ATRP Fundamentals

Controlled radical polymerization (CRP, also termed reversible-deactivation radical polymerization, RDRP) is a versatile method for the preparation of well-defined polymers [1]. Unlike conventional radical polymerizatirai with its slow continuous initiation, fast propagation, and inevitable radical termination, CRP creates and exploits a dynamic equilibrium between growing radicals and dormant species [2]. In this system, the active radicals are deactivated after adding one or several monomer units and converted back to the dormant state. This approach allows preparation of polymers with precise control over molecular weight (MW), molecular weight distribution (MWD), polymer composition, topology, and functionahty. 

Atom transfer radical polymerization (ATRP) is one of the most robust and widely used CRP techniques for polymerization of a broad range of commercially available functional monomers [3-5]. It is attractive because of the simple experimental setup, with readily available initiators and catalysts that can be used in a range of solvents under a broad spectrum of reaction conditions, allowing precise control over final polymer MW and architecture [6]. 

ATRP is a catalytic process and can be mediated by many redox-active transition metal complexes. The most frequently used metal is Cu however, ATRP has also been successfully carried out using Ru, Fe, Mo, Os, etc. [7]. The key limitation of normal ATRP (as it was initially defined) is the large amount of catalyst loading (up to ca. 1 mol%) compared with monomer. This residual metal creates difficulties in purification of the final product [8]. Also, in ATRP, as in any radical process, radical termination occurs but involves only about 1-10% of all chains. Radical termination leads to irreversible transformation of a fraction of the activator to deactivator, leading to a decrease in the reaction rate. 

Surface-initiated ATRP (SI-ATRP) follows the same mechanism and is controlled by the same factors as a regular ATRP however, there are several unique requirements, which are discussed in detail in Sect. 4. 

---

The simplicity of the polymerization reaction is the result of intense research carried out by several groups on the importance and the fundamentals of each parameter. In particular, Matyjaszewski et al. have spent great effort on the construction of numerous comparison charts on the activity of initiators and ligands that are used in ATRP [30-32]. These published comparison tables represent the summary of hundreds of single experiments and are now a very important and reliable source of data for the ATRP technique. 

Although ATRP behaves differently from conventional free radical polymerization, the fundamental reactions involved are very similar and include initiation, propagation, transfer and termination (see Scheme 6). Since chain termination does not occur in a truly living polymerization, the living character of the chains in ATRP derives from the fact that chain propagation is first order with respect to radical concentration and irreversible bi-molecular termination is second order. As such, the concentration of the radicals is kept very low, the rate of bi-molecular termination is greatly reduced, and typically less than 10% of all of the chains will terminate. Unlike conventional free radical polymerization, where the rate is dictated by a steady state between the initiation and termination rates, the rate and concentration of propagating radicals in ATRP is controlled entirely by the equilibrium between activation and deactivation [255]. 

Hong SC, Matyjaszewski K (2002) Fundamentals of supported catalysts for Atom Transfer Radical Polymerization (ATRP) and apphcation of an immobUizEd/soluble hybrid catalyst system to ATRP. Macromolecules 35 7592... 

ATRP was applied to the copolymerization of a monovinyl monomer and a divinyl cross-linker to study the experimental gelation behavior. The fundamental features of ATRP, including fast initiation and reversible deactivation reactions, resulted in a retarded gelation and the formation of a more homogeneous network in the ATRP process compared to gel formation in a conventional radical polymerization. The experimental gel point based on the monomer conversion in the ATRP reaction occurred later than the calculated value based on Flory-Stockmayer s mean-field theory, which was mainly ascribed to intramolecular cyclization reactions. The dependence of the experimental gel points on several parameters was systematically studied, including the ratio of cross-linker to initiator, the concentration of reagents, reactivity of vinyl groups, initiation efficiency of initiators, and polydispersity of primaiy chains. 

Carnegie Mellon University was the first to protect an ATRP in which the four components, shown in Scheme 1, are added to the reaction flask or are formed in situ. US Patent 5,763,548 (26) was filed on March 31, 1995 and issued on June 9, 1998 has been cited by 147 issued patents. This fundamental patent broadly defines suitable initiators, transition metals and ligands. Other... 

As a result of CMU s IP focus, 21 of the 28 issued US patents, pins several active applications, protect the fundamental ATRP process or improvements in the fundamental process, (26,27,30,33-48) while only five of the twenty eight address novel polymer compositions. (28,29,36,49,50) Nevertheless, these early material-focused patents disclose a nnmber of materials that were not prepared by other CRP procednres until a later date. The difference in numbers is due to the fact that two issued patents are directed towards improvements in nitroxide mediated polymerization. (20,51) The first discloses an atom transfer radical addition reaction to form an alkoxyamine that has fonnd nse in ATRP kinetic studies, and the other focnses on rate enhancement of a NMP. 

K. Matyjaszewski in Fundamentals of ATRP Reaction, Matyjaszewski Polymer Group, Pittsburgh, PA, USA. 

NMP, ATRP and RAFTcurrently are the most commercially promising LRP techniques and many of the fundamental kinetic mechanisms and issues have been elucidated [60-66]. These processes are now at the sta where companies are actively pursuing... 

Mechanistic studies provide the fundamental foundation required to develop a comprehension of the critical parameters for an ATRP. ° The studies generated the knowledge required to develop more environmentally benign ATRP procedures ° ° and remain crucial to any future developments in ATRP, since they generate the kinetic data that provide the underpinnings for chemical engineers to scale up the processes to industrial-scale production of specialty materials. 

Other language has been used to describe this process. Indeed many authors have intermittently used other names/abbrevia-tions for reactions utilizing the same components to clarify specific aspects of the reaction. " This multiplicity of nomenclature may have aeated confusion as to the fundamental similarity, or indeed identical nature of the reactions being discussed. A recent recommendation by lUPAC clarifies this position by recommending that specific reversible-deactivation radical polymerizations (RDRPs) should adopt terminology consistent with that in lUPAC documents, specifically that the controlled RDRP procedures in which the deactivation of the radicals involves catalyzed reversible atom transfer or reversible group transfer usually, though not exclusively, by transition metal complexes be named atom transfer radical polymerization, ATRP. °... 

The fundamental idea of ATRP is the halogen exchange in the polymerizing system between the halogen terminated growing polymer chain/Cu(l) dNbP complex and macroradical/Cu(II) dNbP complex. Chain propagation is a first order process while termination is a second order reaction. For conventional bulk or solution ATRP the equilibrium is strongly shifted to the left. The free... 

This review summarizes the recent progress on the synthesis and application of covalently crosslinked hydrogels prepared using ATRP techniques. Section 2 briefly covers the fundamentals of the ATRP mechanism with particular emphasis on the recent progress to understand the structure/reactivity relationship and the development of new ATRP conditions that decrease the amount Cu catalyst. In Sect. 3, theoretical gel points and experimental gel points based on vinyl conversions are described. Emphasis focuses on understanding of the gelation process in... 

---