C O M M U N I C A T I O N S
Figure 2. (A) Mn and Mw/Mn values of PPP as a function of monomer conversion in the polymerization of 1 with 0.88 mol % of Ni(dppe)Cl2 in THF at
room temperature ([1]0 ) 0.080 M). (B) Mn and Mw/Mn values of PPP, obtained with 1 and Ni(dppe)Cl2 in THF at room temperature, as a function of the
feed ratio of 1 to Ni(dppe)Cl2 ([2]0 ) 0.10 M). [Ni(dppe)Cl2]0 ) 0.7-3.0 mM; conversion of 1 ) 85-95%. (C) GPC profiles of the monomer addition
experiment: (a) prepolymer ([1]0/[Ni(dppe)Cl2]0 ) 34), conversion of 1 ) 76%, Mn ) 8000 and Mw/Mn ) 1.22; (b) postpolymer ([remaining and added
1]0/[Ni(dppe)Cl2]0 ) 49), conversion of 1 ) 80%, Mn ) 20400 and Mw/Mn ) 1.25.
to LiCl, which is effective in the halogen-magnesium exchange
to the synthesis of well-defined polythiophenes, but also to that of
PPP. We have also found that LiCl is necessary for optimizing the
chain-growth polymerization leading to well-defined PPP. Synthesis
of block copolymers of different conjugated polymers by this
polymerization method is now under way.
i
reaction between PrMgCl and electron-rich aromatic halides.13
Accordingly, the polymerization of 1 with the above three Ni
catalysts was carried out in the presence of 1 equiv of LiCl. When
Ni(dppp)Cl2 and Ni(dppe)Cl2 were used, the polymerization
proceeded faster than that without LiCl, the multimodal peaks in
the low-molecular-weight region of the GPC chromatogram disap-
peared, and only a narrow monomodal peak was observed in the
high-molecular-weight region (Table 1, entries 4, 5), as we had
expected. When Ni(dppf)Cl2 was used, LiCl was not effective (entry
6).
Acknowledgment. This work was supported in part by a Grant-
in-Aid (Grants 17550120 and 18750106) for Scientific Research
and a Scientific Frontier Research Project from the Ministry of
Education, Culture, Sports, Science and Technology, Japan, and
Sekisui Chemical Grant Program for Research Projects Based on
Learning from Nature in 2005.
Since the polymerization of 1 with Ni(dppe)Cl2 in the presence
of LiCl gave PPP with the narrowest molecular weight distribution,
the Mn and Mw/Mn values of the crude PPP (without purification
by precipitation or fractionation) at each conversion in this
polymerization were analyzed by GPC to evaluate the polymeri-
zation in detail. The Mn values increased in proportion to the
conversion, and the Mw/Mn ratios were less than 1.18 over the whole
conversion range (Figure 2A), indicating that 1 polymerized in a
chain-growth polymerization manner.14 Furthermore, when the
polymerization of 1 was carried out with various feed ratios of 1
to Ni(dppe)Cl2, the Mn values of the polymer increased linearly in
proportion to the feed ratio (Figure 2B). This polymerization
behavior indicates that PPP with any desired Mn up to at least 30000
can be obtained by appropriately controlling the feed ratio of 1 to
the Ni catalyst, as was the case in catalyst-transfer polycondensation
for polythiophene.5 The crude products contained small amounts
of unreacted 2 and 1-bromo-2,5-dihexyloxybenzene formed by
quenching of 1, and those low-molecular-weight compounds were
easily washed out with MeOH. For example, the crude product
(Mn ) 19000, Mw/Mn ) 1.17), obtained by the polymerization with
[converted 1]0/[Ni(dppe)Cl2]0 of 53, was washed with MeOH to
give pure PPP in 79% yield (Mn ) 19600, Mw/Mn ) 1.14).
The chain-growth nature of this polymerization was also
examined by means of a monomer addition experiment, in which
a fresh feed of 1 was added to the prepolymer (Mn ) 8000, Mw/Mn
) 1.22) in the reaction mixture. As shown in Figure 2C, the GPC
profile of the product clearly shifted toward the higher-molecular-
weight region (Mn ) 20400, Mw/Mn ) 1.25), and the prepolymer
did not remain. This result indicates that the added 1 was
polymerized from the propagating end of the prepolymer owing to
the chain-growth nature of this polymerization, and that block
copolymers composed of different conjugated polymers could be
synthesized by successive addition of a different monomer.
In conclusion, we have demonstrated that catalyst-transfer chain-
growth polymerization with a metal catalyst is applicable not only
Supporting Information Available: Synthesis and polymerization
of monomer 1 and 1H NMR spectrum of PPP. This material is available
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J. AM. CHEM. SOC. VOL. 128, NO. 50, 2006 16013