A radical polymerization method has been a well-known method for polymerizing vinyl monomers to obtain a vinyl polymer. Generally, a radical polymerization method has the disadvantage of the difficulty in controlling the molecular weight of the obtained vinyl polymer. Further, there is the disadvantage that the obtained vinyl polymer is a mixture of compounds having various molecular weights, and thus it is difficult to obtain a vinyl polymer having narrow molecular weight distribution. Specifically, even if the reaction is controlled, the ratio of weight-average molecular weight (Mw) and number-average molecular weight (Mn), (Mw/Mn), can be only reduced to about 2 to 3.
As a method for eliminating the aforementioned disadvantages, since around 1990, a living radical polymerization method has been developed. Specifically, according to the living radical polymerization method, it is possible to control the molecular weight. It is also possible to obtain a polymer having narrow molecular weight distribution. Specifically, a polymer having Mw/Mn of 2 or less can easily be obtained. Therefore, this method has come into the limelight as a method for producing a polymer used in an advanced technology such as nanotechnology.
Oxygen is known as a material which inhibits a radical reaction (polymerization inhibitor) in a radical polymerization method. Therefore, in general, a reaction is carried out in an atmosphere in which there is no oxygen. Also in a living radical polymerization, in general, a polymerization reaction has been carried out while the atmosphere in the reaction vessel is replaced with an inert gas such as nitrogen gas or argon. That is, in order to carry out a living radical polymerization reaction, it has been believed that it is preferred to eliminate the oxygen in the atmosphere as much as possible. Those skilled in the art did not believe that it is possible to use oxygen positively in a living radical polymerization. Further, it could not be expected at all by those skilled in the art that a living radical polymerization can be controlled by controlling the concentration or amount of the oxygen.
Furthermore, it was conventionally believed that it is necessary to add a compound which can control the progression of a polymerization reaction as a catalyst into a reaction solution in order to control a living radical polymerization. Therefore, it was conventionally practiced to control a living radical polymerization by adding a compound which acts as a catalyst into a reaction solution. That is, in a conventional and general living radical polymerization method, a method is performed in which a polymerization is controlled by adding a catalyst into the reaction solution in an inert gas atmosphere.
Catalysts which are currently used in living radical polymerization methods include transition metal complex-type catalysts.
For transition metal complex-type catalysts, complexes in which a ligand is coordinated to a compound having a central metal of Cu, Ni, Re, Rh, Ru, or the like have been used. Such catalysts are described in the following documents for example.
Patent Document 1 (Japanese Laid-open Publication No. 2002-249505) discloses that a complex in which Cu, Ru, Fe, Ni or the like is a central metal, is used as a catalyst.
Patent Document 2 (Japanese Laid-open Publication No. 11-322822) discloses that a hydrido rhenium complex is used as a catalyst.
Non-Patent Document 1 (Journal of The American Chemical Society 119, 674-680 (1997)) discloses that a compound in which 4,4′-di-(5-nonyl)-2,2′-bipyridine is coordinated with copper bromide, is used as a catalyst.
However, when such a transition metal complex catalyst is used, it is necessary to use a large amount of the catalyst. This is disadvantageous as it is not easy to completely remove the large amount of the catalyst used, from the products after the reaction. Another disadvantage is environmental problems which may occur by the disposal of the catalyst. The transition metal for the living radical polymerization method includes many toxic metals. The disposal of a large amount of such toxic metals causes environmental problems. Furthermore, there are cases where toxicities of catalysts remaining in products cause environmental problems. Due to the toxicity, it is difficult to use the transition metal catalysts for the production of food packages, material for living body, and medical material. Additionally, there is a problem associated with a high electroconductivity of the transition metal remaining in polymer, rendering the polymer conductive and hence unsuitable for use in electronic material such as resist material, organic electrochemical luminescence material, fuel cell, solar cell, lithium-ion cell. Furthermore, the transition metal-type catalysts do not dissolve in a reaction solution unless they form a complex. Therefore, it is necessary to use a ligand as an additive to form a complex. This causes problems, i.e., an increase of the cost of production and also an increase of the total weight of the catalyst used. Further, a ligand is usually expensive and requires a complicated synthesis method. Furthermore, the polymerization reaction requires a high temperature (for example, 110° C. or higher). (For example, in aforementioned Non-patent document 1, the polymerization reaction is performed at 110° C.).
It is noted that a living radical polymerization methods, which do not require a catalyst, have also been known. For example, a nitroxyl-type method and dithioester-type method have been known. However, these methods have the following disadvantages. A special protecting group must be introduced to the polymer growing chain. The protecting group is very expensive. Further, the polymerization reaction requires a high temperature (for example, 110° C. or higher). Further, the produced polymer is likely to have undesirable properties. For example, the produced polymer is likely to be colored differently from the natural color of the polymer. Further, the produced polymer is likely to have an odor.
On the other hand, Non-Patent Document 2 (Polymer Preprints 2005, 46(2), 245-246) and Patent Document 3 (Japanese Laid-open Patent Publication No. 2007-92014) disclose that compounds having Ge, Sn or the like as a central metal are used as catalysts. Patent Document 4 (International Publication WO2008/139980) discloses that compounds having nitrogen or phosphorus as a central metal are used as catalysts. Non-Patent Document 3 (Polymer Preprints 2007, 56(2), 2452, The Society of Polymer Science, Japan, 56th Symposium on Macromolecules) discloses that a compound having a central metal of phosphorous is used as a catalyst.
In regard to the copper complex catalyst described in Non-Patent Document 1, the cost for the catalyst required to polymerize 1 kg of a polymer sums up to approximately several thousand yen. On the other hand, in regard to a germanium catalyst, the cost is cut down to about one thousand yen. Thus, the inventions of Non-Patent Document 2 and Patent Document 3 markedly decrease the cost for the catalyst. However, in order to apply living radical polymerization to general-purpose resin products and the like, a further less expensiveness is demanded. The inventions of Non-Patent Document 3 and Patent Document 4 further decreased the cost for a catalyst.
However, there is no description in Patent Documents 1 to 4 and Non-Patent Documents 1 to 3 regarding a method for obtaining a polymer having narrow molecular weight distribution by controlling the living radical polymerization without using a catalyst.
As such, in the prior art, it was believed that when an inexpensive and general-purpose protecting group such as halogen is used, it is absolutely necessary to add some compound, which act as a catalyst for reversibly producing a radical from a dormant species, into a reaction solution. The reason for it is as follows. It was believed that since the fundamental principle of a living radical polymerization is to reversibly produce a radical from a dormant species, it is a natural prerequisite to add a compound which reversibly controls the production of a radical from a dormant species into the reaction solution for controlling the reaction. That is, it was a common technical knowledge that it is impossible to perform a living radical polymerization without adding a catalyst when an inexpensive and general-purpose protecting group such as halogen is used. The aforementioned Patent Documents 1 to 4 and Non-Patent Documents 1 to 3 are all described based on such a common technical knowledge.