Self-assembly And Application Of Block Copolymer And Metallo-TPPS&Synthesis Of Macrocyclic Copolymer

Metalloporphyrins have received much attention because of their important roleas catalysts and sensitizers in a wide variety of biochemical and photochemicalreactions. For example, photocatalysis, photoredox systems, photochemical synthesis,solar cells and photodynamic therapy. However, ZnTPPS, MgTPPS, CdTPPS, etc, areapt to be demetallized in acidic solutions due to hydrolysis. Besides, ZnTPPS is apt toform aggregates at a high concentration, which leads to fluorescence self-quenchingeffect of the excited state. For these reasons, application of metalloporphyrins asphotosensitizers and photocatalysts is significantly limited. Therefore, it is desirableto enhance the stability of metalloporphyrins against demetallization and aggregation.It is well-known that self-assembled block copolymer micelles can encapsulatesubstances in their core, which can effectively protect the encapsulated substancesagainst invasion by molecules and ions in the solution. Here we report a novel andeffective method to enhance the stability of ZnTPPS in acidic aqueous solutions.Block copolymer PEG-b-P4VP and ZnTPPS form complex micelles in acidicsolutions through electrostatic interaction between positively charged pyridyls onpolymer and negatively charged sulfonte groups on ZnTPPS. The microenvironmentprovided by micelles effectively reduces the opportunities of H+replacing Zn2+, andthus prevents demetallization of the encapsulated ZnTPPS. Furthermore, fluorescencequenching is restrained to a certain extent due to segregation of ZnTPPS in themicelle core.Metallo-meso-5,10,15,20-tetrakis-(4-sulfonatophenyl)porphyrins (metallo-TPPS)have been widely used as photosensitizers. However, their vulnerability tophotodegradation significantly limits their applications. In this contribution, wedemonstrate a method to enhance the photostability of metallo-TPPS while retaintheir photoactivity by encapsulating them in the cores of complex micelles. Poly(ethylene glycol)-b-poly(4-vinylpyridine)(PEG-b-P4VP) and metallo-TPPS canform complex micelles in acidic solution through electrostatic interactions and thenundergo axial coordination with the pyridine moieties of PEG-b-P4VP when the pH isadjusted to7.4. Metallo-TPPS were entrapped in the hydrophobic and compactmicellar core, which effectively prevented photodegradation of the metallo-TPPS thatwould otherwise occur in aqueous solution due to attack by species such as oxygenmolecules. To further understand the mechanism of the improved photostability ofZnTPPS by their comicellization with PEG-b-P4VP in aqueous media, the stability ofmetallo-TPPS in the presence of other block polymers was tested. Complex micelleswere formed using PEG-b-PDMAEMA through electrostatic interactions betweenZnTPPS and DMAEMA under conditions identical to that of PEG-b-P4VP/ZnTPPSsystems. The resulting complex micelles provided some degree of protection, butwere less efficient than PEG-b-P4VP. In addition, the photodebromination of2,3-dibromo-3-phenylpropionate (DPP) sensitized with Zn(II)TPPS has been used as amodel reaction to study the photosensitive activity of ZnTPPS entrapped in complexmicelles. The entrapped ZnTPPS exhibit pronounced activity and have much higherefficiency and faster photosensitive reaction rates compared to free ZnTPPS, due toprotection of ZnTPPS by polymeric micelles and enrichment of DPP in the micellarcore.Poly(2-hydroxyethyl methacrylate)-b-poly(tert-butyl acrylate)-b-poly(2-hydroxyethyl methacrylate)(PHEMA-b-PtBA-b-PHEMA) was synthesized.The hydroxyethyl compounds in the PHEMA blocks were used to react with thecinnamoyl chloride, to magnify the chains. The macrocycles could be prepared by theassociation of PtBA blocks in a selective solvent for the PCEMA block, in which thePCEMA blocks collapsed together to decrease the interfacial energy. The polymerconcentration was kept below its CMC, which to prevent inter-chain coupling. Thenthe PCEMA block was photo-crosslinked to get macrocycles. Synthesis of cyclicmolecular brushes by click reactions between alkyne-modified PAA chains and azido terminated PtBA. The macrocycles and the linear chains of the crosslinked productscan be determined by comparing retention time of the peaks for GPC. Meanwhile, thepopulation of the macrocyclic brushes product would be determined from GPC andAFM images.