Porphyrin initiator

Accelerated Living Polymerization of Methacrylonitrile with Aluminum Porphyrin Initiators by the Activation of Monomer... 

Photodynamic therapy (PDT) with various photosensitizers has been proven to be highly efficient in the treatment of skin tumors. However, most valid data are available for ALA only, which has been shown to effectively sensitize solar keratoses and superficial basal cell carcinomas with porphyrins. Initial squamous cell carcinomas also show good response to ALA-PDT. Treatment of psoriasis by PDT is a subject under investigation. During the last decade numerous studies on PDT for dermatological diseases have been published, the more important ones are reviewed here. 

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... 

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-... 

The G-SmA-N-I transition temperatures of syndiotactic poly(6-[4 -(4"- -bu-toxyphenoxycarbonyl)phenoxyl)phenoxy]-hexyl methacrylate prepared by aluminum porphyrin initiated polymerizations also level off at approximately 25 repeat units [91]. Similarly, the glass and nematic-isotropic transition temperatures of poly[6-(4 -methoxy-4"- Z-methylstilbeneoxy)hexyl methacrylate] prepared by group transfer polymerization become independent of molecular weight at approximately 20 repeat units [48]. Both polymethacrylates reach the same transition temperatures as the corresponding polymers prepared by radical polymerizations, which have nearly identical tacticities. 

Sugimoto, H., Saika, M., Hosokawa, Y., Aida, T., and Inoue, S., 1996, Accelerated Living Polymerization of Methacrylonitrile with Aluminum Porphyrin Initiators ity Activation of Monomer or Growing Species. Controlled Synthesis and Properties of Poly(medtyl methactylate-b-methaciylonitrile)s, Macromolecules, 29 3359... 

The wide applicability of aluminum porphyrin initiators (1) leads to a variety of tailored block copolymers such as polymethacrylate-polyether and polymethacrylate-polye-ster, as well as polymethacrylate-polymethacrylate and polymethacrylate-polyacrylate, that can be synthesized by sequential living polymerization of the corresponding monomers.- For example, when 1,2-epoxypropane (11, R = Me) is added to a polymerization mixture of methyl methacrylate (21, R = Me) with la at 100% conversion of 21, the polymerization of 11 takes place from the enolate growing end (32 ) to give a narrow MWD polymethacrylate-polyether block copolymer having an alcoholate growing terminal (Table 4). Likewise, the aluminum enolate species (32 ) can also react with lactones (14,15), thereby allowing the formation of a poly(methyl methacrylate)-polyester block copolymer with narrow MWD. 

The principle of immortal polymerization indicates the possibility of recycling the aluminum porphyrin initiator. In order to facilitate isolation of the polymer and regeneration of the initiator, an aluminum porphyrin bound to a cross-linked polystyrene (4) has been developed. - For example, the immortal polymerization of 1,2-epoxypropane (11, R = Me) initiated with 4 can be repeated 9 times without any loss of the initiator activity and without any broadening of polymer MWD. 

Recently, it was found that Lewis acid assisted polymerizations of methyl methacrylate with aluminum porphyrin initiators yield living polymers. The polymerizations of methacrylate esters with al laluminum porphyrin initiators occur through formations of enolate aluminum porphyrin intermediate as the growing species. For the sake of illustration, the methylaluminum porphyrin molecules (see Fig. 3.3) can be designated as... 

Sugimoto, H. Kuroki, M. Watanabe, T. Kawamura, C. Aida, T. Inoue, S. High-speed living anionic polymerization of methacrylic esters with aluminum porphyrin initiators. Organoaluminum compounds as Lewis acid accelerators. Macromolecules 1993, 26, 3403-3410. 

Sugimoto, H. Saika, M. Hosokawa, Y. Aida, T. Inoue, S. Accelerated living polymerization of methacrylonitrile with aluminum porphyrin initiators by activation of monomer or growing species. Controlled synthesis and properties of poly(methyl methacrylate-i>-methacrylonitrile)s. Macromolecules 1996, 29, 3359-3369. 

Inoue and co-workers were the first to report the use of bulky Lewis acid as additives to aluminum porphyrin initiators to enhance the polymerization rate. This was shown in particular for methacrylic esters and For... 

Scheme 36 Bulky Lewis acid (MAIBP) used with aluminum porphyrin initiators for the monomer-activated polymerization of epoxides.

Braune and Okuda have shown the possibility to substitute porphyrin initiators by simpler systems based on the association of ammonium salts with a bulky aluminum bis(phenolate) electrophile. According to their study, the ring-opening polymerization of PO cannot occur at simple Lewis add centers, but that nucleophilic ate complexes must be present at the same time. However, so far, only the synthesis of PPO oligomers with MWs less than 5000 has been reported for this system. The important contribution of Braune and Okuda is that they confirmed the Vandenberg binudear mechanism earlier proposed for epoxide polymerization (see Figure 12). °- ... 

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