4.2 Star Polymer
Star-shaped condensation polymers have been prepared by copolycondensation of
an An monomer with an AB-type monomer or with A 2 and B 2 monomers [63–66],
in which the arm lengths of the polymers obtained are not controlled. Well-defined
star-shaped condensation oligomers have been synthesized by the coupling reaction
between an An monomer and linear oligomers with monodispersity, which were
prepared by a sequential condensation procedure [67–69]. We synthesized starshaped aromatic polyamides with low polydispersity by a core-first method, which
was the CGCP of 14a from 1,3,5-tris(4-phenyloxycarbonylbenzyloxy)benzene (28)
having the benzyloxy spacers (Scheme 30) [70]. The benzyloxy linkages of the core
of the star polymer were cleaved by hydrogenolysis to yield a polymer with low
polydispersity. The M n of this polymer was one-third of that of the star polymer,
indicating that the star polymer possesses exactly three arm chains of a uniform and
controlled length. However, the polymerization at higher feed ratios of [14a] 0 /[28] 0
afforded not only the three-armed polymer but also a linear polymer formed by selfpolycondensation of 14a. In the polymerization of 14a with a monofunctional initiator, no self-polycondensation of 14a takes place as long as the feed ratio of [14a] 0 /
[initiator] 0 is 100 or less [20]. In the polymerization with trifunctional initiator 28,
however, the self-polycondensation occurred even at the feed ratio of ([14a] 0 /[initiator
site of 28] 0 ) 33, which is much less than 100. Easy occurrence of the selfpolycondensation in the polymerization of 14a with multifunctional initiator 28 is
presumably ascribed to the low local concentration of the initiator site in the whole
solution except for the area around 28. Thus, monofunctional initiators homogeneously exist in the solution, whereas multifunctional initiators occur in both the
area of high local concentration of the initiator units and the area of low local
concentration where self-polycondensation would be liable to occur.
Accordingly, we optimized polymerization conditions for the synthesis of star
polyamides by CGCP with LiHMDS by using the porphyrin-cored tetrafunctional
initiator 29 (Scheme 31) [71]. Since the target star polymer has an absorption at
430 nm due to the porphyrin ring, whereas the linear polymer formed as a
Scheme 29 Synthesis of amphiphilic linear-hyperbranched block copolymer of PEG and HBPA
214
Y. Ohta and T. Yokozawa
Star-shaped condensation polymers have been prepared by copolycondensation of
an An monomer with an AB-type monomer or with A 2 and B 2 monomers [63–66],
in which the arm lengths of the polymers obtained are not controlled. Well-defined
star-shaped condensation oligomers have been synthesized by the coupling reaction
between an An monomer and linear oligomers with monodispersity, which were
prepared by a sequential condensation procedure [67–69]. We synthesized starshaped aromatic polyamides with low polydispersity by a core-first method, which
was the CGCP of 14a from 1,3,5-tris(4-phenyloxycarbonylbenzyloxy)benzene (28)
having the benzyloxy spacers (Scheme 30) [70]. The benzyloxy linkages of the core
of the star polymer were cleaved by hydrogenolysis to yield a polymer with low
polydispersity. The M n of this polymer was one-third of that of the star polymer,
indicating that the star polymer possesses exactly three arm chains of a uniform and
controlled length. However, the polymerization at higher feed ratios of [14a] 0 /[28] 0
afforded not only the three-armed polymer but also a linear polymer formed by selfpolycondensation of 14a. In the polymerization of 14a with a monofunctional initiator, no self-polycondensation of 14a takes place as long as the feed ratio of [14a] 0 /
[initiator] 0 is 100 or less [20]. In the polymerization with trifunctional initiator 28,
however, the self-polycondensation occurred even at the feed ratio of ([14a] 0 /[initiator
site of 28] 0 ) 33, which is much less than 100. Easy occurrence of the selfpolycondensation in the polymerization of 14a with multifunctional initiator 28 is
presumably ascribed to the low local concentration of the initiator site in the whole
solution except for the area around 28. Thus, monofunctional initiators homogeneously exist in the solution, whereas multifunctional initiators occur in both the
area of high local concentration of the initiator units and the area of low local
concentration where self-polycondensation would be liable to occur.
Accordingly, we optimized polymerization conditions for the synthesis of star
polyamides by CGCP with LiHMDS by using the porphyrin-cored tetrafunctional
initiator 29 (Scheme 31) [71]. Since the target star polymer has an absorption at
430 nm due to the porphyrin ring, whereas the linear polymer formed as a
Scheme 29 Synthesis of amphiphilic linear-hyperbranched block copolymer of PEG and HBPA
214
Y. Ohta and T. Yokozawa