Polymerizations with Metalloporphyrins

Aluminum porphyrins initiate controlled ring-opening polymerizations of oxiranes [67-69] ]3-lactones [70-72], 5-valerolactone [74], -caprolactone [74] and D-lactide [75], as well as controlled addition polymerizations of methacrylates [76] and methacrylonitrile [77] (Eq. 15). As shown in Eq. (16), propagation occurs by a coordinative anionic mechanism 

III Molecular Engineering of Side Chain Liquid Crystalline Polymers 

Although the aluminum porphyrin initiated polymerizations take days or weeks to go to complete monomer conversion in the presence of a proton source, their rate is increased substantially by adding a sterical-ly hindered Lewis acid such as methylalu- 

In contrast to the heterocyclic polymerizations, (TPP)AICI does not initiate polymerization of methacrylates [76] and me-thacrylonitrile [77], whereas (TPP)AlMe initiation requires irradiation by visible light. These extremely slow polymerizations are also accelerated by addition of 

Photoirradiated (TPP)AlMe was used to polymerize 6-[4 -(4 -methoxyphenoxy-carbonyl)phenoxy]hexyl methacrylate [90], 6-[4 -(4 -n-butoxyphenoxycarbonyl)phen-oxylhexyl methacrylate [91] and 6-[4 -(4 -cyanophenoxycarbonyl)phenoxy]hexyl methacrylate [92] in order to determine the effect of molecular weight and tacticity [(rr) =0.75]. Although high speed conditions [84-89] were not used, the polymerizations reached 76-92% conversion in 10 h to produce polymers with the expected molecular weight (DP 35) and pdi= 1.09-1.35. In contrast, that of 6-[4 -(4 -cyanophenyla-zo)phenoxy] hexyl methacrylate (Eq. 18) 

---

INOUE AND AIDA Polymerization with Metalloporphyrin Catalysts... 

Table 1. Monomers for Controlled Polymerization with Metalloporphyrins...

Compared to polymerization with metalloporphyrins of nontransition metals, those with transition-metal complexes have not been well explored to date. Nevertheless, some metalloporphyrins of transition metals have been found to serve as initiators for controlled polymerization. Examples include manganese porphyrins for controlled ring-opening polymerization and organo-cobalt and -rhodium porphyrins for controlled addition polymerization. " -... 

From propylene oxide these catalysts yield crystalline, isotactic polymers. Living polymerizations with metalloporphyrin derivatives are difficult to terminate and are therefore called by some immortal Catalysts like (C6H5)3-SbBr2-(C2H5)3N in combination with Lewis acids also yield crystalline poly(propylene oxide). Others, like pentavalent organoantimony halides, are useful in polymerizations of ethylene oxide. 

Control of Addition and Ring-Opening Polymerizations with Metalloporphyrin Catalyst... 

The activity of metalloporphyrins as initiator is dependent on (1) nucleophilicity of metal-axial ligand bond. (2) Lewis acidity of metal. (3) electronic and steric effect of porphyrin group, and (4) photo-excitation of porphyrin group. In this article will be described some of the recent advances in our studies on polymerizations with metalloporphyrins. 

Wang E., Meyerhoff M.E., Anion selective optical sensing with metalloporphyrin-doped polymeric films, Anal. Chim. Acta 1993, 283 673. 

In contrast to the above polymerizations via anionic and/or coordination anionic mechanisms, radical polymerization initiated with metalloporphyrins remains to be studied. The only example of controlled radical polymerization by metalloporphyrins has been reported by Wayland et al. where the living radical polymerization of acrylic esters initiated with cobalt porphyrins was demonstrated. In this section the radical polymerization of MMA initiated with tin porphyrin is discussed. 

Takeuchi D, Watanabe Y, Aida T, Inoue S (1995) Maclomolecules 28 651 Recent reviews (a) Aida T (1994) Prog Polym Sci 19 469 (b) Inoue S, Aida T (1998) Controlled polymer synthesis with metalloporphyrins. In Vogl O, Hatada K (eds) Molecular design of polymeric materials. Dekker, New York, in press (a) Kuroki M, Watanabe T, Aida T, Inoue S (1991) J Am Chem Soc 113 5903 (b) Sugimoto H, Kuroki M, Watanabe T, Kawamura C,Aida T, Inoue S (1993) Macromolecules 26 3403 (c) Sugimoto H,AidaT, Inoue S (1994) Macromolecules 27 3672 (d) Sugimoto H, Kawamura C, Kuroki M, Aida T, Inoue S (1994) Macromolecules 27 2013 (e) Akatsuka M, Aida T, Inoue S (1994) Macromolecules 27 2820 Inoue, S, Aida T (1994) Chem tech 24 28... 

A key to achieving living polymerization is to develop well-behaved initiators, because the initiator affects both the relative rate of initiation to propagation and the potential for side reactions. - In this chapter, we focus on the precision synthesis of macromolecules with metalloporphyrins as initiators. Extensive initial studies by Inoue and Aida, - and more recent studies by Wayland et al.have shown that the metalloporphyrins which are applicable to controlled macromolecular synthesis include compounds of aluminum (1-4), zinc (5), manganese (6), cobalt (7) and rhodium (8). The first two nontransition metal metalloporphyrins are effective for anionic (nucleophilic) polymerization while... 

Some monomers give the polymers with narrow molecular weight distribution even in the presence of a protic compound as a chain transfer agent. This is different from living polymerization, and a new concept of immortal polymerization has been presented . Living and immortal polymerizations initiated with metalloporphyrins can provide various types of block copolymers and end reactive polymers . ... 

A polymeric structure can be generated by intermolecular coordination of a metalloporphyrin equipped with a suitable ligand. Fleischer (18,90) solved the crystal structure of a zinc porphyrin with one 4-pyridyl group attached at the meso position. In the solid state, a coordination polymer is formed (75, Fig. 30). The authors reported that the open polymer persists in solution, but the association constant of 3 x 104 M 1 is rather high, and it seems more likely, in the light of later work on closed macrocycles (see above), that this system forms a cyclic tetramer at 10-3 M concentrations in solution (71,73). 

The metalloporphyrin-initiated polymerizations are accelerated by the presence of steri-cally hindered Lewis acids [Inoue, 2000 Sugimoto and Inoue, 1999]. The Lewis acid coordinates with the oxygen of monomer to weaken the C— O bond and facilitate nucleophilic attack. The Lewis acid must be sterically hindered to prevent its reaction with the propagating center attached to the prophyrin structure. Thus, aluminm ortho-substituted phenolates such as methylaluminum bis(2,6-di-/-butyl-4-methylphenolate) accelerate the polymerization by factors of 102-103 or higher. Less sterically hindered Lewis acids, including the aluminum phenolates without ortho substituents, are much less effective. 

In the most important series of polymers of this type, the metallotetraphenylporphyrins, a metalloporphyrin ring bears four substituted phenylene groups X, as is shown in 7.19. The metals M in the structure are typically iron, cobalt, or nickel cations, and the substituents on the phenylene groups include -NH2, -NR2, and -OH. These polymers are generally insoluble. Some have been prepared by electro-oxidative polymerizations in the form of electroactive films on electrode surfaces.79 The cobalt-metallated polymer is of particular interest since it is an electrocatalyst for the reduction of dioxygen. Films of poly(trisbipyridine)-metal complexes also have interesting electrochemical properties, in particular electrochromism and electrical conductivity.78 The closely related polymer, poly(2-vinylpyridine), also forms metal complexes, for example with copper(II) chloride.80... 

In terms of availability, number, and nature of surface groups, surface area, pore size, pore volume, and form and size of the particles, silica has been undoubtedly the most preferred inorganic support. Suitable modification is possible via the surface silanol groups, which can react either directly with an appropriate metal complex or with an intermediate ligand group. Direct surface bonding has often been practiced, e. g., for the anchoring of metal carbonyl complexes [14] (eq. (11)), carbonyl clusters [26], polymerization catalysts [21, 62], or other special systems, e. g., 7r-allyl complexes [63] or metalloporphyrins [64]. 

---