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Manufacturing Techniques for Nanoparticles in Drug Delivery
Published in Yasser Shahzad, Syed A.A. Rizvi, Abid Mehmood Yousaf, Talib Hussain, Drug Delivery Using Nanomaterials, 2022
Daniel Real, María Lina Formica, Matías L. Picchio, Alejandro J. Paredes
Like ATRP and NMP, RAFT is another reversible-deactivation radical polymerization technique but is more versatile and allows for better control over molecular weights and polydispersity (Keddie, 2014). This polymerization method relies on adding a chain transfer agent (RAFT agent) to a conventional free radical polymerization medium. RAFT polymerization is characterized by four different steps: initiation, addition–fragmentation, reinitiation, and equilibration. In the first step, free radicals are generated from the initiator, and the subsequent addition of monomer creates active polymer chains (Pn•). In the addition-fragmentation step, the polymer chains combine with the RAFT agent, giving an active intermediate and releasing a homolytic leaving group (R•). This step is reversible, and the active intermediate can lose either the cleavable group (R•) or the polymeric chain (Pn•). Re-initiation can start with R• by addition of a monomer and forming a new active polymer (Pm•). This active chain goes through the addition–fragmentation or equilibration steps. Active polymer chains (Pn• and Pm•) are in equilibrium between the active and dormant (bound to the thiocarbonyl compound) stages. Thus, when one polymer chain is in the dormant stage, the other chain is active in polymerization.
Synthesis and Functionalization of Magnetic Particles
Published in Jeffrey N. Anker, O. Thompson Mefford, Biomedical Applications of Magnetic Particles, 2020
Erika C. Vreeland, Dale L. Huber
There is another approach to radical polymerization termed “controlled radical polymerization,” or more formally “reversible-deactivation radical polymerization” (Jenkins, Jones, and Moad 2010), which refers to a collection of methods that modifies the kinetics of a radical polymerization to yield a more controlled product. These reactions include the well-known techniques of atom-transfer radical polymerization (ATRP) and reversible-addition-fragmentation chain-transfer polymerization (RAFT), as well as a number of related techniques. These reactions are often referred to as “living radical polymerizations” in the literature, although that term is no longer preferred (Jenkins, Jones, and Moad 2010). These methods share a common approach to controlling the polymerization reaction, and all have a reversible reaction that deactivates the free radical. The precise details of the various approaches are described elsewhere (Matyjaszewski and Spanswick 2005) but can be envisioned simplistically as a free radical reacting with some capping species to form a metastable dormant species that periodically dissociates reforming the active radical. In a well-controlled reaction, the vast majority of the radicals are in the dormant, deactivated state at any given time, while a small proportion of the radicals are in an active state and are adding monomer (if it is available). Individual radicals freely convert between the dormant and active state, while maintaining a low, equilibrium concentration of the active state throughout the reaction.
Mechanical characterization of additively manufactured photopolymerized polymers
Published in Mechanics of Advanced Materials and Structures, 2023
Roberto Brighenti, Liviu Marsavina, Mihai P. Marghitas, Mattia P. Cosma, Matteo Montanari
The light radiation going inside the material is absorbed by photo-initiators, photo-absorbers, and by other light reactive species present in the matrix, leading to a liquid (monomers) solid (polymer chains) transformation. Such a solidification process takes place thanks to the following phenomena: the light radiation induces the photo-initiators molecules whose concentration in the liquid monomer bath is to be converted into free radicals () whose concentration is Then, free radicals react with monomer molecules () inducing the appearance of functional groups (), which constitute growing chains whose evolution in space (chain growth: ) is interrupted either when the polymer chain encounters a free radical (in symbols: ) or when it joints to another chain encountered along its growing path [22]. In the above relations and are reaction rate parameters, depending on the environmental conditions, such as the temperature, characterizing the chemical process. For the sake of simplicity, in the following all the involved chemical processes are assumed to be isothermal so the above rates are constant. The above-described photopolymerization kinetics is representative of a conventional radical polymerization, which is the polymerization mechanism considered in the present study. It is worth mentioning that, in contrast to conventional radical polymerization, the so-called reversible deactivation radical polymerization (RDRP) exists. This latter chemical-physical transformation, whose kinetic mechanism is governed by the same laws of the conventional radical polymerization, allows obtaining a polymer characterized by a “living” or “controlled” radical polymerization. RDRP typically exploits the ability of some chemical species inserted in the resin being printed, of reversibly controlling the propagation of radical species (and thus of polymer chains), allowing for a precise tuning of the polymer chains architecture. This makes possible to obtain intriguing post-synthesis transformations level such as self-healing, network alterations, etc., taking place at the molecular level [29, 30].