JP2000143728A - Vinyl ester oligomer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain the subject oligomer capable of giving highly adhesive crosslinked products by designing itself to use a specific alkyl vinyl ester as the main component and bear carboxyl groups on both the molecular terminals. SOLUTION: This oligomer with a number-average molecular weight of 500-100,000 is obtained by designing itself to use a 1-20C, pref. 1-14C alkyl vinyl ester as the main component and bear carboxyl groups on both the molecular terminals; wherein the 1-20C alkyl vinyl ester is e.g. vinyl acetate, vinyl formate, vinyl propionate, vinyl butyrate; for introducing carboxyl groups onto both the terminals of this oligomer, the following process may be cited: the alkyl vinyl ester is polymerized in the presence of an iodine compound and a radical source and the iodine atoms on the terminals being in growth are substituted by a carboxyl-bearing chain transfer agent such as 2,2'-dithioglycolic acid.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a vinyl ester oligomer having functional groups at both ends.

[0002]

2. Description of the Related Art In recent years, in view of environmental problems, demands for solvent-free in the fields of adhesives, sealing materials, paints, sealing materials and the like have been increasing. Therefore, a low molecular weight polymer (oligomer) having a crosslinkable functional group is being studied. Since this oligomer has a low molecular weight and a low viscosity at room temperature or high temperature due to low molecular weight, coating without solvent, sheet molding, casting molding,
It is applicable to reaction extrusion molding and the like, and impregnation, dipping and the like are also possible. Furthermore, a polymer having excellent elongation and tensile strength can be obtained by introducing a functional group into the oligomer and reacting it with a crosslinking agent to extend or crosslink the oligomer chain.

[0003] Among these, oligomers having a functional group at the terminal, that is, telechelic oligomers, can efficiently form a chain by cross-linking between the terminals, so that a high molecular weight product can be formed and heat resistance and elongation can be reduced. Thus, a crosslinked body having excellent physical properties such as the above is obtained.

With respect to the above telechelic oligomer,
There have been many reports on synthetic examples of rubber-based compounds by the living polymerization method. Specifically, synthesis of telechelic polybutadiene by the living anionic polymerization method (Japanese Rubber Association, Vol. 48, No. 5, p. 263; 1975
And the synthesis of polychloroprene having hydroxyl groups at both ends by the iniferter method (Japanese Patent Application Laid-Open No. 3-287613). Oligomers such as polypropylene glycol in which both ends of a propylene glycol skeleton are hydroxyl groups are also used as a raw material for urethane adhesives and sealing materials, and as a modifier for epoxy-based adhesives.

[0005] However, the crosslinked product obtained by crosslinking these oligomers has poor adhesiveness alone, and requires the addition of a plasticizer or a tackifying resin in order to be used as an adhesive or an adhesive. Furthermore, since the tack at low temperatures is poor, there is a problem in using it as a raw material for adhesives, adhesives, sealing materials, etc. in cold regions.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to provide a vinyl ester oligomer having a carboxyl group at both ends or a hydroxyl group at both ends, which gives a crosslinked product having excellent adhesion.

[0007]

The vinyl ester oligomer according to the first aspect of the present invention is a vinyl ester oligomer mainly composed of an alkyl vinyl ester having an alkyl group having 1 to 20 carbon atoms, and has a molecular terminal at both ends. Is characterized by having a carboxyl group.

The vinyl ester oligomer according to the invention of claim 2 of the present application is a vinyl ester oligomer containing an alkyl vinyl ester having 1 to 20 carbon atoms as a main component and having hydroxyl groups at both ends. And

Hereinafter, the present invention will be described. The carbon number of the alkyl vinyl ester used in the present invention is limited to 1 to 20, preferably 1 to 14, from the viewpoint of adhesiveness. Examples of the alkyl vinyl ester in which the alkyl group has 1 to 20 carbon atoms include vinyl acetate, vinyl formate, vinyl propionate, vinyl butyrate, vinyl n-caproate, vinyl isocaproate, vinyl caprylate,
Examples thereof include vinyl caprate, vinyl octylate, vinyl laurate, and vinyl myristate, and these may be used alone or in combination of two or more. Among the above-mentioned alkyl vinyl esters, those having an alkyl group having 5 or more carbon atoms are preferable because of excellent adhesiveness.

[0010] If necessary, other vinyl ester monomers may be used in combination with the alkyl vinyl ester. Other vinyl ester monomers include, for example, vinyl palmitate, vinyl stearate, vinyl pivalate, vinyl trimethyl acetate, vinyl chloroacetate, vinyl trichloroacetate, vinyl trifluoroacetate,
And vinyl benzoate.

When the above-mentioned alkyl vinyl ester is used in combination with another vinyl ester monomer, the proportion of the other vinyl ester monomer is preferably at most 40% by weight, more preferably at most 30% by weight. When the proportion of the other vinyl ester monomer is 40% by weight or more, the adhesiveness of the crosslinked product is impaired. The above-mentioned other vinyl ester monomers may be added to the polymerization solution in their entirety from the beginning of the polymerization, or may be added sequentially as the polymerization proceeds.

As a method for introducing carboxyl groups at both molecular terminals of the alkyl vinyl ester oligomer,
Basically, a synthesis method utilizing a living polymerization method is preferable,
For example, as described in JP-A-10-60021, it can be synthesized by a vinyl ester polymerization method using an iodine compound.

Specifically, an alkyl vinyl ester is polymerized in the presence of an iodine compound (I) and a radical generator,
A method of substituting iodine at the growing end with a chain transfer agent (I) having a carboxyl group such as 2,2'-dithioglycolic acid, or a method of polymerizing an alkyl vinyl ester in the presence of a polyfunctional iodine compound and a radical generating source. And a method of converting iodine at the growth end to a carboxyl group.

As the above-mentioned iodine compound (I), a compound having at least one carboxyl group and at least one carbon-iodine bond in the molecule is used. Such iodine compounds include, for example, iodoacetic acid, o-
Examples thereof include iodobenzoic acid, m-iodobenzoic acid, p-iodobenzoic acid, 3-iodopropionic acid, and 3,5-diiodosalicylic acid. These may be used alone or in combination of two or more.

Examples of the polyfunctional iodine compound include diiodomethane, 1,2-diiodoethane, iodoform, 1,2-diiodoperfluoroethane, 1,4
-Diiodoperfluorobutane, 1,6-diiodoperfluorohexane and the like. These may be used alone or in combination of two or more.

Examples of the chain transfer agent (I) include compounds having at least one carboxyl group in the molecule, for example, 2-mercaptoacetic acid, 2-mercaptopropionic acid, dimercaptosuccinic acid, 2,2'-dithioglycolic acid. , 3,3'-dithiopropionic acid, 2,2'-dithiobenzoic acid, and the like. These may be used alone or in combination of two or more. Among them, 2,2′-dithioglycolic acid, which does not generate hydrogen radical,
Disulfide compounds such as 3'-dithiopropionic acid and 2,2'-dithiobenzoic acid are preferred.

The vinyl ester oligomer of the present invention preferably has a number average molecular weight of 500 to 100,000, more preferably 700 to 70,000, and still more preferably 1000 to 50000.
It is ten thousand. If the number average molecular weight is less than 500, a large amount of a crosslinking agent is required for the chain extension reaction, and if it exceeds 100,000, the chain extension reaction itself is very unlikely to occur, so that unreacted vinyl ester oligomers remain. The heat resistance of the crosslinked product will be impaired.

The vinyl ester oligomer can be polymerized by causing the carboxyl groups at the molecular terminals to react with each other by a crosslinking agent, whereby a chain extension reaction and a crosslinking reaction proceed. In this case, as a crosslinking agent,
Polyfunctional isocyanate compound, polyfunctional epoxy compound,
Examples include polyfunctional aziridine compounds, amino resins, melamine resins, and the like.

Examples of the polyfunctional isocyanate compound include tolylene diisocyanate, hydrogenated tolylene diisocyanate, tolylene diisocyanate adduct of trimethylolpropane, triphenylmethane triisocyanate, methylenebis (4-phenylmethane) triisocyanate, and isophorone. Diisocyanate, hexamethylene diisocyanate and the like. These may be used alone or in combination of two or more.

Examples of the polyfunctional epoxy compound include bisphenol A, epichlorohydrin type epoxy resin, ethylene glycol diglycidyl ether,
1,6-hexanediol diglycidyl ether, triglycidyl ether, trimethylolpropane triglycidyl ether, diglycidylaniline, diglycidylamine, N, N, N, N-tetraglycidyl m-xylenediamine, 1,3-bis ( (N, N-diglycidylaminomethyl) cyclohexane and the like. These may be used alone or in combination of two or more.

Examples of the above polyfunctional aziridine compound include N, N-hexamethylene-1,6-bis (1-aziridinecarboxamide), trimethylolpropane-tri-β-aziridinylpropionate, bisisopropane Phthaloyl-1- (2-methylaziridine) and the like. These may be used alone or in combination of two or more.

Next, the case where hydroxyl groups are introduced into both ends of the molecule of the above alkyl vinyl ester will be described.

As a method for introducing hydroxyl groups at both molecular ends of the above-mentioned alkyl vinyl ester oligomer, a synthesis method utilizing a living polymerization method is basically preferred as in the case of introducing the carboxyl group described above.
As described in JP-A-60021, the compound can be synthesized by a method for polymerizing a vinyl ester using an iodine compound (II).

Specifically, the above-mentioned alkyl vinyl ester is polymerized in the presence of an iodine compound (II) and a radical generating source, and the iodine at the growth end is converted to a chain transfer agent having a hydroxyl group such as 2-hydroxyethyl disulfide (II). ) Or a method of polymerizing an alkyl vinyl ester in the presence of a polyfunctional iodine compound and a radical generating source to convert iodine at the growth end into a hydroxyl group.

As the above-mentioned iodine compound (II), a compound having at least one hydroxyl group and at least one carbon-iodine bond in the molecule is used. Examples of such an iodine compound (II) include 2-iodobenzyl alcohol, 3-iodobenzyl alcohol, 2-iodoethanol, and 4-iodo-2-methylphenol. These may be used alone or in combination of two or more.

As the above-mentioned chain transfer agent (II), compounds having at least one hydroxyl group in the molecule, for example, mercaptophenol, 3-mercapto-1,2-propanediol, 3-mercaptoethanol, bis (2- (Hydroxyethyl) disulfide, bis (3-hydroxypropyl) disulfide and the like. These may be used alone or in combination of two or more. Among them, disulfide compounds that do not generate hydrogen radicals, such as bis (2-hydroxyethyl) disulfide and bis (3-hydroxypropyl) disulfide, are preferred.

As the above polyfunctional iodine compound, the same compounds as those in the case where a carboxyl group is introduced are used.

The obtained vinyl ester oligomer can be polymerized by causing hydroxyl groups at the molecular terminals to react with each other by a crosslinking agent, whereby a chain extension reaction and a crosslinking reaction proceed. As the cross-linking agent, the same one as in the case of introducing a carboxyl group is used.

When the above alkyl vinyl ester is polymerized with the iodine compound (I) or (II), it is preferable to use a radical polymerization initiator. The radical polymerization initiator is not particularly limited as long as it can generate a radical and react with an alkyl vinyl ester or an iodine compound to start a polymerization reaction. It is selected from compounds that generate a radical by the action of a reaction or the like.

Examples of the radical polymerization initiator include azo compounds, organic peroxides, inorganic peroxides, organometallic compounds, and photopolymerization initiators. More specifically, azo compounds such as azobisisobutyronitrile (AIBN), azobisisobutyric acid ester and hyponitrite, benzoyl peroxide (BPO), lauroyl peroxide,
Organic peroxides such as dicumyl peroxide and dibenzoyl peroxide; inorganic peroxides such as potassium persulfate and ammonium persulfate; hydrogen peroxide-ferrous iron, BPO
Redox polymerization initiators such as dimethylaniline-based and cerium (IV) salt-alcohol-based. Among them, AIBN and BPO are particularly preferably used.

Two or more of the above radical polymerization initiators may be used in combination as long as the interaction does not adversely affect the progress of polymerization. Further, a combination in which the polymerization is advanced to some extent by the action of heat and then the polymerization is completed by light may be used.

As a polymerization method of the above-mentioned alkyl vinyl ester and the iodine compound, a conventionally known method can be adopted.
For example, bulk polymerization, solution polymerization, suspension polymerization, emulsion polymerization and the like are suitably used. In particular, bulk polymerization is preferably used from the viewpoint of suppressing chain transfer and obtaining a better controlled polymer. Further, continuous polymerization using an extruder can also be used.

[0033]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be specifically described based on embodiments.

Example 1 AIBN1. Was placed in a 1000 ml flask equipped with a reflux condenser and a stirring blade and purged with nitrogen.
64 g (10 mmol), 3.93 g of iodoform (1
0 mmol), 100 g of vinyl caproate (39 mmol)
l) and 1 ml of n-heptane were charged and mixed uniformly, and then nitrogen was bubbled for 30 minutes to remove dissolved oxygen in the reaction solution. Then, while supplying nitrogen, the temperature of the reaction solution was raised to 70 ° C. to start polymerization. The polymerization conversion of the reaction solution after 6 hours was 97%. Further, 3.09 g of 2,2′-dithioglycolic acid (20 mm
ol) and reacted at 100 ° C. for 2 hours to complete the polymerization. After completion of the polymerization, the above reaction solution was washed with a 10% aqueous solution of sodium thiosulfate, the aqueous phase was separated, the organic layer was dehydrated with sodium sulfate, and the remaining monomer was removed from the organic layer by evaporation to remove the liquid resin. Obtained. The liquid resin was dried under vacuum to obtain a vinyl polycaproate oligomer.

Ethylene glycol diglycidyl ether was added to 50 g of the above oligomer so that epoxy group / carboxyl group = 1.0 (molar ratio), 4 g of piperidine was further added and stirred, and the mixture was subjected to corona discharge treatment.
It was coated on a polyethylene terephthalate (PET) film having a thickness of μm, and then the coated material was dried in an oven at 150 ° C. for 20 minutes to obtain a sheet on which a crosslinked oligomer having a thickness of 25 μm was laminated.

When the number average molecular weight (Mn) and molecular weight distribution (Mw / Mn) of the oligomer were determined, Mn = 9.
900, Mw / Mn = 1.57. When the carboxyl group in the oligomer was quantified, it was 0.85% by weight, and it was estimated that the oligomer had an average of two carboxyl groups per molecule.

The polymerization conversion and the number average molecular weight (M
n), the molecular weight distribution (Mw / Mn) and the measurement of the carboxyl group were measured by the following methods, and the following Examples and Comparative Examples were similarly measured.・ Polymerization conversion rate The polymerization conversion rate was measured using n-heptane as an internal standard.
The measurement was performed by gas chromatography (“GC” manufactured by Shimadzu Corporation, column: PEG 1500). -Number average molecular weight (Mn), molecular weight distribution (Mw / Mn) The molecular weight was determined by gel permeation chromatography (GPC) (manufactured by Shodex, column: KF-80M).
× 2) in terms of standard polystyrene. The measurement was performed using THF as an eluent at a flow rate of 1 ml / min. The detection was performed using a refractometer, and after measuring the number average molecular weight (Mn) and the weight average molecular weight (Mw), the weight average molecular weight (M
w) and the number average molecular weight (Mn).
w / Mn) was calculated. -Measurement of carboxyl group The oligomer was dissolved in a mixed solvent of methanol / toluene, and titrated with a methanol solution of sodium hydroxide using a cresol red indicator to determine the amount of the carboxyl group.

Example 2 7.86 g of iodoform (2
0 mmol), and polymerization was carried out in the same manner as in Example 1 except that 100 g of vinyl caprate was used instead of vinyl caproate. The polymerization conversion of the reaction solution after 6 hours was 98%. Then, the same treatment as in Example 1 was performed except that 6.18 g (40 mmol) of 2,2′-dithioglycolic acid was added to the reaction solution, to obtain a vinyl polycaprate oligomer. Mn of this oligomer
= 5100, Mw / Mn = 1.36. The amount of the carboxyl group of the oligomer is 0.17% by weight, and it is estimated that the oligomer has an average of two carboxyl groups per molecule. Next, the above oligomer 5
0 g, isocyanate group / carboxyl group = 1.0
(Molar ratio), diphenylmethane diisocyanate was added thereto, and 5 g of a 2% toluene solution of dibutyltin dilaurate was further added and stirred. The resulting mixture was subjected to corona discharge treatment and 38 μm-thick polyethylene terephthalate (PET).
Coating was performed on the film, and the coated product was dried in an oven at 150 ° C. for 20 minutes to obtain a 25 μm-thick laminated sheet of a crosslinked oligomer.

Example 3 In place of vinyl caproate, 95 g of vinyl myristate and 5 g of vinyl pivalate were used.
Polymerization was carried out in the same manner as in Example 1 except that g was used. The polymerization conversion of the reaction solution after 6 hours was 94%.
Next, the same treatment as in Example 1 was performed on the reaction solution to obtain a poly (vinyl myristate / vinyl pivalate) oligomer. Mn = 11,000, Mw of this oligomer
/Mn=1.61. The amount of the carboxyl group in the oligomer was 0.79% by weight, and it is estimated that the oligomer has an average of two carboxyl groups per molecule. Next, N, N-hexamethylene-1,6-bis (1-aziridinecarboxamide) was added to 50 g of this oligomer so that the aziridine group / carboxyl group = 1.0 (molar ratio), and the mixture was stirred. Was coated on a 38 μm-thick polyethylene terephthalate (PET) film that had been subjected to corona discharge treatment.
The sheet was dried in an oven at 0 ° C. for 20 minutes to obtain a sheet in which a crosslinked oligomer having a thickness of 25 μm was laminated.

Example 4 AIBN1. Was placed in a 1000 ml flask equipped with a reflux condenser and stirring blades and purged with nitrogen.
64 g (10 mmol), 3.93 g of iodoform (1
0 mmol), 100 g of vinyl caproate (39 mmol)
l) and 1 ml of n-heptane were charged and mixed uniformly, and then nitrogen was bubbled for 30 minutes to remove dissolved oxygen in the reaction solution. Then, while supplying nitrogen, the temperature of the reaction solution was raised to 70 ° C. to start polymerization. The polymerization conversion of the reaction solution after 6 hours was 97%. Further, 3.09 g (20 mmol) of bis (2-hydroxyethyl) disulfide was added to the reaction solution, and the mixture was reacted at 100 ° C. for 2 hours to complete the polymerization. After completion of the polymerization, the same treatment as in Example 1 was performed to obtain a vinyl polycaproate oligomer.

Mn = 9500, Mw /
Mn was 1.4. The amount of hydroxyl groups in the oligomer was 0.33% by weight, and it is estimated that the oligomer has an average of two hydroxyl groups per molecule. The amount of hydroxyl groups in the oligomer was determined by acetylating the oligomer with acetic anhydride in the presence of pyridine, and then adding water to convert the excess acetic anhydride to acetic acid. It was determined by titration using an indicator (measured in the same manner in the following examples).

Diphenylmethane diisocyanate was added to 50 g of the above oligomer so that isocyanate group / hydroxyl group = 1.0 (molar ratio), and 5 g of a 2% toluene solution of dibutyltin dilaurate was added and stirred. Coating was performed on a 38 μm-thick polyethylene terephthalate (PET) film that had been subjected to corona discharge treatment, and then the coating was dried in a 150 ° C. oven for 20 minutes to obtain a 25 μm-thick laminated sheet of an oligomer crosslinked product. .

Example 5 Instead of iodoform, 2
Using 2.58 g (15 mmol) of iodoethanol and vinyl caprylate instead of vinyl caproate;
Polymerization was carried out in the same manner as in Example 4 except that 0 g was used. The polymerization conversion of this reaction solution after 6 hours is 98%.
Met. Then, 4.63 g (30 mmol) of bis (2-hydroxyethyl) disulfide was added to the reaction solution, and 6.18 g of 2,2′-dithioglycolic acid (40 mmol) was added.
Except that l) was added, the same treatment as in Example 4 was carried out to obtain a polyvinyl polycaprylate oligomer. Mn = 7100 and Mw / Mn of this oligomer were 1.35. The amount of hydroxyl groups in the oligomer was 0.45% by weight, and it is estimated that the oligomer has an average of two hydroxyl groups per molecule.

Next, diphenylmethane diisocyanate was added to 50 g of the above oligomer so that isocyanate group / hydroxyl group = 1.0 (molar ratio), 5 g of a 2% toluene solution of dibutyltin dilaurate was added, and the mixture was stirred. Coating on a 38 μm-thick polyethylene terephthalate (PET) film subjected to corona discharge treatment,
Next, the coated product was dried in an oven at 150 ° C. for 20 minutes to obtain a sheet in which a crosslinked oligomer having a thickness of 25 μm was laminated.

Example 6 Instead of vinyl caproate, 90 g of vinyl laurate and 10 g of vinyl caprate were used.
Polymerization was carried out in the same manner as in Example 4 except that g was used. The polymerization conversion of this reaction solution after 6 hours was 98%. Further, 3.09 g (20 mmol) of bis (2-hydroxyethyl) disulfide was added to the reaction solution, and the mixture was reacted at 100 ° C. for 2 hours to complete the polymerization. After completion of the polymerization, the same treatment as in Example 4 was performed to obtain a poly (vinyl laurate / vinyl caprate) oligomer. Mn = 10500, Mw / Mn = 1.50 of this oligomer
Met. The amount of hydroxyl groups in the oligomer is 0.31% by weight, and it is estimated that the oligomer has an average of two hydroxyl groups per molecule.

Next, hexamethylene diisocyanate was added to 50 g of the above oligomer so that isocyanate groups / hydroxyl groups = 1.0 (molar ratio), and 5 g of a 2% toluene solution of dibutyltin dilaurate was added and stirred. Is coated on a 38 μm-thick polyethylene terephthalate (PET) film subjected to corona discharge treatment,
Next, the coated product was dried in an oven at 150 ° C. for 20 minutes to obtain a sheet in which a crosslinked oligomer having a thickness of 25 μm was laminated.

Comparative Example 1 Polypropylene glycol having hydroxyl groups at both ends (Mn = 10000, hydroxyl value KO)
Hmg / g = 11.4) 50 g of isocyanate groups /
Hexamethylene diisocyanate was added so that the hydroxyl group was 1.0 (molar ratio), 5 g of a 2% toluene solution of dibutyltin dilaurate was added, and the mixture was stirred, followed by corona discharge treatment. Coating was performed on the film, and the coated product was dried in an oven at 150 ° C. for 20 minutes to obtain a 25 μm-thick laminated sheet of a crosslinked oligomer.

Comparative Example 2 Polybutadiene having hydroxyl groups at both ends (Mn = 3090, hydroxyl value KOH mg / g =
30.4) In 50 g, isocyanate groups / hydroxyl groups = 1.
0 (molar ratio), hexamethylene diisocyanate was further added, and 5 g of a 2% toluene solution of dibutyltin dilaurate was further added and stirred. The mixture was subjected to corona discharge treatment and a 38 μm-thick polyethylene terephthalate (PE).
T) Coating was performed on a film, and the coated product was dried in an oven at 150 ° C. for 20 minutes to obtain a sheet having a 25 μm-thick laminated oligomer cross-linked body.

The sheet on which the crosslinked body obtained above was laminated was evaluated for the following properties, and the results are shown in Table 1. (1) Ball tack test A ball tack test was performed at 0 ° C. in accordance with JIS Z 0237. (2) 180 ° peel test The crosslinked oligomer was cut into a 25 mm width,
Under a condition of 5% RH, a 2 kg roller was reciprocated once and pressed to adhere to a stainless steel plate. After 20 minutes, a 180 ° peel test was performed by a tensile test at a speed of 300 mm / min to measure a peel force. did.

[0050]

[Table 1]

[0051]

The vinyl ester oligomer of the present invention is
The present invention provides a crosslinked product having the above-described structure, which is excellent in adhesiveness and low-temperature characteristics as compared with a conventional crosslinked product of a polybutadiene oligomer or a polypropylene glycol oligomer.

Continued on the front page F term (reference) 4J002 BF011 CD012 CD052 CD132 ER006 EU016 FD142 FD146 GH00 GJ00 4J011 NA25 NA26 NB02 NB05 SA76 SA79 SA85 4J100 AG02P AG03P AG04P AG05P AG05Q AG06Q AG08Q BA03H BA16H BA51QBA52 BC33 JA01 JA03

Claims (3)

[Claims] 1. A vinyl ester oligomer comprising an alkyl vinyl ester having 1 to 20 carbon atoms in an alkyl group as a main component, having a carboxyl group at both molecular terminals. 2. A vinyl ester oligomer mainly comprising an alkyl vinyl ester having 1 to 20 carbon atoms in an alkyl group, wherein the vinyl ester oligomer has hydroxyl groups at both molecular terminals. 3. The vinyl ester oligomer according to claim 1, wherein the number average molecular weight is from 500,000 to 100,000.