Mechanisms and Catalyst Development
Understanding the Mechanistic Parameters of Atom Transfer Radical Polymerization (ATRP): Research conducted in the Matyjaszewski group in the area of developing an understanding of mechanistic parameters of transition metal catalyzed ATRP covers several topics:
On this page we provide a summary of the mechanism of ATRP and in other pages we discuss in greater detail the chemistry involved in some Controlled Radical Polymerization (CRP) processes, in particular ATRP, and the spectrum of new materials that can now be prepared by CRP processes.
The ultimate aim of these projects is to establish a structure-property correlation for the catalytic system and explore the potential of utilizing ATRP for the polymerization of an expanding range of monomers thereby allowing the preparation of materials with properties that closely match those required for specifically targeted applications.
The group has historically focused on copper based catalysts, and this work forms the foundation for most of the observations/discussions/conclusions on these pages, although other transition metals have been examined and continue to be examined both within the group and by many other researchers throughout the world.
This means that one catalyst does not work for every monomer: selection of an appropriate catalyst system, reaction medium, and reaction conditions are required.
For instance if conditions are defined employing a lower activity catalyst complex, such as a copper/N-(n-alkyl)-2-pyridylmethanimine, they will not provide a controlled polymerization with a catalyst complex that is a million times more active, such as a complex formed with Me6TREN. This difference does not mean a different mechanism is operating only that process conditions have to take into account the reaction parameters.
See Starting Points
Other language can, and has been used to describe this process, such a transition metal mediated living radical polymerization or OSET-LRP, but the basic principles underlying all processes using the direct addition or in situ formation of these four components are the same.
One or more of these functions can be combined in a single molecule, e.g. an initiator and monomer, which directly forms a (hyper)branched structure when (co)polymerized.
The general mechanism of ATRP is shown below.
The existence of a radical, (Pn*), here we are using IUPAC nomenclature, has been proposed in copper-mediated ATRP and is based on several experimental observations. (1)
Mechanistically, ATRP is based on an inner sphere electron transfer process, (3) which involves a reversible homolytic (pseudo)halogen transfer between a dormant species, an added initiator or the dormant propagating chain end, (Pn-X) and a transition metal complex in the lower oxidation state (Mtm/Ln) resulting in the formation of propagating radicals (R*) (4) and the metal complex in the higher oxidation state with a coordinated halide ligand (e.g. X-Mtm+1/Ln).
The active radicals form at a rate constant of activation (kact), subsequently propagate with a rate constant (kp) and reversibly deactivate (kdeact), but also terminate (kt). As the reaction progresses, radical termination is diminished as a result of the persistent radical effect, (PRE), (5) and the equilibrium is strongly shifted towards the dormant species (kact << kdeact).
Addition of the persistent radical, (X-Mtm+1/Ln), to the initial reaction medium increases the efficiency of initiation by avoiding the need to form the persistent radical by early stage termination reactions. This results in better polymerization control and higher initiation efficiency.
There are presently several ways to set up the ATRP equilibrium and they will be addressed in greater detail on various pages throughout the site.
The equilibrium can be approached from both sides:
There is abundant evidence that ATRP operates via a radical mechanism, Such as:
Activation rate constants (kact), for a specific ATRP, are typically determined from model studies in which the transition metal complex is reacted with a model alkyl halide in the presence of a radical trapping agent, such as TEMPO. (30) The rates are typically determined by monitoring the rate of disappearance of the alkyl halide in the presence of excess activator (CuIX/Ln) and excess TEMPO, which traps radicals faster than CuIIX2/Ln. Under such conditions, the activation rate constant can be kinetically isolated from the deactivation rate constant and is given by ln([RX]0/[RX]t) = -kact[CuIX/Ln]0t .
The values were also determined for some polymeric systems using GPC techniques and similar kinetic expressions. However, both methods are limited in measuring fast rate constants with the maximum upper limit of approximately 2 M-1s-1. This prompted us to consider the stopped-flow technique for measuring very fast activation rate constants for model systems in copper-mediated ATRP, (31) since the mixing time (ca. 1 ms) and speed of data collection (one full spectrum ca. every 1 ms for diode-array UV-visible spectrophotometers) allow the measurement of pseudo-first order rate constants up to ca. 1.5*102 s-1 (t1/2 = ca. 5 ms). The stopped-flow technique allowed determination of activation rate constants which were previously not accessible due to the limitations in measuring fast reaction rates with current GC and NMR techniques and demonstrated the effectiveness of the stopped-flow technique to measure fast reaction rates opening up a new way to systematically determine activation rate constants and activation parameters for other highly active ATRP complexes. Unpublished results have expanded the range of both techniques and it is possible to push the upper limit of GPC techniques to ~10 M-1s-1 and stop flow measurements to 7.5 *103 s-1.
The activity of the catalyst complexes formed with this spectrum of ligands covers a wide spectrum of activity as detailed in the following pages and summarized in the interactive kinetic data section and in a recent paper (37) and a review on catalyst performance (38) which describes catalyst complexes with different ligands displaying a range of activity exceeding 106.
A similar series of activation rate constants (kact) have been determined for a variety of initiators for Cu-mediated ATRP under the same conditions. (39) The ratio of the activation rate constants for the studied alkyl (pseudo)halides also exceeds 106 times