Photochemical routes to hyperbranched polymers

Light plays an important role in many processes in nature, such as atmospheric ozone production and photosynthesis. In the light-induced reactions, absorption of light generates excited state species that may take place in a number of different reactions such as isomerization, dissociation, abstraction, electron transfer and bond formation. Such light induced processes represent several advantages over conventional thermal techniques regarding the level of control since they can be accurately moderated in terms of wavelength, polarization direction and intensity, allowing spatial and temporal of the reactions. Light is used also in polymerization reactions following different mechanisms including radical, cationic, anionic and step-growth polymerization routes. In recent years, light induced processes are successfully employed also in controlled/living polymerization and click reactions for the formation of various complex macromolecular architectures. Hyperbranched polymers (HBPs), a unique class of dendritic macromolecules, have received continuous interest from macromolecular scientists due to their inherent properties such as high level of functional terminal units, high solubility and low viscosity. Despite enormous efforts devoted to the synthesis of HBPs by traditional methods, there are limited attempts to employ light induced processes. Until now, numerous strategies for the synthesis of HBPs have been reported. Depending on the number of monomers used in the polymerization, they can be classified in two categories; single-monomer methodology (SMM) and double-monomer methodology (DMM). SMM is the synthetic method that allows the syntheses of hyperbranched macromolecules by polymerization of only one type of monomers (ABn or latent ABn type). The SMM method covers various specific approaches: (a) polycondensation of ABn monomers; (b) self-condensing vinyl polymerization (SCVP); (c) self-condensing ring-opening polymerization (SCROP) and proton-transfer polymerization (PTP) and (d) chain-walking polymerization; and (e) dialkyne polycyclotrimerization. In 1995, Fréchet et. al. reported a new versatile synthetic strategy, SCVP which considers the polymerization of AB* type vinyl monomers. This strategy can also be defined as 'inimer route'. Inimers (AB*) are specific vinyl monomers consisting of pendant groups that can be converted into an initiating moiety by an external stimulus. In AB*structure, the A group serves as a vinyl moiety and the B* group signifies the initiating site. The chain initiation is realized through the addition of an initiating B* group to the double bond of another monomer. Hence, a dimer form comprised of two active sites B* and A* with one double bond is generated. The created dimer acts as an AB2 monomer, and lead to yield hyperbranched structure. This method fundamentally relies on the difference between the activities of chain propagation of the growth sites and the initiating sites. This behavior results a lower degree of branching (DB) in comparison to the DB of the HBP formed by polycondensation of AB2 monomers. The theoretical maximum DB of SCVP reaches 46.5%. The SCVP approach has been readily applicable to diverse type of controlled polymerization methods, such as living ionic, group transfer polymerization, ring opening and controlled radical polymerization (CRP) by suitably selected inimer structure variations. Taking account of the unique advantages of SCVP, we focused on the development of light induced processes for the synthesis of hyperbranched polymers with various architectures.In the first part of the thesis, a self-condensing vinyl polymerization for the preparation of hyperbranched polymers is explored on the basis of Type II photoinitiated system using benzophenone (BP) and camphorquinone (CQ) under UV and visible light. In this approach, 2-hydroxy ethyl methacrylate (HEMA) or 2-(dimethylamino)ethyl methacrylate (DMAEMA), and methyl methacrylate (MMA) were used as hydrogen donating inimers and comonomer, respectively.In the second part, branched polymers with different branching densities and their cross-linked analogues were synthesized by photoinduced self-condensing vinyl xxi polymerization (SCVP) via benzodioxinone photochemistry. Thus, MMA was copolymerized with two different comonomers, namely HEMA and DMAEMA in the presence bisbenzodioxinone (BBNZ) under UV light. Upon irradiation, BBNZ undergoes irreversible decomposition leading to the formation of benzophenone photosensitizer and bisketene. The released benzophenone is further photoexcited at the same wavelength to give the initiating radicals through hydrogen abstraction from the inimer. In the case of HEMA, additional branching sites were formed by the reaction of bisketen with the hydroxyl functionalities of HEMA.Finally, a novel photoinduced metal free ATRP approach was developed for the preparation of HBPs by SCVP. The approach relies on the copolymerization of MMA and styrene (S) with respective structurally antagonist inimers; 2-(2- bromoisobutryryloxy)ethyl methacrylate (BIBEM) and 4-vinylbenzylbromide (VBB) in the presence of perylene under visible light. In the process, excited state perylene directly reduces alkyl bromides to form carbon-centered radicals capable of iniaiting the polymerization of MMA and S, while BIBEM and VBB inimers contain both polymerizable vinyl moiety and bromide groups to generate branching points. As the halide groups remain at the chain ends, the methodology was shown to be a suitable candidate to form core-shell type block copolymers carrying hyperbranched and linear structures at the inner and outer spheres, respectively.