JP5301752B2 - Active energy ray-curable coating composition - Google Patents

Description

  The present invention is an active energy ray curing that forms a cured coating film having a low curing shrinkage rate and a high hardness by using a urethane acrylate resin containing a polyol having a condensed polycyclic structure as an essential component. The present invention relates to a mold paint composition.

  Recently, due to social needs, there has been an increasing demand for the development of paints with higher productivity and higher performance. Such high-performance paints are active in terms of (1) fast curability, (2) low energy cost, and (3) non-pollution by solvent-free. An energy beam curable resin is exemplified.

As the active energy ray curable resin, for example, unsaturated polyester is mentioned, and it is widely used in the field of woodwork coatings, etc. because it has a good gloss and fleshiness and gives a beautiful finish. There is a problem in that the curability by irradiation with active energy rays is relatively poor.

  Epoxy acrylate has high hardness and excellent curability, and is used in the fields of woodworking paints and plastic paints. However, when there is a problem in terms of high cure shrinkage and low adhesion to the substrate. There are many.

  Therefore, although there is a problem in terms of cost, urethane acrylate has begun to be widely used in the field of building material coating as an active energy ray-curable resin having excellent curability as compared with unsaturated polyester.

  In the field of woodwork painting, imported plywood, which is weak in material and cheap, is increasing. This imported plywood has a large internal distortion, and cracks easily occur under severe cold conditions. Therefore, when a general active energy ray-curable resin paint that easily generates internal distortion is applied, there is a problem that cracks are remarkably generated in the thermal cycle test.

  Further, in the field of plastic coating, a high hardness, high energy active energy ray curable coating is widely used as a surface protection for a substrate, and the coating has a high double bond concentration in order to develop a high hardness. . However, since the double bond concentration is high, there is a problem that the curing shrinkage rate is higher than that of a general-purpose active energy ray-curable coating material, and cracks due to internal strain tend to occur.

Problems to be solved by the invention

  The problem to be solved by the present invention is a composition for an active energy ray-curable coating material which has a low curing shrinkage rate when irradiated with active energy rays and can obtain a cured film having a high hardness and does not generate cracks. Is to provide things.

Means for solving the problem

  Therefore, the present inventors have made extensive studies while focusing on the problems to be solved by the invention as described above, and as a result, urethane acrylate resins comprising a polyol component having a condensed polycyclic structure as a raw material component. As a result of using an active energy ray-curable composition comprising as an essential component, the inventors have found that the intended purpose can be achieved, and have completed the present invention.

That is, the present invention requires a urethane acrylate resin (A) comprising a polyol (a) having a condensed polycyclic structure, a norbornane diisocyanate (b), and a compound (c) containing a hydroxyl group and a (meth) acryloyl group. The present invention provides an active energy ray-curable coating composition characterized by containing as a component.

The present invention is described in detail below.
The polyol (a) having a condensed polycyclic structure used in the present invention is not particularly limited. For example, a compound obtained by hydroxyalkylating indene, naphthalene, azulene, anthracene, etc .; bicyclo [5,3,0] decandimethanol, bicyclo [4,4,0] decanedimethanol, bicyclo [4,3,0] nonanedimethanol, norbornanedimethanol, tricyclodecanedimethanol, 1,3-adamantanediol (1,3-dihydroxytricyclo [3. 3.1.1 ^3,7 ] decane) and the like.

  Among these, a polyol having an alicyclic condensed polycyclic structure is preferable, and tricyclodecane dimethanol is most preferable from the viewpoint of the performance of the cured coating film of the final coating composition.

Polyisocyanate bets used in the present invention are norbornane diisocyanate (b).

  Examples of the compound (c) containing a hydroxyl group and a (meth) acryloyl group used in the present invention include 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, 2-hydroxybutyl (meth) acrylate, 4 -Various hydroxyalkyl (meth) acrylates such as hydroxybutyl (meth) acrylate; various such as di (meth) acrylate of trimethylolpropane, tri (meth) acrylate of pentaerythritol, hexa (meth) acrylate of dipentaerythritol Polyacrylates of a hydroxyl group-containing compound; adducts of the above various hydroxyl group-containing monomers with ε-caprolactone, adducts of alkylene oxide, and the like. These compounds (c) can be used alone or in combination of two or more.

  Of these compounds (c), compounds having one hydroxyl group and one (meth) acryloyl group are preferred, and among them, 2-hydroxypropyl (meth) acrylate and hydroxyethyl (meth) acrylate have low cure shrinkage and high It is preferable because a cured coating film having hardness can be formed. Pentrierythritol triacrylate is preferable from the viewpoint of obtaining a coating film with higher hardness.

In producing the urethane acrylate resin used in the present invention, first, the total hydroxyl group (OH) of the polyol (a) having a condensed polycyclic structure and the compound (c) containing a hydroxyl group and a (meth) acryloyl group, and norbornane. The polyisocyanate may be reacted so that the equivalent ratio (OH / NCO) of the diisocyanate (b) to the isocyanate group (NCO) is 0.9 / 1 to 1 / 0.9, preferably 1/1. After reacting the polyol (a) having a structure with the norbornane diisocyanate (b), the compound (c) containing a hydroxyl group and a (meth) acryloyl group may be reacted, or the hydroxyl group and the (meth) acryloyl group may be reacted. after reacting the compound containing (c) and norbornane diisocyanate (b), may be reacted with the polyol (a) having a condensed polycyclic structure Further, may be reacted with a polyol having a condensed polycyclic structure (a) and the hydroxyl group and (meth) norbornane diisocyanate to the mixture of compounds containing acryloyl group (c) (b), in particular for these reaction sequence There is no limit.

  As a catalyst used by the said reaction, various catalysts, such as a dibutyltin laurate and a dibutyltin acetate, can be used, for example, It can manufacture on the conditions of the urethanation reaction performed normally. Further, if necessary, a solvent such as ethyl acetate, butyl acetate, methyl isobutyl ketone, toluene, xylene, or a radical polymerizable monomer that does not contain a site that reacts with isocyanate, for example, a radical polymerizable monomer described later Of the bodies (B), those which do not contain a hydroxyl group or an amino group may be used as the solvent.

  In this invention, you may contain radically polymerizable monomers (B) other than urethane acrylate resin (A). Examples of such radical polymerizable monomers (B) include the following.

  Examples of the monofunctional monomer include N-vinylcaprolactam, N-vinylpyrrolidone, N-vinylcarbazole, vinylpyridine, N, N-dimethyl (meth) acrylamide, acrylamide, acryloylmorpholine, 7-amino-3,7-dimethyl. Octyl (meth) acrylate, isobutoxymethyl (meth) acrylamide, t-octyl (meth) acrylamide, diacetone (meth) acrylamide, dimethylaminoethyl (meth) acrylate, diethylaminoethyl (meth) acrylate, 2-ethylhexyl (meth) acrylate , Ethyldiethylene glycol (meth) acrylate, lauryl (meth) acrylate, dicyclopentadienyl (meth) acrylate, dicyclopentenyloxyethyl (meth) acrylate Dicyclopentenyl (meth) acrylate, tetrachlorophenyl (meth) acrylate, 2-tetrachlorophenoxyethyl (meth) acrylate, tetrahydrofurfuryl (meth) acrylate, tetrabromophenyl (meth) acrylate, 2-tetrabromophenoxyethyl (meth) ) Acrylate, 2-trichlorophenoxyethyl (meth) acrylate, tribromophenyl (meth) acrylate, 2-tribromophenoxyethyl (meth) acrylate, phenoxyethyl (meth) acrylate, butoxyethyl (meth) acrylate, pentachlorophenyl (meth) ) Acrylate, pentabromophenyl (meth) acrylate, bornyl (meth) acrylate, methyltriethylenediglycol (meth) acrylate, iso Runiru (meth) monoacrylate monomer such as acrylate; bisphenol F mono (meth) acrylate, mono (meth) acrylate of alkylene oxide addition bisphenol F;

Various phosphoric acid group-containing vinyl monomers such as mono {2- (meth) acryloyloxyethyl} acid phosphate; vinylsulfonic acid, allylsulfonic acid, 2-methylallylsulfonic acid, 4-vinylbenzenesulfonic acid, 2- Various sulfonic acid group-containing vinyl monomers such as (meth) acryloyloxyethanesulfonic acid, 3- (meth) acryloyloxypropanesulfonic acid, 2-acrylamido-2-methylpropanesulfonic acid;

_{CH 2 = CHCOO (CH 2)} 3 [Si (CH 3) 2 O] n Si (CH 3) 3, CH 2 = C (CH 3) COOC 6 H 4 _{[Si (CH 3) 2 O)} n Si ( _{CH 3) 3, CH 2 =} C (CH 3) COO (CH 2) 3 [Si (CH 3) 2 O] n Si (CH 3) 3, CH 2 = C (CH 3) COO (CH 2) 3 [Si (CH ₃ ) (C ₆ H ₅ ) ○] _n Si (CH ₃ ) ₃ , or CH ₂ ═C (CH ₃ ) COO (CH ₂ ) ₃ [Si (C ₆ H ₅ ) ₂ O] _n Si (CH ₃ ) ₃ (provided that n in each formula is an integer of 0 or 1 to 130) and other polysiloxane bond-containing monomers represented by a general formula such as:

γ- (meth) acryloxypropyltrimethoxysilane, γ- (meth) acryloxypropyltriethoxysilane, γ- (meth) acryloxypropylmethyldimethoxysilane, γ- (meth) acryloxypropylmethyldiethoxysilane, γ -(Meth) acryloyloxypropyltriisopropenyloxysilane, vinyltrimethoxysilane, vinyltriethoxysilane, vinyl (tris-β-methoxyethoxy) silane, vinyltriacetoxysilane, vinyltrichlorosilane or N-β- (N- Vinylbenzylaminoethyl) -γ-aminopropyltrimethoxysilane and its hydrochloride; 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, polyethylene glycol mono (meth) acrylate, (Meth) acrylic acid esters having a hydroxyl group such as polypropylene glycol mono (meth) acrylate, and ε-caprolactone adducts of these monomers;

Various vinyl ethers having tertiary amines such as 2-dimethylaminoethyl vinyl ether, 2-diethylaminoethyl vinyl ether, 4-dimethylaminobutyl vinyl ether, 4-diethylaminobutyl vinyl ether, 6-dimethylaminohexyl vinyl ether; 2-hydroxyethyl vinyl ether, 4 -Vinyl ethers having various hydroxyl groups such as hydroxybutyl vinyl ether and 6-hydroxyhexyl vinyl ether; or 2-hydroxyethoxyallyl ether, 4-hydroxybutoxyallyl ether, trimethylolpropane mono-, di-allyl ether, pentaerythritol Allyl ethers having a hydroxyl group such as mono- or di-allyl ether, and ε-caprolactone of these monomers Addendum; methyl vinyl ether, ethyl vinyl ether, n- propyl vinyl ether, n- butyl vinyl ether, iso- butyl vinyl ether, 2-ethylhexyl vinyl ether, cyclopentyl vinyl ether, and vinyl ethers such as cyclohexyl vinyl ether.

  Examples of the bifunctional monomer include ethylene glycol di (meth) acrylate, dicyclopentenyl di (meth) acrylate, triethylene glycol di (meth) acrylate, tetraethylene glycol di (meth) acrylate, and tricyclodecane di (meta). ) Acrylate, dimethylol tricyclodecane diacrylate, tripropylene glycol di (meth) acrylate, 1,4-butanediol di (meth) acrylate, 1,6-hexanediol di (meth) acrylate, polyethylene glycol di (meth) Acrylate, polypropylene glycol di (meth) acrylate, neopentyl glycol di (meth) acrylate, di (meth) acrylate of hydroxypivalic acid neopentyl glycol ester, bisphenol Di (meth) acrylate of A or F, Di (meth) acrylate of alkylene oxide-added bisphenol A or F; Divalent carboxylic acids such as various unsaturated dibasic acids such as maleic acid, fumaric acid, itaconic acid, citraconic acid Examples include divinyl esters of acids.

  The amount of these radically polymerizable monomers (B) used is not particularly limited, but in particular when a radically polymerizable monomer having a large cure shrinkage such as pentaerythritol triacrylate or dipentaerythritol hexaacrylate is used. Is preferably used in an amount of 50 parts by weight or less, more preferably 20 parts by weight or less, in 100 parts by weight of the radical polymerizable monomer (B) so as not to impair the balance between the curing shrinkage and the hardness.

  As a blending ratio of the urethane acrylate resin (A) and the radical polymerizable monomer (B), the weight ratio (A) / (B) is 20 in order to make the coating viscosity appropriate and improve the hardness of the cured coating film. The range of / 80 to 90/10 is preferable, and the range of 30/70 to 80/20 is particularly preferable.

  Moreover, a photoinitiator can also be used as needed. Various photoinitiators can be used, and examples thereof include compounds that generate radicals by hydrogen abstraction, such as benzophenone, benzyl, Michler ketone, thioxanthone, and anthraquinone. Generally, tertiary amines such as methylamine, diethanolamine, N-methyldiethanolamine and tributylamine are used in combination with these compounds.

  Examples of other photoinitiators include compounds that generate radicals by intramolecular splitting, and include benzoin, dialkoxyacetophenone, acyloxime ester, benzyl ketal, hydroxyalkylphenone, halogenoketone, and the like.

  The blend ratio of the photoinitiator in the active energy ray-curable coating composition of the present invention is such that the cured film is not colored and can be sufficiently cured. It is preferable that it is 0.1-10 weight part in 100 weight part per minute.

  The coating composition of the present invention as described above uses an organic solvent such as ethyl acetate, methyl ethyl ketone, and toluene, or a radical polymerizable monomer (B) as a dilution monomer, if necessary, and has an appropriate coating viscosity. Can be cured by being irradiated with an active energy ray.

  The active energy ray means an electromagnetic wave or charged particle beam having an energy quantum capable of polymerizing and cross-linking molecules. Usually, ultraviolet rays, electron beams, etc. are used. In the case of ultraviolet rays, an ultrahigh pressure mercury lamp is used. A light source such as a high-pressure mercury lamp, a low-pressure mercury lamp, a carbon arc, a black light lamp, or a metal halide lamp can be used. As the ultraviolet wavelength, a wavelength range of 1900 to 3800 angstroms is mainly used.

  In the case of an electron beam, it is equipped with an irradiation source such as a cockroft Walton type, a bandegraph type, a resonant transformer type insulated core transformer type, or various electron beam accelerators such as a linear type, a dynamitron type, and a high frequency type. An apparatus can be used, and electrons having an energy of 100 to 1000 KeV, preferably 100 to 300 KeV are irradiated. As an irradiation dose, about 0.5-30 Mrad is preferable normally.

  Depending on the purpose, other various vinyl ester resins, polyisocyanate compounds, polyepoxides, acrylic resins, alkyd resins, urea resins, melamine resins, polyvinyl acetate, vinyl acetate copolymers , Polybutadiene elastomers, saturated polyesters, saturated polyethers; cellulose derivatives such as nitrocelluloses and cellulose acetate butyrate; Natural or synthetic polymer materials; active energy ray-curable resins such as unsaturated polyester resins, urethane acrylate resins, epoxy acrylate resins; calcium carbonate, talc, mica, clay, silica powder, colloidal silica, aluminum hydroxide, stearic acid Various fillers such as lead, zinc white, titanium white, bengara, azo pigments; pigments; polymerization inhibitors such as hydroquinone, benzoquinone, tolhydronoquinone, paratertiary butyl catechol, etc. can also be added .

  The active energy ray-curable coating composition of the present invention thus obtained can be applied using a known coating method such as conventional air spray, airless spray, electrostatic coating, roll coater, flow coater, etc. It can be used as a building interior or exterior material.

  Hereinafter, the present invention will be described in more detail with reference to examples. In the following, all parts and percentages are based on weight unless otherwise specified.

Comparative example 4 (Preparation example of active energy ray-curable coating composition)
In a 1 liter flask equipped with a stirrer, a gas inlet tube, a condenser, and a thermometer, 310.8 parts of isophorone diisocyanate, 425.6 parts of diacrylate of hydroxypentylglycolate, 2.1 parts of tertiary butylhydroxytoluene Then, 0.3 part of methoxyhydroquinone and 0.15 part of dibutyltin diacetate were added, the temperature was raised to 70 ° C., and 182 parts of 2-hydroxypropyl acrylate was added dropwise over 1 hour. After dropping, the mixture was reacted at 70 ° C. for 3 hours, and then 145.6 parts of tricyclodecane dimethanol was charged in portions over 2 hours. After dropping, the mixture was reacted at 70 ° C. for 3 hours, and then reacted until the infrared absorption spectrum of 2250 cm ⁻¹ showing the isocyanate group disappeared, and then 1-hydroxy-cyclohexyl-phenyl ketone (Irgacure 184, manufactured by Ciba Geigy Corporation). ) 30 parts was uniformly mixed to obtain an active energy ray-curable coating composition comprising urethane acrylate resin (A) and a diacrylate of hydroxypivalate neopentyl glycol which is a radical polymerizable monomer (B). .

Example 2 (Preparation example of active energy ray-curable coating composition)
In a 1 liter flask equipped with a stirrer, a gas introduction tube, a condenser, and a thermometer, 206 parts of norbornane diisocyanate (NBDI, manufactured by Mitsui Chemicals, Inc.), 1.5 parts of tertiary butylhydroxytoluene, methoxyhydroquinone 0.2 And 0.15 part of dibutyltin diacetate were added, the temperature was raised to 70 ° C., and 463.6 parts of pentaerythritol triacrylate (Aronix M-305, manufactured by Toagosei Co., Ltd.) was added dropwise over 1 hour. After the dropwise addition, the mixture was reacted at 70 ° C. for 3 hours, and then 98.2 parts of tricyclodecane dimethanol (TCD alcohol DM, manufactured by Celanese) were dividedly charged over 2 hours. After dropping, the mixture was reacted at 70 ° C. for 3 hours, and then reacted until the infrared absorption spectrum of 2250 cm ⁻¹ showing the isocyanate group disappeared, and then 1-hydroxy-cyclohexyl-phenyl ketone (Irgacure 184, manufactured by Ciba Geigy Corporation). ) After 30 parts of uniform mixing, add 329 parts of neopentyl glycol hydroxypivalate diacrylate, diacrylate of neopentyl glycol hydroxypivalate which is urethane acrylate resin (A) and radical polymerizable monomer (B) An active energy ray-curable coating composition comprising:

Comparative Example 5 (Preparation example of active energy ray-curable coating composition)
In a 1 liter flask equipped with a stirrer, gas inlet tube, condenser, and thermometer, 266.4 parts isophorone diisocyanate, 364.8 parts acryloylmorpholine, 1.5 parts tertiary butylhydroxytoluene, 0.2 parts methoxyhydroquinone Then, 0.15 part of dibutyltin diacetate was added, the temperature was raised to 70 ° C., and 156 parts of 2-hydroxypropyl acrylate was added dropwise over 1 hour. After dripping, after reacting at 70 ° C. for 3 hours, 88.3 parts of tricyclodecane dimethanol was charged in portions over 2 hours. After dropping, the mixture was reacted at 70 ° C. for 3 hours, and then reacted until the infrared absorption spectrum of 2250 cm ⁻¹ showing the isocyanate group disappeared, and then 1-hydroxy-cyclohexyl-phenyl ketone (Irgacure 184, manufactured by Ciba Geigy Corporation). ) 30 parts were uniformly mixed to obtain an active energy ray-curable coating composition comprising urethane acrylate resin (A) and acryloylmorpholine which is a radically polymerizable monomer (B).

Comparative Example 1 (Preparation example of active energy ray-curable coating composition)
In a 1 liter flask equipped with a stirrer, a gas introduction tube, a condenser, and a thermometer, 353 parts of norbornane diisocyanate (NBDI, manufactured by Mitsui Chemicals), 300 parts of neopentyl glycol diacrylate hydroxypivalate, tertiary butylhydroxy 2 parts of toluene, 0.3 part of methoxyhydroquinone and 0.15 part of dibutyltin diacetate were added, the temperature was raised to 70 ° C., and 223 parts of 2-hydroxypropyl acrylate was added dropwise over 1 hour. After the dropwise addition, the mixture was reacted at 70 ° C. for 3 hours, and then 124 parts of cyclohexanedimethanol was charged in portions over 2 hours. After dropping, the mixture was reacted at 70 ° C. for 3 hours, and then reacted until the infrared absorption spectrum of 2250 cm ⁻¹ showing the isocyanate group disappeared, and then 1-hydroxy-cyclohexyl-phenyl ketone (Irgacure 184, manufactured by Ciba Geigy Corporation). ) 30 parts was uniformly mixed to obtain an active energy ray-curable coating composition.

Comparative Example 2 (Preparation example of active energy ray-curable coating composition)
In a 1 liter flask equipped with a stirrer, gas inlet tube, condenser and thermometer, 227 parts isophorone diisocyanate, 400 parts neopentyl glycol diacrylate hydroxypivalate, 2 parts tertiary butylhydroxytoluene, methoxyhydroquinone 0.3 Part, 0.15 part of dibutyltin diacetate was added, the temperature was raised to 70 ° C., and 133 parts of 2-hydroxypropyl acrylate was added dropwise over 1 hour. After dropping, the mixture was reacted at 70 ° C. for 3 hours, and then 239 parts of polypropylene glycol triol (MN-700, manufactured by Mitsui Chemicals, Inc.) having a molecular weight of about 700 was dividedly charged over 2 hours. After dropping, the mixture was reacted at 70 ° C. for 3 hours, and then reacted until the infrared absorption spectrum of 2250 cm ⁻¹ showing the isocyanate group disappeared, and then 1-hydroxy-cyclohexyl-phenyl ketone (Irgacure 184, manufactured by Ciba Geigy Corporation). ) 30 parts was uniformly mixed to obtain an active energy ray-curable coating composition.

Comparative Example 3 (Preparation example of active energy ray-curable coating composition)
In a 1-liter flask equipped with a stirrer, a gas introduction tube, a condenser, and a thermometer, 600 parts of epoxy acrylate resin (Unidic V-5500, manufactured by Dainippon Ink and Chemicals), neopentyl glycol hydroxypivalate 400 parts of diacrylate and 70 parts of 1-hydroxy-cyclohexyl-phenyl ketone (Irgacure 184, manufactured by Ciba Geigy) were uniformly mixed to obtain an active energy ray-curable coating composition.

<Evaluation method>
Each active energy ray-curable coating composition obtained in Examples 1 to 3 and Comparative Examples 1 to 3 was applied by the following method and evaluated. The evaluation results are shown in Table 1.

(1) Coating method: Bar coater No. 30, each resin was applied to a glass plate, and one high-pressure mercury lamp, intensity 80 W / cm, conveyor speed 5 m / min with an ultraviolet irradiation device (manufactured by Nippon Battery Co., Ltd.) Irradiated until dry to the touch.
(2) Curing shrinkage: Using a pycnometer, the resin specific gravity before curing at 25 ° C. and the specific gravity of the cured product were measured and calculated by the following formula.
Curing shrinkage (%) = 100 × (cured product specific gravity−resin specific gravity) / (cured product specific gravity)
(3) Hardness: Pencil hardness was measured (destructive hardness).
(4) Heat resistance: In order to promote the occurrence of cracks, the state of the coating film after heating the cured coating film at 150 ° C. for 6 hours was determined by visual evaluation.
Evaluation criteria ○: No crack
×: Crack generation

Effect of the invention

  According to the present invention, by using a urethane acrylate resin containing a polyol having a condensed polycyclic structure as an essential component, an active energy ray-curable coating material having a low curing shrinkage rate and a high hardness cured coating film can be obtained. A composition can be provided.

Claims (6)

Containing a urethane acrylate resin (A) comprising a polyol (a) having a condensed polycyclic structure, a norbornane diisocyanate (b), and a compound (c) containing a hydroxyl group and a (meth) acryloyl group as essential components. An active energy ray-curable coating composition characterized by the above.
The active energy ray-curable coating composition according to claim 1, wherein the polyol (a) containing a condensed polycyclic structure is a polyol having an alicyclic condensed polycyclic structure.
The active energy ray-curable coating composition according to claim 1, wherein the polyol (a) containing a condensed polycyclic structure is tricyclodecane dimethanol.
The active energy ray-curable coating material according to any one of claims 1 to 3 , wherein the compound (c) containing a hydroxyl group and a (meth) acryloyl group is a compound having one hydroxyl group and one (meth) acryloyl group. Composition.
The active energy ray-curable coating composition according to any one of claims 1 to 3 , wherein the compound (c) containing a hydroxyl group and a (meth) acryloyl group is a triacrylate of pentaerythritol.
Furthermore, the composition for active energy ray hardening-type coating materials of any one of Claims 1-5 containing a radically polymerizable monomer (B).