Block copolymers porphyrin polymers

Pyridine in place of methylaluminum bis(2,6-di-tert-butyl-4-methylphenolate) (3e) was also effective for the polymerization of MAN from the living PMMA (2). An example is shown by the polymerization of MAN under irradiation with the 2 (Mn=4000,Mw/Mn=1.09 prepared with [MMA]o/[l]o=40,100% conversion)-pyridine system at the initial mole ratio [MAN]q/[2] q/[pyridine]q of 100/1.0/0.6. MAN was polymerized up to 63% conversion in 65 h [Fig. 25 ( )], where the GPC peak of the polymer formed was observed to shift clearly towards the higher molecular-weight region (Mn=7600), retaining the narrow MWD (Mw/Mn=1.26), and the peak corresponding to the prepolymer of MMA was not observed. These facts clearly demonstrate the successful polymerization of MAN from 2, affording a PMMA-PMAN block copolymer. In sharp contrast to the polymerization of MAN, polymerization of MMA with aluminum porphyrin 2 was retarded by pyridine. For example, in the presence of 2 equiv of pyridine with respect to 2, the polymerization of 100 equiv of MMA proceeded very slowly to attain 25% conversion in 18 h under irradiation, while in the absence of pyridine, the polymerization of MMA with 2 was complete within 12 h under otherwise identical conditions. 

Moreover, the reactivation of a cobalt-terminated polymer in the presence of second monomer leads to block copolymerization. In this respect, CMRP has aheady contributed to the preparation of the valuable copolymers listed in Table 4.1. For example, well-defined poly(acrylate) block copolymers were prepared via a sequential polymerization of acrylic monomers with cobalt porphyrin la or cobaloximes 2 [14, 20]. The synthesis of well-defined poly (acrylate)-b-poly(VAc) block copolymers was also achieved with complex la [26]. Co(acac)2 (3a see Figure 4.1) is the most prolific complex for the preparation of block copolymers, until now. Indeed, the sequential CMRP of VAc with NVP [33], AN [48], or vinyl pivalate (VPi) [49] leads to the corresponding block copolymers, in controlled fashion. Throughout the polymerization, the experimental conditions were necessarily adjusted, taking into consideration the reactivity of the second monomer. As an illustration of this, well-defined PVAc-b-poly(acrylonitrile) (PAN) copolymers could only be prepared via a bulk polymerization of VAc at 30 °C, followed by the AN polymerization at 0°C in solution in DMF [48]. In this case, the DMF not only serves as the solvent but also binds the metal and adjusts its reactivity. As a rule, the PVAc sequences of these copolymers were hydrolyzed in order to provide poly(vinyl alcohol) (PVA)-containing derivatives, such as hydrosoluble PVA-b-poly... 

A described example is a reaction conducted at 60°C in deoxygenated benzene, using neopentylcobalt with tetramesityl-porphyrin ligand and methyl acrylate monomer [236]. A slow polymerization yields 66% conversion in 38 h. The product is a narrow molecular weight distribution polymer of = 144,000. The polymerization is even slower with less hindered phenyl substituents on the porphyrin ligand. Both homopolymers and block copolymers can be formed. 

Multifunctional initiators based on, for example, cyclotriphosphazine [106], silesquioxane [107], porphyrin [108] and bipyridine metal complex [109, 110] cores were also successfully used for the living cationic ring-opening (co)polymerization of 2-oxazolines, resulting in star-shaped (co)polymers. The use of polymeric initiators also allowed the construction of well-defined complex macro molecular architectures, such as triblock copolymers with a non-poly(2-oxazo-line) middle block that is used to initiate the 2-oxazoHne polymerization after functionalization with tosylate end-groups [111-113]. In addition, poly(2-oxazoline) graft copolymers can be prepared by the inihation of the CROP from, for example, poly(chloromethylstyrene) [114, 115] or tosylated cellulose [116]. 

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