Summary: A supported and highly recyclable catalyst complex, CuBr/HMTETA physically adsorbed to silica gel, was used for the ATRP of MMA to elucidate the nature of the catalytic site. In some polymerizations, the reaction solutions were filtered and compared with their unfiltered references for catalytic activity. The filtered systems had high catalyst activity indicating the presence of active catalyst sites in solution. These sites are the primary catalytic contributors.

ABSTRACT:Atom transfer radical polymerization (ATRP) is a controlled/living radical polymerization process developed a decade ago that allows the synthesis of tailored macromolecules. It has been widely used in the laboratory for polymer synthesis since but little use has been made of it at the industrial scale for polymer production. This is due to the low activity of the ATRP catalyst that is central to the process. Much work has been done over the years to overcome this challenge, and the greatest successes have been achieved through catalyst supporting and recycling. We present here a historical account of the development of supported ATRP catalysts while shedding light on their present and future challenges.

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The solubilities of heterogeneous atom transfer radical polymerization (ATRP) catalysts are determined for the first time and used to calculate previously unquantified ATRP equilibrium constants (K ATRP). These new data are essential for understanding, modeling, and designing ATRP processes with low catalyst concentrations. K ATRP values for CuIBr/1,1,4,7,10,10-hexamethyltriethylenetetramine (CuIBr/HMTETA) and CuIBr/N,N,N‘,N‘,N‘ ‘-pentamethyldiethylenetriamine (CuIBr/PMDETA) are 8.66 × 10-6 and 1.44 × 10-6, respectively. The limited solubility of the CuIBr/HMTETA catalyst explains why large reductions in metal salt concentration can be made without affecting polymerization rates. Catalyst solubility and polymerization rate increase with ligand concentration. In contrast, increasing the metal salt concentration in excess of the ligand's causes a drop in catalyst solubility. This unexpected observation is attributed to the formation of insoluble catalyst networks (gels). The solubility data point to differences in the ionic character of the catalyst complexes formed. The following solubility trend is observed at ATRP conditions (toluene, 90 °C):  CuIBr/PMDETA ≫ CuIIBr2/PMDETA > CuIIBr2/HMTETA > CuIBr/HMTETA.

Supported catalysts are used in atom transfer radical polymerization (ATRP) to facilitate catalyst recovery and recycling. These catalysts consist of catalytic sites that are covalently tethered to larger supporting particles. It is generally believed that supported ATRP is a surface-mediated polymerization process; that is, both activation and deactivation reactions take place at the surface of the particles. However, recent experiments show that this may not be the case. This paper provides a theoretical analysis to test the concept of surface-mediated ATRP. It is found that deactivation at the surface is unlikely. The geographic isolation of catalytic sites, rather than polymer diffusivity limitation, is primarily responsible for this infeasibility. A trace amount of free catalyst in solution that minimizes the geographic isolation is essential for mediating supported ATRP.