A number of procedures have been developed for the initiation of an ATRP reaction.
The primary differences between the procedures are:
- the oxidation state of the transition metal complex added to the reaction flask or formed in situ from an added transition metal and the selected ligand;
- when the more oxidatively stable higher oxidation state transition metal is added to the reaction flask the procedures used to activate the higher oxidation state transition metal complex and,
- with more recently developed procedures exploiting low concentrations of catalyst the absolute amount of transition metal complex added to the reaction and the procedures used to continuously reactivate the higher oxidation state transition metal complex.
There are no mechanistic differences betrween the polymerization procedures irrespective of the abreviation used to indicate differences in methods for the initiation of the controlled polymerization procedure. This is true even when transition metal in zero oxidation state is employed to reactivate the catalyst in ARGET systems.
Selecting the correct conditions for initiation of an ATRP reaction is the obvious first step that has to be taken in order to be able to carry out any well controlled ATRP. Control over the initiation procedure is critical for the preparation of materials where exploitation of other aspects of controlled material synthesis is desired. As noted in first set of the bullet points below, seven different acronyms have been created to describe the procedures that have been developed for initiation of an ATRP, together with two procedures discussing conditions for improving cross-propagation kinetics when conducting a chain extension reaction from a reactive macroinitiator to polymerize a less active monomer, e.g. chain extension from a poly(methacrylate) to a styrene or acrylate, and a discussion on conditions for conducting an ATRP from initiators tethered to solid surfaces.
In the following discussion the seven initiation procedures are listed in the order in which they were developed and do not indicate a preference for a specific procedure for initiation of an ATRP. In certain circumstances the “normal ATRP” procedure is still employed whereas ARGET or ICAR should be considered for reactions that could be scaled up to industrial scale in existing equipment since they are much more environmentally benign.
The procedure termed “halogen exchange” provides a tool that allows one to alter the order of addition of monomers to a sequential block copolymerization from that dictated by monomer reactivity thereby allowing increased freedom in designing polymer architecture. This is a unique advantage of ATRP and cannot be used in SFRP or RAFT but requires addition of stoichiometric amounts of catalyst compared to initially added initiator. Therefore, halogen exchange cannot work with the development of procedures that use low levels of catalyst, and another procedure was developed to overcome this limitation: preparation of a copolymer in the second block which is in many ways the equivalent of halogen exchange in AGET or ARGET ATRP.
In order exploit the benefits of a CRP initiation should be fast and complete at low monomer conversion since this provides control over molecular weight, PDI, structure, functionality. When applied to preparation of materials that may result in large scale production initiation procedures should be robust, inexpensive, and provide a clean ATRP. Such features are an inherent part of ARGET and ICAR ATRP and these new systems circumvent the oxidation problem present in a normal ATRP system and employ small amount of catalysts that are added to the reaction in the stable higher oxidation state. Only, 1-50ppm of high-oxidation-state transition metal complex is needed to mediate a controlled polymerization and therefore greatly simplifies the procedures available for ATRP making it applicable for industrial production of functional polymeric materials.
Procedues for initiation of an ATRP reaction include:
Simultaneous Reverse and Normal Initiation (SR&NI)
Activator Generated by Electron Transfer (AGET)
Activator ReGenerated by Electron Transfer (ARGET)
Initiators for Continuous Activator Regeneration (ICAR)
Initiators Containing Radically Transferable Groups
"Equivalent" of "Halogen Exchange" in ARGET Chain Extension