WO2017102526A1

WO2017102526A1 - Thermochromic methacrylate copolymers

Info

Publication number: WO2017102526A1
Application number: PCT/EP2016/080175
Authority: WO
Inventors: Sarav JHAVERI
Original assignee: Basf Coatings Gmbh
Priority date: 2015-12-15
Filing date: 2016-12-08
Publication date: 2017-06-22

Abstract

Thermochromic methacrylate copolymers are disclosed. The copolymers comprise methyl methacrylate, a hydroxyalkyl (meth)acrylate, and a crosslinking monomer. The copolymers exhibit a visually observable color at a temperature of about 75°C to about 100°C or lower and are colorless at higher temperatures.

Description

THERMOCHROMIC METHACRYLATE COPOLYMERS

FIELD OF THE INVENTION

[0001] The present invention relates to thermochromic methacrylate copolymers that demonstrate a visually observable color at temperatures up to about 75°C to about 100°C and are colorless at higher temperatures. The thermochromic copolymers comprise methyl methacrylate, a hydroxyalkyl (meth)acrylate, and a crosslinking monomer.

BACKGROUND OF THE INVENTION

[0002] Articles of manufacture based on a (meth)acrylate polymer typically require the inclusion of a dye or pigment in order to provide a desired color for the article. Dyes and pigments are expensive and often difficult to formulate into the polymeric article. In some applications, the use of a dye or pigment poses toxicity or environment problems that preclude their inclusion into an article of manufacture that could benefit by having a desired color.

SUMMARY OF THE INVENTION

[0003] The present invention is directed to methacrylate copolymers that demonstrate a desired color and obviate the inclusion of a pigment or dye to impart color to an article of manufacture.

[0004] More particularly, the present invention is directed to a methacrylate copolymer that demonstrates thermochromic properties. The present copolymers exhibit a visually observable color up to a temperature of about 75°C to about 100°C, and the copolymer undergoes a color transition to colorless at higher temperatures. The color transition is reversible and the visually observable color returns when the copolymer is cooled. The thermochromic properties of the present copolymers are observed both when the copolymers are in solution or in the solid state.

[0005] A present thermochromic methacrylate copolymer comprises: (a) about 50% to about 95%, by weight, of methyl methacrylate (MMA); (b) about 5% to about 15%, by weight, of a hydroxyalkyl (meth)acrylate; (c) about 0.1% to about 5%, by weight, of a crosslinking monomer; and (d) 0% to about 20%, by weight, of one or more additional monomer having a vinyl moiety.

[0006] In one embodiment, a present thermochromic copolymer comprises methyl methacrylate (MMA), hydroxyethyl methacrylate (HEMA), and hexanediol diacrylate as a crosslinking monomer (HDD A). In another embodiment, a thermochromic copolymer comprises MMA, HEMA, HDDA, and one or both of styrene and ureido methacrylate (UMA).

[0007] In various embodiments, the present thermochromic copolymers have weight average molecular weight (Mw) of about 2,000 to about 120,000, as measured by gel permeation chromatography (GPC). The thermochromic copolymers also exhibit a number average molecular weight (M_n) of about 500 to about 40,000, as measured by GPC. The thermochromic copolymers exhibit a polydisperability index (PDI) of about 2 to about 10.

[0008] Another aspect of the present invention is to provide articles of manufacture comprising a present thermochromic polymer, wherein the article exhibits a visually observable color without the inclusion of a dye or pigment.

[0009] These and other aspects and embodiments of the invention will become apparent from the following nonlimiting description of preferred embodiments.

BRIEF DESCRIPTION OF THE FIGURES

[0010] Figure 1 is a DSC plot of Heat Flow (W/g) vs. Temperature (°C) for the thermochromic copolymer of Example 2 showing a glass transition temperature of 83.9°C;

[0011] Figure 2 is a UV absorbance plot of intensity of absorbance vs. wavelength, over time, for the thermochromic copolymer of Example 2;

[0012] Figure 3 is a DSC plot of Heat Flow (W/g) vs. Temperature (°C) for the thermochromic copolymer of Example 3 showing a glass transition temperature of 70.6°C and an apparent second glass transition temperature of about 95.8°C; and

[0013] Figure 4 is a UV absorbance plot of intensity of absorbance vs. wavelength, over time, for the thermochromic copolymer of Example 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] A present thermochromic copolymer comprises:

[0015] (a) about 50% to about 95%, by weight, of methyl methacrylate;

[0016] (b) about 2% to about 20%, by weight, of a hydroxyalkyl (meth)acrylate;

[0017] (c) about 0.1% to about 5%, by weight, of a crosslinking monomer; and

[0018] (d) 0% to about 20%, by weight, of one or more additional monomer having a vinyl moiety. [0019] The present thermochromic copolymers exhibit a visually observable color up to a temperature about 75°C to 100°C. At higher temperatures, the copolymer is colorless. The color of a thermochromic copolymer varies with the amounts of components (a) to (c) and the presence or absence of component (d).

[0020] A present thermochromic copolymer has a highly branched structure. In solution, and as a solid coating, domains of the thermochromic copolymer, theorized to be liquid crystal-type domains, are present. These crystal-like domains of the copolymer diffract and scatter light to yield a visually observable color at a temperature up to about 100°C. At elevated temperatures, the crystal-like domains disassemble, and the visually observable color disappears. The appearance and disappearance of a visually observable color is reversible. The domains therefore reform in the copolymer gel when the gel is cooled, and the visually observable color reforms.

[0021] The color transition temperature for a specific thermochromic copolymer varies between about 75°C and 100°C and is related to the chemical composition of the copolymer, Tg, Mw, and degree of branching/crosslinking, for example.

[0022] The present thermochromic copolymer contains methyl methacrylate (MMA) in an amount of about 50% to about 95%, by weight, of the polymer, and preferably, about 60% to about 95%, by weight. In most preferred embodiments, the thermochromic polymer contains about 70% to about 90%, by weight, MMA.

[0023] A present thermochromic copolymer also contains a hydroxyalkyl (meth)acrylate in an amount of about 2% to about 20%, or about 5% to about 15%, by weight, of the thermochromic copolymer, and preferably about 6% to about 15%, by weight. In most preferred embodiments, the thermochromic copolymer contains about 7% to about 12%, by weight, of the hydroxyalkyl (meth)acrylate.

[0024] The hydroxyalkyl (meth) acrylate can be a hydroxyalkyl acrylate, a hydroxyalkyl methacrylate, or a mixture thereof. The alkylene group of the hydroxyalkyl (meth)acrylate contains 1 to about 6 carbon atoms. Examples of the hydroxyalkyl (meth)acrylate include, but are not limited to, 2-hydroxyethyl methacrylate (HEMA), 4-hydroxybutyl acrylate, 3- hydroxypropyl acrylate, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 2-hydroxypropyl methacrylate, 3-hydroxypropyl methacrylate, 4-hydroxybutyl methacrylate, and mixtures thereof.

[0025] A present thermochromic copolymer is a branched, crosslinked polymer containing about 0.1% to about 5%, and preferably 0.5% to about 4%, by weight of the copolymer, of a crosslinking monomer. In most preferred embodiments, the thermochromic copolymer contains about 0.75% to about 3%, by weight of the copolymer, of a crosslinking monomer. The crosslinking monomer contains at least two carbon-carbon double bonds, i.e., vinyl moieties, and polymerizes with the MMA and hydroxyalkyl (meth)acrylate via a free radical reaction to form a present thermochromic copolymer.

[0026] The crosslinking monomer can contain two, three, or four vinyl moieties.

Typically, a crosslinking monomer contains two vinyl moieties. In some embodiments, a combination of crosslinking monomers is used, i.e., a crosslinking monomer containing two vinyl monomers is used in conjunction with one or more crosslinking monomer containing three or four vinyl moieties.

[0027] The selection of a particular crosslinking monomer or crosslinking monomers, and their amounts, relates to the degree of branching and crosslinking desired in the

thermochromic copolymer and is readily determined by persons skilled in the art. The amount of crosslinking monomers present in the thermochromic polymer is sufficiently high such that the viscosity of the polymer is not too low; and is the amount sufficiently low such that the thermochromic polymer is not too hard. Typically, a present thermochromic copolymer has a weight average molecular weight (M_w) of about 2,000 to about 120,000, as determined by gel permeation chromatography using polystyrene as a standard, preferably 5,000 to about 60,000, or about 10,000 to about 40,000, and more preferably about 15,000 to about 30,000.

[0028] The present thermochromic copolymers also exhibit a number average molecular weight (M_n) of about 500 to about 40,000, preferably about 1,000 to about 25,000, or about 1,500 to about 15,000, and more preferably about 2,000 to about 10,000. The copolymers have a polydispersity index (PDI) of about 2 to about 10, preferably about 2.25 to about 8, or about 2.5 to about 5, more preferably about 3 to about 4.

[0029] The glass transition temperature of a present thermochromic copolymer is about 30°C to about 120°C, preferably about 40°C to about 110°C, and more preferably about 50°C to about 100°C.

[0030] Crosslinking monomers useful in the present thermochromic copolymers are not limited, and include, but are not limited to, 1,4-butanediol diacrylate, 1,4-butanediol dimethacrylate, 1,3-butylene glycol diacrylate, 1,3-butylene glycol dimethacrylate, diethylene glycol diacrylate, diethylene glycol dimethacrylate, ethylene glycol dimethacrylate, 1,6- hexanediol diacrylate, 1,6-hexanediol dimethacrylate, 1,10-decanediol diacrylate, neopentyl glycol dimethacrylate, polyethylene glycol diacrylate, polyethylene glycol dimethacrylate, triethylene glycol diacrylate, triethylene glycol dimethacrylate, tripropylene glycol diacrylate, tetraethylene glycol diacrylate, tetraethylene glycol dimethacrylate, trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, Ν,Ν'-methylenebisacrylamide, Ν,Ν'- methylenebismethacrylamide, and mixtures thereof.

[0031] Preferred crosslinkers are 1,6-hexanediol diacrylate (HDD A), 1,6-hexanediol dimethacrylate, 1,10-decanediol diacrylate, 1,4-butanediol diacrylate, 1,4-butanediol dimethacrylate, 1,3-butylene glycol diacrylate, 1,3-butylene glycol dimethacrylate, neopentyl diacrylate, and mixtures thereof.

[0032] A monomer solution or dispersion containing MMA, the hydroxyalkyl

(meth)acrylate, and the crosslinking monomer can further contain one or more additional monomer having a carbon-carbon double bond, i.e., a vinyl moiety. The additional monomers are polymerized via a free radical mechanism.

[0033] The additional monomers are included in an amount of 0% to about 20%, by weight, of the thermochromic copolymer, and preferably in an amount of 0% to about 18%, by weight. In some embodiments, the thermochromic copolymer contains about 2% to about 18%, by weight, of the additional monomer. In one embodiment, the additional monomer includes, but is not limited to, one or more of a C_1-8, and preferably a Ci-₄, ester of acrylic acid or a C_2-8, and preferably a C₂-₄ ester of methacrylic acid, for example n-butyl acrylate, n- butyl methacrylate, ethyl acrylate, isopropyl acrylate, isopropyl methacrylate, n-propyl methacrylate, ethyl methacrylate, isobutyl acrylate, t-butyl acrylate, t-butyl methacrylate, methyl acrylate, hexyl acrylate, 2-ethylhexyl acrylate, cyclohexyl acrylate, and mixtures thereof.

[0034] In another embodiment, the additional monomer includes, but is not limited to, one or more of ureido methacrylate, benzyl methacrylate, phenyl acrylate, phenyl methacrylate, and methyl vinyl ether. In yet other embodiments, the additional monomer is a styrenic compound having a formula:

[0035] wherein R is hydrogen or a Ci-6alkyl group, and wherein the phenyl ring optionally is substituted with one to four Ci-₄alkyl and/or hydroxy groups. Specific examples of the styrenic monomers include, but are not limited to, styrene, a-methylstyrene, p-methylstyrene, t-butyl styrene, and the like, and mixtures thereof.

[0036] The additional monomers can be used individually or in any combination. It has been found that additional monomers, such as styrene and ureido methacrylate, can alter the color of a thermochromic copolymer, which allows tailoring of the color of the copolymer.

[0037] In the preparation of a present thermochromic copolymer, the monomers are solubilized or dispersed in a solvent. The solvent is hydrophobic, typically an aromatic or aliphatic solvent having a boiling point sufficiently high such that an exothermic free radical polymerization can be conducted. Preferably, the solvent is an aromatic solvent having a boiling point of about 120°C to about 200°C. One nonlimiting example of an aromatic solvent is Solvesso™ 100, available from ExxonMobil, Houston, TX. The concentration of monomers in the monomer solution or dispersion is about 40% to about 70%, by weight, of the solution or dispersion.

[0038] The monomer solution or dispersion also contains at least one initiator. The initiator or combination of initiators is present in a sufficient amount to provide a present thermochromic copolymer having a desired weight average molecular weight.

[0039] Any of the various polymerization initiators known for use in a free radical polymerization of vinyl-containing monomers can be used in the present invention.

Examples of useful initiators include, but are not limited to, t-butyl hydroperoxide; di-t-butyl peroxide; t-butyl perbenzoate; t-butyl peroxy isopropyl carbonate; l,l-di-t-butylperoxy-3,3,5- trimethylcyclohexane; benzoyl peroxide, dicumyl peroxide; caprylyl peroxide; acetylacetone peroxide; methyl ethyl ketone peroxide; cumene hydroperoxide; tert-amyl perpivalate; tert- butyl perpivalate; tert-butyl perneohexanote; tert-butyl perisobutyrate; tert-butyl per-2- ethlhexanoate; tert-butyl perisononanoate; tert-butyl permaleate; tert-butyl perbenzoate; tert- butyl per-3,5,5-tri-methylhexanoate; and tet-amyl perneodecanoate.

[0040] Thermal initiators also are useful. Nonlimiting examples of thermal initiators include, but are not limited to, azobisisobutyronitrile; 4-t-butylazo-4'-cyanovaleric acid; 4,4'- azobis(4-cyanovaleric acid); 2,2'-bis(2-amidinopropane) dihydrochloride; 2,2'-azobis(2,4- dimethylvaleronitrile); dimethyl 2,2'-azobisisobutyrate; 2,2'-azodimethyl bis(2,4- dimethylvaleronitrile); (1-phenylethyl) azodiphenylmethane; 2,2'-azobis(2- methylbutyronitrile) ; 1 , 1 '-azobis( 1 -cyclohexanecarbonitrile) ; 2-(carbamoylazo)

isobutyronitrile; 2,2'-azobis(2,4,4-trimethylpenta-2-phenylazo-2,4-dimethyl-4- methoxyvaleronitrile; 2,2'-azobis (2-methylpropane); 2,2'-azobis (N,N'dimethyleneisobutyramidine) dihydrochlonde; 4,4'azobis (4-cyanopentanoic acid); 2,2'- azobis(2-methyl-N-[l,l-bis (hydroxymethyl)-2-hydroxyethyl]propionamide); 2,2'-azobis(2- methyl-N-[l,l-bis(hydroxymethyl)ethyl]propionamide); 2,2'-azobis[2-methyl-N- (hydroxyethyl)propionamide] ; 2,2'-azobis(isobutyramide)dihydrate; and other thermal initiators known to persons skilled in the art.

[0041] Initiators can be used singularly or in suitable combination. The initiators can be used together with a chain transfer agent to control the molecular weight of the

thermochromic copolymer. Typically chain transfer agents are sulfur-containing compounds, such as dodecyl mercaptan, 2-mercaptoethanol, n-octyl mercaptan, and butyl mercaptan.

[0042] A present thermochromic copolymer is prepared by forming a solution or dispersion of the MMA, hydroxyalkyl (meth)acrylate, optional additional monomer, and crosslinking monomer in a suitable solvent. The resulting solution or dispersion then is subjected to a free radical polymerization to yield a present thermochromic copolymer.

[0043] In an alternative embodiment of the present invention, a solution or dispersion of MMA, the hydroxyalkyl (methacrylate), and the crosslinking monomer are polymerized to form a copolymer reaction product, then an additional monomer is added to the resulting copolymer reaction product, and the additional monomer is polymerized in the presence of the copolymer reaction product to provide a thermochromic copolymer. The copolymer reaction product exhibits a first visually observable color and the final thermochromic copolymer exhibits a second visually observable color, which is different from the first visually observable color of the copolymer reaction product.

[0044] The following examples illustrate present thermochromic copolymers and their method of preparation.

EXAMPLE 1

[0045] Aromatic Solvesso™ S100 solvent (550 g) was heated to 120°C in a 3-neck round bottom flask equipped with a stirrer under a nitrogen atmosphere. A mixture containing methyl methacrylate (MMA) (576 g), 25% uriedo methacrylate (UMA) in methyl

methacrylate (72 g), 2-hydroxyethyl methacrylate (HEMA) (57.6 g), hexanediol diacrylate (HDD A) (14.4 g), and an initiator (tert-butyl peroxy-2-ethylhexanoate, 23.11 g) was added to the flask over a period of 90 minutes. Upon completion of the feed, the mixture container was rinsed with S100 solvent (14.78 g). The resulting reaction mixture was held at 120 °C for 15 minutes, then tert-butyl peroxy-2-ethylhexanoate initiator (0.3 g) mixed in S100 solvent (20 g) was added. The resulting mixture again was held for 15 minutes at 120°C, then cooled by adding S100 (478.48 g). N-Butanol (n-BuOH, 136.01 g) then was added, and the reaction mixture was poured into a storage container at 80°C -90°C.

[0046] When the reaction mixture cooled, the resulting copolymer gel was hazy and opalescent, emitting a blue-yellow color. The addition of further n-BuOH cleared and solubilized the copolymer. The thermochromic copolymer of Example 1 was viscous at room temperature, i.e., about 25°C. Heating the copolymer gel above 80°C allowed the gel to flow and the copolymer gel became clear and colorless. The blue-yellow color of the

thermochromic copolymer of Example 1 returned when the copolymer was cooled.

[0047] The copolymer of Example 1 had a number average molecular weight of 7,250; a weight average molecular weight of 24,720; and a polydispersity index (PDI) of 3.41.

EXAMPLE 2

[0048] Aromatic Solvesso™ 100 solvent (550 g) was heated to 120°C in a 3-neck round bottom flask equipped with a stirrer under a nitrogen atmosphere. A mixture containing methyl methacrylate (648 g), 2-hydroxyethyl methacrylate (57.6 g), HDDA (14.4 g), and an initiator (tert-butyl peroxy-2-ethylhexanoate, 14.78 g) was added to the flask over a period of 90 minutes. Upon completion of the feed, the mixture container was rinsed with S100 solvent (14.78 g). The resulting reaction was held at 120°C for 15 minutes, then tert-butyl peroxy-2-ethylhexanoate initiator (0.3 g) mixed in S100 solvent (20 g) was added. The mixture again was held for 15 minutes at 120°C and a sample was removed (Sample 1) prior to cooling the reaction mixture. The reaction mixture then was cooled to room temperature by adding S100 solvent (200 g).

[0049] Sample 1, before cooling, exhibited thermochromic behavior emitting a strong blue color. When solvent was added and the reaction mixture cooled, the copolymer product was hazy and opalescent, emitting a blue-yellow opalescence. Heating the copolymer product above 80°C cleared the gel and the gel was colorless. The color of the copolymer returned when the copolymer was cooled. Example 2 shows that neither ureido groups in the polymer nor a butanol solvent are required for the copolymer to exhibit thermochromic behavior. The glass transition temperature (Tg) of the copolymer was 83.9°C, as shown in the DSC plot of Fig. 1.

[0050] UV absorbance curves for the thermochromic copolymer of Example 2 were obtained. The thermochromic copolymer of Example 2 exhibited a blue-yellow- white opalescent color. A sample of the copolymer of Example 2 was heated to 100°C and allowed to cool naturally to room temperature over a period of 20 minutes while observing changes in the photonic behavior of the sample. The absorbance spectra were measured over time after cooling began at t=0 up to t=20 minutes. As seen in Fig. 2, the initial curve at t=0 min, when the sample is hot at 100°C, did not exhibit any absorbance above 400nm wavelength. As the copolymer solution cooled, the copolymer began to absorb between 400nm-1090nm, and the intensity of absorbance increased over time until absorbance began to saturate after 20 minutes, when the sample had cooled to room temperature.

[0051] The copolymer of Example 2 had a number average molecular weight (M_n) of 6,450; a weight average molecular weight (M_w) of 20,610; and a polydispersity index (PDI) of 3.19.

EXAMPLE 3

[0052] Aromatic Solvesso™ S100 solvent (507.67 g) was heated to 125°C in a 3 neck round bottom flask equipped with a stirrer under a nitrogen atmosphere. A mixture containing methyl methacrylate (486.9 g), 2-hydroxyethyl methacrylate (64.9 g), hexanediol diacrylate (13.21 g), and an initiator (tert-butyl peroxy-2-ethylhexanoate, 22.20 g) was added into the flask over a period of 90 minutes. The reaction mixture was held at 120°C for 15 minutes, then a mixture containing styrene (103.9 g) and initiator (tert-butyl peroxy-2- ethylhexanoate, 8.03 g) was added to the flask over a period of 20 minutes. After completion of the feed, the mixture container was rinsed with SI 00 solvent (15.90 g). tert-Butyl peroxy- 2-ethylhexanoate initiator (0.58 g) mixed in S100 solvent (19.87 g) then was added to the reaction mixture. The resulting mixture was held for 30 minutes at 120°C, then was cooled to room temperature.

[0053] The thermochromic copolymer of Example 3 exhibited a blue -red

emission/opalescence. Heating the copolymer above 80°C to 100°C cleared the gel and the color disappears. The color of the copolymer returned when the copolymer cooled. The addition of styrene changed the scattering wavelength, and accordingly the color of the copolymer solution. It is theorized, but not relied upon, that the emission color/wavelength changed because the refractive index of the structured copolymer changed due to an alteration of the copolymer composition and morphology. The glass transition temperature of the copolymer of Example 3 was at 70.6°C (Fig. 3), although a second apparent glass transition temperature was observed at a temperature of about 100°C.

[0054] UV absorbance curves for the thermochromic copolymer of Example 3 also were obtained. The thermochromic copolymer of Example 3 had a distinct blue-red color. The intensity of absorption of Example 3 at different wavelengths was measured as a sample of the copolymer was allowed to cool from 100°C to room temperature. For the thermochromic copolymer of Example 3, the intensity of absorption increased between 310nm-580nm with a drop in temperature (Fig. 4). The increase in the intensity saturated after about 12 minutes.

[0055] The thermochromic copolymers of Examples 1 through 3 also were applied onto a substrate as a coating. After solvent evaporation and drying, the resulting films exhibited the same reversible thermochromic behavior as the copolymers in solution.

[0056] As demonstrated above, the present thermochromic copolymers scatter and diffract light at various wavelengths due to Bragg's scattering phenomenon. As also demonstrated above, the light scattering and resulting color can be altered according to the monomers used to prepare to copolymer, and how the copolymer is prepared, i.e., a stepwise polymerization as in Example 3.

[0057] The observable color exhibited by a present thermochromic copolymer can be modified by admixing the copolymer with a dye or pigment, or by adding an aromatic compound, such as triphenyl phosphine, perylene, pyrene, or similar aromatic compounds. In this case, the resulting product has a first color at a temperature of about 75°C to about 100°C, then under a transition to a second color at elevated temperature.

[0058] The present thermochromic copolymers also possess the benefit of having pendant hydroxy functionalities that can be used for crosslinking in coating applications. Additional benefits include a readily scalable process, and practical applications in the fields of coatings for reflective signs, photonic plexiglass, non-pigmented photoactive plastics, electronic and optically active materials, colored plastics, plastics replacing pigments, and coatings and plastics having novel optical properties, for example.

Claims

WHAT IS CLAIMED:

1. A thermochromic copolymer comprising:

(a) about 50% to about 95%, by weight, of methyl methacrylate;

(b) about 2% to about 20%, by weight, of hydroxyalkyl (meth)acrylate;

(c) about 0.1% to about 5%, by weight, of a crosslinking monomer; and

(d) 0% to about 20%, by weight, of one or more additional monomer having a vinyl moiety.

2. The thermochromic copolymer of claim 1 having an M_w of about 2,000 to about 120,000, an M_n of about 500 to 40,000; a PDI of about 2 to about 10; and a Tg of about 30°C to about 120°C.

3. The thermochromic copolymer of claim 1 comprising about 60% to about 95%, by

weight, of methyl methacrylate.

4. The thermochromic copolymer of any of the preceding claims comprising about 6% to about 15%, by weight, of the hydroxyalkyl (meth)acrylate.

5. The thermochromic copolymer of any of the preceding claims comprising about 0.5% to about 4%, by weight, of the crosslinking monomer.

6. The thermochromic copolymer of any of the preceding claims having a weight average molecular weight of about 10,000 to about 40,000.

7. The thermochromic copolymer of any of the preceding claims wherein the additional monomer comprises a C_1-8 ester of acrylic acid or a C2-₈ester of methacrylic acid.

8. The thermochromic copolymer of any of the preceding claims wherein the additional monomer comprises one or more of n-butyl acrylate, n-butyl methacrylate, ethyl acrylate, isopropyl acrylate, isopropyl methacrylate, n-propyl methacrylate, ethyl methacrylate, isobutyl acrylate, t-butyl acrylate, t-butyl methacrylate, methyl acrylate, hexyl acrylate, 2-ethylhexyl acrylate, cyclohexyl acyrlate, ureido methacrylate, benzyl methacrylate, phenyl acrylate, phenyl methacrylate, methyl vinyl ether, a styrenic compound having a formula:

wherein R is hydrogen or a Ci-6alkyl group, and wherein the phenyl ring optionally is substituted with one to four Ci-₄alkyl and/or hydroxy groups, and mixtures thereof.

9. The thermochromic copolymer of any of the preceding claims, wherein the copolymer exhibits a visually observable color up to a temperature of about 75°C to 100°C and is colorless at higher temperatures.

10. The thermochromic copolymer of any of the preceding claims wherein the methyl methacrylate, hydroxyalkyl methacrylate, and crosslinking monomer are polymerized in an aromatic solvent.

11. A composition comprising a thermochromic copolymer of claim 1 and at least one of an aromatic compound, a dye, and a pigment.

12. The composition of claim 11 wherein the aromatic compound comprises

triphenylphosphine, perylene, or pyrene.

13. An article of manufacture comprising a thermochromic copolymer of any of the

preceding claims.

14. A method of preparing a thermochromic copolymer comprising

(a) providing a mixture of methyl methacrylate, a hydroxyalkyl (meth)acrylate, and a crosslinking monomer;

(b) subjecting the mixture of (a) to a free radical polymerization to provide a reaction mixture comprising a thermochromic copolymer having a first color;

(c) adding an additional monomer having a vinyl moiety to the reaction mixture of (b);

(d) subjecting the mixture of (c) to a free radical polymerization to provide a

thermochromic copolymer having a second color.

15. The method of claim 14 wherein the mixture of (a) further comprises a hydrophobic solvent.