EP-3992214-B1 - VINYL ALCOHOL-BASED BLOCK COPOLYMER AND METHOD FOR PRODUCING SAME

Inventors

• TAKAYAMA, Takumi
• MISHIMA, Kazutoshi
• KONISHI, HIROYUKI
• MAEKAWA, KAZUHIKO

Dates

Publication Date

20260506

Application Date

20200625

Claims (7)

1. A vinyl alcohol-based block copolymer that is a block copolymer composed of a vinyl alcohol-based polymer block (B-b) and a copolymer block (B-c) comprising a vinyl alcohol-based monomer unit and an acrylic acid-based monomer unit, wherein a content (Z) of the acrylic acid-based monomer unit relative to all monomer units is 0.05 to 20.0 mol%, the block copolymer has a saponification degree of 80 to 99.99 mol%, measured as disclosed in the description, the block copolymer has a number-average molecular weight (Mn a ) of 20,000 to 440,000, measured as disclosed in the description, the block copolymer has a molecular weight distribution (Mw a /Mn a ) of 1.05 to 1.95, and a ratio (DP b /DP a ) of the number-average polymerization degree (DP b ) of the vinyl alcohol-based polymer block (B-b) to the number-average polymerization degree (DP a ) of the block copolymer is 0.010 to 0.999.
2. The block copolymer according to claim 1, wherein the vinyl alcohol-based polymer block (B-b) has a number-average polymerization degree (DP b ) of 450 to 5,000.
3. The block copolymer according to claim 1 or 2, wherein a content (R) of the acrylic acid-based monomer unit relative to all monomer units in the copolymer block (B-c) is 0.1 to 50.0 mol%.
4. The block copolymer according to any one of claims 1 to 3, wherein a content (Z) [mol%] of the acrylic acid-based monomer unit and a crystal melting temperature (Q) [°C], measured as disclosed in the description, of a polymer obtained by re-saponifying the block copolymer to a saponification degree of 99 mol% or more satisfy a formula (1) below: 2 Z + Q ≥ 225
5. The block copolymer according to any one of claims 1 to 4, wherein a molar ratio (V) of a lactone ring to a total of an acrylic acid monomer unit and a lactone ring [a lactone ring/a total of an acrylic acid monomer unit and a lactone ring] in a polymer obtained by heat-treating the block copolymer in an acidic aqueous solution and then drying the block copolymer is 0.70 or more.
6. A method for producing the block copolymer according to any one of claims 1 to 5, the method comprising: a polymerization step of performing polymerization of a vinyl ester and copolymerization of a vinyl ester and an acrylic acid ester by controlled radical polymerization in the presence of a radical initiator and an organic cobalt complex to obtain a vinyl ester-based block copolymer composed of a vinyl ester polymer block (B-b1) and a copolymer block (B-c1) comprising a vinyl ester monomer unit and an acrylic acid ester monomer unit; and a saponification step of saponifying the vinyl ester-based block copolymer obtained in the polymerization step to obtain a vinyl alcohol-based block copolymer composed of a vinyl alcohol-based polymer block (B-b) and a copolymer block (B-c) comprising a vinyl alcohol-based monomer unit and an acrylic acid-based monomer unit.
7. The method for producing the vinyl alcohol-based block copolymer according to claim 6, wherein the acrylic acid ester is methyl acrylate.

Description

TECHNICAL FIELD The present invention relates to a vinyl alcohol-based block copolymer superior in handleability in an aqueous solution state and capable of forming a film superior in mechanical strength, and a method for producing the same. BACKGROUND ART Polyvinyl alcohol (hereinafter, sometimes abbreviated as PVA) resins are crystalline water-soluble polymer materials and are widely applied to emulsifiers, suspensions, surfactants, fiber treating agents, various binders, paper treating agents, adhesives, films, etc. using their superior water solubility and film properties (strength, grease resistance, film formability, oxygen gas barrier properties, etc.). Conventionally, PVAs differing in saponification degree or polymerization degree have been used according to applications. Various modified PVAs with special functions by introducing a functional group into PVAs are proposed. Incidentally, since PVA is a crystalline polymer, if it has an excessively high saponification degree, its solubility in water is low. In order to ensure solubility in water; there are known a method of lowering the saponification degree and a method of decreasing crystallinity by randomly copolymerizing a vinyl ester with another monomer. It, however, is known that a film obtained by forming a PVA prepared by such a method into a film shape and then drying it has insufficient performance such as mechanical strength, barrier property, and water resistance because of its low crystallinity. As a measure for preventing such deterioration of the performance of a film, a method for improving water resistance by introducing an acrylic acid-based unit into a PVA by random copolymerization to form ionic crosslinking has been proposed (Patent Document 1). However, the PVA obtained by this method is low in crystallinity, so that a resulting film is still insufficient in water resistance or film strength. As a method for achieving both solubility in water and film properties of PVA, a method using a block copolymer of a PVA block and a hydrophilic block is conceivable. Patent Document 2 proposes a block copolymer in which a PVA block and a block formed by copolymerizing a hydrophilic monomer and a hydrophobic monomer are linked by a hydrophobic linker. However, since the block polymer has a hydrophobic part, there is a concern about an effect of improving solubility in water. In addition, in the invention described in Patent Document 2, the end of the polyvinyl ester is halogenated by a chain transfer reaction to form the starting point of block copolymerization, but the chain transfer reaction has a drawback that an increased molecular weight and an increased halogenation efficiency cannot be achieved simultaneously. Specifically, attempting to increase the halogenation efficiency promotes chain transfer to interfere the molecular weight of a PVA block from becoming large, so that sufficient film properties cannot be obtained. Non-Patent Document 1 describes a complete block copolymer of a PVA block and a polyacrylic acid block. However, since the polyacrylic acid block has excessively high pH sensitivity, for example, there is a concern that the block copolymer is gelled in an aqueous alkali solution and cannot be handled as a solution or the block copolymer has poor solution stability, so that the method of its use is greatly limited. Non-Patent Document 2 describes a block copolymer of a vinyl alcohol-acrylic acid ester copolymer block and a PVA block obtained by saponifying a block copolymer of a vinyl acetate-acrylic acid ester copolymer block and a polyvinyl acetate block in a dehydrated solvent. There is a concern with the block copolymer described in Non-Patent Document 2 that since the proportion of the vinyl alcohol-acrylic acid ester copolymer block to the PVA block is large, PVA-derived film properties cannot be expected. PRIOR ART DOCUMENTS PATENT DOCUMENTS Patent Document 1: JP 09-278827 APatent Document 2: JP 2010-209336 A NON-PATENT DOCUMENTS Non-Patent Document 1: Highly Stretchable Free-Standing Poly(acrylic acid)-block-poly(vinyl alcohol) Films Obtained from Cobalt-Mediated Radical Polymerization, Macromolecules, 2017, vol.50, p. 6054-6063 Non-Patent Document 2: Mn2(CO)10-Induced Controlled/Living Radical Copolymerization of Vinyl Acetate and Methyl Acrylate: Spontaneous Formation of Block Copolymers Consisting of Gradient and Homopolymer Segments, Journal of Polymer Science: Part A: Polymer Chemistry, 2009, vol.47, p1343-1353 SUMMARY OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION The present invention has been devised in order to solve the above problems, and an object thereof is to provide a vinyl alcohol-based block copolymer superior in solubility in water, aqueous solution stability, and strength and a method for producing the same. MEANS FOR SOLVING THE PROBLEMS The problem described above is solved by providing a vinyl alcohol-based block copolymer that is a block copolymer composed of a vinyl alcohol-bas