Photoinitiated atom transfer radical polymerization

The ATRP is one of the most frequently used controlled radical polymerizations due to its simplicity, broad applicability and ability to prepare previously inaccessible well-defined polymers with complex architecture. This process is based on a redox reaction involving a transition metal complex in which a halide atom (especially Cl or Br) is reversibly transferred between a growing radical and a dormant species. It is noted that the atom transfer step is the key step in the reaction responsible for uniform polymer chain growth. The ATRP was independently reported by the Matyjaszewski, Sawamoto and Percec groups in 1995 [WAN 95a, WAN 95b, KAT 95, PER 95]. 

A series of well-defined PMMA were successfully prepared by reduction of Cu(II) to Cu(I) upon UV irradiation and subsequent activation of alkyl halide. A linear relationship was observed between monomer consumption and polymerization time and evolution of molecular weight versus 

MMA could be initiated in the absence of alkyl halide, loss of control over the polymerization process was observed. The type and concentration of photoinitiators directly effects the photoinduced reverse ATRP. Photoinitiated SR NI ATRP was successfully applied to MMA in the presence of alkyl halide. The molecular weights increased with conversion, and they were in good agreement with the theoretical values (Diagram 3.4) [TAS 1 la, DAD 14b, DON 08], 

Diagram 3.4. Proposed mechanism for dye-sensitized SR NI ATRP using Type I or Type IT photoinitiating systems (PI photoinitiator PS photosensitizer and R-H hydrogen donor) 

Diagram 3.5. Mechanistic scheme for photoinduced reverse ATRP using camphorquinone/benzhydrol 

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As mentioned above, the cleavage of Si-Si-bonds in polysilanes upon irradiation with UV-light is very efficient and results in the formation of silicon radicals. These radicals react with olefins to initiate radical polymerizations. Especially the polymerization of methylmethacrylate and styrene with a variety of polysilanes as photoinitiators has been studied in detail [104]. The advantage of this kind of initiation is the possibility to prepare polysilane-polyolefin hybrids [105]. All modifications of radical polymerizations, such as the atom transfer radical polymerization (ATRP), are possible with polysilanes as photoinitiators [106]. 

Photo-induced polymerization of hydroxypropyl-P-CD/methyl methacrylate complex, i.e. HP-P-CD/MMA was performed in aqueous solution, at room temperature with Irgacure 2959, i.e. 4-(2-hydroxyethoxy)phenyl-(2-hydroxy-2-propyl)ketone serving as a photoinitiator [75]. The atom transfer radical polymerization (ATRP) and reversible addition fragmentation chain transfer (RAFT) [76] methods are also used to perform polymerization of the CD/guest monomer complexes in aqueous solutions. 

Tertiary amines with an a-hydrogen are among the most effective electron donors other electron donors include alcohols, amides, amino acids, and ethers. A third process, direct hydrogen atom transfer from RH to the ketone, is not common hut does occur with some photoinitiators. The overall result is the same as the electron-transfer process. Although two radicals are produced by photolysis of the photoinitiator, only one of the radicals is typically active in initiation—the aroyl and amine radicals in Eqs. 3-48 and 3-49, respectively. The other radical may or may not initiate polymerization, hut is active in termination. The decrease in photoinitiator concentration during polymerization is referred to as photo-bleaching. 

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