Porphyrins polymerization processes

Another very important visible light-initiated reaction of alkyl aluminum porphyrins is their 1,4-addition to alkyl methacrylates to produce ester enolate species [Eq. (4)]. This enolate then acts as the active species in the subsequent polymerization of the acrylate monomer. For example, Al(TPP)Me acts as a photocatalyst to produce polymethylmethacrylate with a narrow molecular weight distribution in a living polymerization process [Eq. (4)]. Visible light is essential for both the initiation step (addition of methylmethacrylate to Al(TPP)Me) and the propagation... 

C. Alkylaluminum Porphyrins as Initiators for Living Polymerization Processes... 

Recently, Takeuchi and coworkers [37] reported the use of molecular imprinting for constructing a highly specific porphyrin-based receptor site. 9-Ethyladenine [37] was chosen as the imprint molecule. Two different functional monomers were utilized to bind 58 during the polymerization process, methacrylic acid (MAA) and a polymerizable zinc-porphorin derivative, 59 (P-53), as shown in Fig. 20. Reference polymers imprinted with 56 were fabricated using either 59 or MAA (P-54 and P-55, respectively) and corresponding nonimprinted, blank polymers were prepared using MAA and 59, MAA, or 57 as functional monomers to form polymers P-56, P-57, and P-58, respectively (Fig. 21). 

From a general point of view, electropolymerization was performed starting from porphyrins decorated with different electropolymerizable units that permit the polymerization process to occur (see reviews on thienyl- [28], pyrrole- [29, 30] or other aryl-appended porphyrins [31, 32] and references herein). An unusual example found in literature concerns the electropolymerization of thienyl units used as ligand of a phosphorus atom complexed in the porphyrin core, this specific electropolymerization process led to one-dimensional poly thiophene wires containing phosphorus porphyrins [33]. 

Regarding the number of SBF units linked to the porphyrin, it only has a weak influence on the porphyrin electrochemical behaviour (but will have important consequences on the polymerization processes, see Sect. 4). Indeed, the CVs recorded for the free porphyrins (SBF)PH2 and (SBF>2PH2 are similar to that of (SBF)4PH2 with only a very small shift in potential (0.59 1.06 for (SBF)PH2 0.6 1.05 V for (SBF>2PH2 and 0.58 0.91 for (SBF)4PH2). 

Without going deeply in the different electropolymerization conditions, one may highlight that the more efficient polymerization was observed for (SBF)4-decorated porphyrins showing the influence of the number of SBF units on the polymerization process. Comparing the nature of the porphyrin, metaUated or not, all (SBF)4P anodic polymerization occurred in significant yields when the accurate potential limit determined along CVs studies was reached. This polymerization is very efficient and independent of the metal inserted within the porphyrin. 

The procedure for electropolymerization of porphyrins presented above required a first step consisting in the synthesis of starting monomeric subunits, for example ZnOEP(bpy). Even considering the apparently simple electrochemical process described above, the synthesis of the monomers requires lime that can be saved by the use of commercial monomers. In this context, Ruhlmann and coworkers proposed an alternative method called Easy Polymerization Of Porphyrins (EPOP) process involving the direct use of commercial ZnOEP and free 4,4 -bipyridine dissolved in the electrolytic solution (Fig. 16) [141]. As before, the electropolymerization is performed by iterative cychc voltammetiy scans. 

This is clo.sely related to the Tertiary radical synthesis" scheme for the preparation of organocobalt porphyrins, in which alkenes insert into the Co—H bond of Co(Por)H instead of creating a new radical as in Eq. (13). If the alkene would form a tertiary cobalt alkyl then polymerization rather than cobalt-alkyl formation is observed. " " " The kinetics for this process have been investigated in detail, in part by competition studies involving two different alkenes. This mimics the chain transfer catalysis process, where two alkenes (monomer and oligomers or... 

Maldotti (96) studied the kinetics of the formation of the pyrazine-bridged Fe(II) porphyrin shish-kebab polymer by means of flash kinetic experiments. Upon irradiation of a deaerated alkaline water/ethanol solution of Fe(III) protoporphyrin IX and pyrazine with a short intense flash of light, the 2 1 Fe(II) porphyrin (pyrazine)2 complex is formed, but it immediately polymerizes with second-order kinetics. This can be monitored in the UV-Vis absorption spectrum, with the disappearance of a band at 550 nm together with the emergence of a new band due to the polymer at 800 nm. The process is accelerated by the addition of LiCl, which augments hydrophobic interactions, and is diminished by the presence of a surfactant. A shish-kebab polymer is also formed upon photoreduction of Fe(III) porphyrins in presence of piperazine or 4,4 -bipyridine ligands (97). 

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