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2,6-Anthracenedicarboxylate-Containing Polyesters and Copolyesters

2,6-Anthracenedicarboxylate-Containing Polyesters and Copolyesters

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COLLARD AND SCHIRALDI



include the high cost of 2,6-naphthalenedicarboxylic acid and the high melt

viscosity of the polymer.

Small amounts of comonomers (e.g., isophthalic acid, diethylene glycol,

1,4-cyclohexane dimethanol) are added to PET, PBT, and PEN to improve

their physical properties. However, other poly(alkylene arenedicarboxylate)

homopolymers have not been commercialized. The vast choice of flexible

and rigid structural units available for incorporation in polyesters has led us

to investigate new copolymers prepared from other aromatic diacids, in

particular 2,6-anthracenedicarboxylic acid. As a homolog of terephthalic

and napthoic acids, we expected the anthracenedicarboxylic acid unit to

provide further enhancement of the physical properties of polyesters. In

addition, while the anthracenedicarboxylic acid is aromatic, and therefore

stable to the harsh conditions required for polyesterification, we also hoped

to exploit the reactivity of the anthracene unit to chemically modify these

new polymers and copolymers. Our work has been extended to a number of

other fused arenes (3-7).

Previous reports of poly(alkylene 2,6-anthracenedicarboxylate)s (PxA)

have been limited to characterization of the polymer prepared by

condensation of dimethyl 2,6-anthracenedicarboxylate with 1,6-hexanediol

(8), and solution phase polymerization of ethylene glycol and a Diels-Alder

adduct of anthracenedicarboxylate followed by heating to effect the retro

Diels-Alder reaction to give poly(ethylene 2,6-anthracenedicarboxylate),

P2A (9). In this chapter we provide an overview of our work to incorporate

2,6-anthracenedicarboxylate into homopolymers and copolymers, how this

unit effects the thermal properties of the polymers, and how it presents new

opportunities to modify polymers by addition reactions.



2. MONOMER SYNTHESIS AND POLYMERIZATION

2.1 Monomer Synthesis

Our synthesis of dimethyl 2,6-anthracenedicarboxylate starts with the

Friedel-Crafts acylation of p-xylene with 4-methylbenzoyl

chloride to give 2,5,4’-trimethylbenzophenone, Figure 1 (10). Heating the

neat product to reflux results in ring closure by the Elbs reaction to give a

mixture of 2,6-dimethylanthracene and 2,6-dimethylanthrone.

Both

components of the mixture obtained from this cyclization undergo oxidation

upon treatment with chromium trioxide to give a single product: 9,10anththraquinone-2,6-dicarboxylic acid. The anthraquinone is reduced to 2,6anthracene-dicarboxylic acid upon treatment with zinc metal in aqueous

ammonium hydroxide. Our initial attempts to perform this reduction made

use of a catalytic amount of copper(II) sulfate. However these reactions led



2,6 ANTHRACENEDICARBOXYYLATE-CONTAINING



239



to the formation of a large amount of the over-reduced product, 9,10dihydroanthracene-2,6-dicarboxylic acid. Omission of the copper sulfate

leads to a high yield of the desired product with no over-reduction. The

diacid is treated with methyl iodide and lithium carbonate in DMF to give

the methyl ester. This method has proved successful for the synthesis of

diesters from diacids that do not undergo Fischer esterification because of

their insolubility in acidic refluxing methanol. The monomer is isolated and

purified by recrystallization from benzene as a yellow crystalline solid

(mp=274-275 °C)



An alternative route was explored to avoid the low conversion of the Elbs

cyclization in the previous procedure and to allow access to the 2,7-isomer.

Diels-Alder reaction between isoprene and benzoquinone, Figure 2, gives a

mixture of 2,6- and 2,7-dimethyltetrahydroanthraquinones. Treatment of the

crude mixture with oxygen in basic ethanol gives a mixture of the

corresponding dimethylanthraquinones that can be separated by repeated

recrystallization from ethanol (11). Oxidation of the separate dimethylanthraquinones with chromium trioxide yields the anthraquinonedicarboxylic acids that can be converted to the dimethyl anthracenedicarboxylates by the methods described above (reduction of the anthraquinone followed by esterification). While this route allows access to both

the 2,6- and 2,7-isomers, the tedious separation of the dimethylanthraquinone presents a severe limitation.

Another attempt to synthesize the 2,6-substituted anthracene core made

use of a Diels-Alder reaction between benzoquinone and 2-carboxy-l,3butadiene. The latter compound can be generated by in situ extrusion of

from 3-carboxysulfolene, which itself is readily prepared from butadiene,

sulfur dioxide and carbon dioxide, Figure 2 (12).

2-Anthracenecarboxylic acid was prepared by reduction of 2-carboxy9,10-anthtraquinone, and converted to its 2-hydroxyethylester to prepare

anthracene end-capped macromers (Section 7), Figure 3.



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COLLARD AND SCHIRALDI



2.2 Polymerization

Dimethyl 2,6-anthracenedicarboylate is amenable to polyesterification

under a variety of conditions appropriate for synthesis of polyesters. In

general, it was incorporated into PET-copolymers, PET-A (Sections 4-6), by

standard a two-step polymerization process. A mixture of ethylene glycol

(2.2 equivalents),

dimethyl

terephthalate and dimethyl

2,6anthracenedicarboylate (one equivalent of dimethyl esters in a ratio to define

the composition of the copolymer), together with antimony trioxide and

manganese acetate is heated to 190-230 °C for 2-3 hours to prepare a

mixture of corresponding bis(2-hydroxyethyl) esters. This is followed by

addition of polyphosphoric acid to deactivate the manganese catalyst and

heating to 250-290 °C under vacuum for 2-3 hours with the removal of

excess ethylene glycol to afford polymer.

Poly(alkylene 2,6-anthracenedicarboxylate)s, PxA (Section 3), were

prepared from dimethyl 2,6-anthracenedicarboxylate and the appropriate

diol,

in the presence of tetra(butoxy)titanium(IV) at 250-290

°C.



2,6 ANTHRACENEDICARBOXYYLATE-CONTAINING



241



3. POLY(ALKYLENE ANTHRACENE 2,6-DICARBOXYLATE)S, PnA

Poly(alkylene 2,6-anthracenedicarboxylate)s (PxA) were prepared from

dimethyl 2,6-anthracenedicarboxylate and

to

explore the effect of extending the aromatic diacid in polyesters from 1,4phenylene (i.e., terephalate) to 2,6-naphthalene to 2,6-anthracene (11).

Poly(ethylene 2,6-anthracenedicarboxylate), P2A, the anthracene analog of

PET and PEN, is an insoluble, intractable solid. The polymer solidifies as it

is formed in the stirred polymerization reactor at 290 °C. An alternate

synthesis of P2A entailed monitoring mass lost during thermal transesterification of bis(hydroxyethyl) 2,6-anthracenedicarboxylate by thermal

gravimetric analysis (TGA). The thermogram indicates no loss of mass up

to 225 °C and then a mass loss of 17.8% by 300 °C, which correlates well

with the removal of one equivalent of ethylene glycol (18 mass%) from the

monomer to form the polymer P2A. Further heating led to decomposition at

temperatures above 400 °C. The mass loss was confirmed by isothermal

gravimetric analysis at 310 °C. The evolution of ethylene glycol was

confirmed by Fourier transform infrared spectroscopy of the gas evolved

from the TGA. The material resulting from this thermolysis is insoluble in

all common organic solvents including those typically used for polyesters

(trifluoroacetic acid, 1,1,1,3,3,3-hexafluoroisopropanol, 2-chlorophenol).

Differential scanning calorimetry (DSC) analysis of the material shows no

thermal transitions up to 400 °C.

The homopolymer derived from dimethyl 2,6-anthracenedicarboxylate

and 1,4-butanediol is also insoluble and infusible, and displays no thermal

transitions below 400 °C by DSC. The higher homologs are more soluble

and display melting points. Homologs from P6A to P12A display two

endotherms upon melting, and a single exotherm upon cooling. As

expected, the melting points of the polymers decrease as the alkyl spacer

length increases. The temperature difference between the two endotherms

also decreases and the relative peak areas of the two endotherms change as

the rate of cooling is varied: The enthalpy associated with the lowertemperature peak increases upon more rapid cooling. This is consistent the

with formation of a polymorphic sample whereby the polymer crystallizes

into two different crystal forms, and it is analogous to the behavior of other

aliphatic-aromatic polyesters, including poly(hexamethylene terephthalate),

P6T, the terephthalate analog of P6A. Examination of the series of

homologous anthracene-containing homopolymers by polarized light

microscopy and x-ray diffraction provides no evidence for the formation of

liquid crystalline phases.

In general, the melting points of the anthracene-containing polymers,

PnA, are 80-90 °C higher than the corresponding terephthalates and 50-60



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COLLARD AND SCHIRALDI



°C higher than the 2,6-naphthalate polymers. There is a pronounced oddeven effect of the number of carbons in the aliphatic diol on the melting

point of the anthracene-containing homopolymers, similar to that found for

the corresponding homologs in the PxT and PxN series.



4. POLY(ETHYLENE 2,6-ANTHRACENEDICARBOXYLATE-co-TEREPHTHALATE)S, PET-A

Incorporation of small amounts of the 2,6-anthracenedicarboxylate

structural unit into copolymers of PET results in an increase in glass

transition temperature and a decrease in melting temperature (10). The

intensity of the melting transition gradually decreases upon increasing the

amount of the anthracene comonomer in the polymer, and copolymers with

greater than 15% of the anthracenedicarboxylate structural unit are

completely amorphous. The copolymers with greater than 20% of the

anthracene comonomer are insoluble in common solvents. We have

explored the effect of the anthracenedicarboxylate in copolyesters on the

post-polymerization chemical modification of polymer structure (Sections 46), on the fluorescent properties of PET fiber (3,4) and on gas barrier

properties of PET films (13).



5. DIELS-ALDER CROSSLINKING AND GRAFTING

REACTIONS OF PET-A

Anthracene undergoes rapid [4+2] cycloaddition with electron deficient

alkenes by the Diels-Alder reaction. This reaction is reversible upon

proceeding to higher temperatures. Reaction of copolyesters, PET-A, with

various maleimides leads to rapid Diels-Alder reactions, Figure 4 (14).

While initial experiments were performed over long times (a number of

hours) in the melt (i.e., >270 °C), we have also shown that this reaction

proceeds at considerably lower temperatures in the solid state (i.e., 150 °C).

Differential scanning calorimetry (DSC) of mixtures of PET-A and Noctadecylmaleimide shows a melting endotherm at 60-70 °C

of maleimide in the mixture) corresponding to the melting of phaseseparated maleimide. This is followed by a broad exotherm with a peak at

approximately 150 °C corresponding to the heat of the Diels-Alder reaction

(approximately -90 kJ per mole Diels-Alder adduct). Subsequent heating and

cooling cycles show only a glass transition at 45 °C,

Further

exidence for the Diels-Alder reaction included changes in the 1HNMR and



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