization (ATRP) and nitroxide mediated polymerization (NMP). A crucial aspect is the use of detailed reaction schemes and the appropriate correction for diffusional limitations. Limited focus has however paid to the impact of penultimate monomer unit (PMU) effects, which can be explained by the complexity of the associated kinetic models with multiple reaction channels and the lack of data on reactivity ratios, in particular for NMP specific reactions. In the present contribution, it is demonstrated that depending on the comonomer pairs and the reaction conditions either a terminal [2] or penultimate model [3] is more suited. For copolymerizations with equimolar conditions for the comonomer amounts the PMU can be very pronounced even if based on the reactivity ratios as such this is not expected (Figure 1). Please click Additional Files below to see the full abstract

A novel interpretation of measured and simulated PLP data

Figure 1 - Simulated ν dependency of the observed kp in vinyl acetate PLP at 323 K. Case 1 (♦): chain length independent head-to-tail prop., Case 2 (■): chain length dependent head-to-tail prop., Case 3 (●): chain length dependent head-to-tail, head-to-head, tail-to-tail, and tail-to-head prop., and Case 4 (▲): Case 3 with backbiting by head and tail radicals, and mid-chain prop. Pulsed laser polymerization (PLP) is an interesting technique to study individual reactions.1-4 In PLP, photoinitiator radical fragments are generated at laser pulses with a frequency ν (or dark time Δt = ν-1). Depending on the PLP conditions and the monomer type, the molar mass distribution (MMD) can possess specific characteristics, allowing the determination of intrinsic rate coefficients. Most known is that under well-chosen conditions a multimodal MMD with inflection points Lj (j = 1, 2, …) is obtained, allowing the determination of the propagation rate coefficient kp ([M]0: initial monomer concentration): (1) In this contribution, kinetic Monte Carlo (kMC) modeling is applied to allow a further understanding and exploitation of PLP. For PLP of acrylates, regression analysis to low frequency inflection point data at various solvent volume fractions is proposed as an additional new method to estimate the backbiting rate coefficient kbb.5 Moreover, it is demonstrated that photodissociation, chain initiation and termination reactivities can be extracted from the complete PLP MMD.6 For the first time, the ratio of MMD peak heights has been used for the fast and reliable estimation of the photodissociation quantum yield,Φ.7 For PLP of vinyl acetate a unique combination of ab initio calculated rate coefficients and kMC simulations is considered to explain the experimental8 ν dependency of the observed kp (cf. Case 4 in Figure 1; Eq. (1) with kpobs). Via a stepwise extension of the kMC model (cf. 4 cases in Figure 1), the ν dependency is attributed to backbiting of tail radicals formed via head-to-head propagation.9 In contrast to acrylates, backbiting of head radicals is shown to be kinetically insignificant in VAc PLP, further highlighting the chemical difference between both vinyl monomer types. Please click Additional Files below to see the full abstract