Synthesis and characterization of four alkyl 2-cyanoacrylate monomers and their precursors for use in latent fingerprint detection

Synthesis and Characterization of Four Alkyl 2-Cyanoacrylate Monomers

and Their Precursors for Use in Latent Fingerprint Detection

MARK TAHTOUH, JOHN R. KALMAN, BRIAN J. REEDY

Department of Chemistry and Forensic Science, University of Technology, Sydney, PO Box 123,

Broadway New South Wales 2007 Australia

Received 6 August 2010; accepted 19 October 2010

DOI: 10.1002/pola.24449

Published online 18 November 2010 in Wiley Online Library (wileyonlinelibrary.com).

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ABSTRACT: Four novel cyanoacrylates, 2-cyanoethyl 2-cyanoacry-

late, 1-cyanoethyl 2-cyanoacrylate, trideuteromethyl 2-cyano-

acrylate and pentadeuteroethyl 2-cyanoacrylate have been

of fingerprints on difficult surfaces. 2010 Wiley Periodicals, Inc.

J Polym Sci Part A: Polym Chem 49: 257–277, 2011

synthesized using

a

Diels-Alder protection/deprotection route

KEYWORDS: adhesives; chemical imaging; cyanoacrylate; 1-cya-

noethyl 2-cyanoacrylate; 2-cyanoethyl 2-cyanoacrylate; finger-

prints; forensic science; FTIR; hyperspectral imaging; infrared;

infrared spectroscopy; pentadeuteroethyl 2-cyanoacrylate; syn-

thesis; trideuteromethyl 2-cyanoacrylate

involving anthracene. The common route for the synthesis of

alkyl 2-cyanoacrylates was found to be unsatisfactory for the pro-

duction of small quantities of the targeted cyanoacrylates, which

have potential as reagents for the mid-infrared spectral imaging

INTRODUCTION Alkyl 2-cyanoacrylate monomers (1) (Fig. 1,

often referred to as Super Glue or superglue) have found

unique application for the instantaneous bonding of a wide

variety of materials such as metals, alloys, plastics, rubbers,

and ceramics.^1–6A large range of cyanoacrylate adhesives

have been marketed, but the most widely used cyanoacrylate

monomer is the ethyl ester (1b). Cyanoacrylates are applied

as liquids and cure within seconds to minutes at room

temperature by reacting in the presence of moisture or

weakly alkaline materials, forming inert, hard, polymeric

solids. Important medical^7–11and pharmaceutical^12–14appli-

cations have also been reported.

enclosure such as a fish tank was used to expose evidentiary

items to low concentrations of the cyanoacrylate vapor.^15,21

A number of techniques have since been developed to

accelerate the fuming process and minimize background

development.^{15,18,21–23}

Cyanoacrylate Synthesis

The most common method of cyanoacrylate synthesis, first

reported by Ardis,²⁴involves the Knoevenagel condensation

of alkyl cyanoacetates (2) with formaldehyde in the presence

of a base catalyst. This reaction should, in principle, yield the

desired monomeric alkyl cyanoacrylate, but the extreme

reactivity of these monomers precludes their direct synthesis

via this method. The actual product formed is an oligomer of

the desired cyanoacrylate, which necessitates the additional

step of thermal depolymerization to isolate the monomeric

alkyl cyanoacrylate.

A lesser known application of cyanoacrylate monomers is for

the detection and enhancement of latent fingerprints on non-

porous surfaces. The use of monomeric cyanoacrylates for

this purpose was first demonstrated by the Criminal Identifi-

cation Division of the Japanese National Police Agency in

1978.¹⁵At this stage, it was seen as a rather exotic method

of detection and was known only to a small section of the

forensic community.¹⁶Since then, the cyanoacrylate fuming

method has gained wide popularity and has become the

method of choice for the development of prints on non-

porous surfaces such as metals, electrical and adhesive tape,

leather, glass, ceramics, and plastics.^15,17–20

A number of improvements to this method have since

been reported.^25–34Alternative routes have been described,

mostly to synthesize cyanoacrylates that are difficult or

impossible to synthesize via the normal condensation-de-

polymerization method. These include fluorinated, higher

alkyl and difunctional or bis-cyanoacrylates. These alterna-

tive routes include:

The cyanoacrylate fuming method involves exposure of the

• the use of acid catalysts for the condensation reaction

between alkyl cyanoacetates and formaldehyde^35–38

• Diels-Alder protection and deprotection^39–49

• esterification of 2-cyanoacryloyl chloride with an alcohol

or diol (for bis cyanoacrylates)⁵⁰

evidential object to cyanoacrylate vapors in

a cabinet

designed for this purpose. The monomeric cyanoacrylate

vapor preferentially polymerizes on the ridges of latent

fingerprints, usually producing a white deposit. Initially, an

Correspondence to: B. J. Reedy (E-mail: brian.reedy@uts.edu.au)

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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 49, 257–277 (2011) 2010 Wiley Periodicals, Inc.

NOVEL ALKYL 2-CYANOACRYLATE MONOMERS, TAHTOUH, KALMAN, AND REEDY

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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA

The R groups were selected to contain a functional group

that would produce a vibrational band in a region of the

mid-infrared that is generally free of absorption bands from

common surfaces on which the fingerprint may lie. The

range from 2500 to 1800 cm^ꢀ1generally contains very few

vibrational bands, a notable exception being triple bonds

such as CBC and CBN. It is desirable to have a reagent that

contains a functional group that absorbs within this ‘‘region

of interest.’’ This would provide the necessary contrast

between the ridge details of the treated fingerprint and the

background on which it may be located. The particular vibra-

tional modes targeted during this work were those produced

by nitrile (CBN) or deuterated (C–D) functional groups due

64

to their absorbances at 2280–2240 cm^ꢀ1

and 2100–

2235 cm^{ꢀ1 65,66}

respectively.

,

FIGURE 1 Structural formula for alkyl 2-cyanoacrylates (1),

ethyl 2-cyanoacrylate (1a), and alkyl cyanoacetates (2).

Compounds that contain the nitrile (CBN) functional group

are immediately obvious candidates for the MIR spectral

imaging of fingerprints. Not only is the stretching frequency

of this functional group (m(CBN)) isolated and free from inter-

ferences but it is also present in conventional cyanoacrylate.

Unfortunately, the intensity of the nitrile band in conventional

cyanoacrylate decreases significantly upon polymerization.

This phenomenon has been noted by several authors^{57,58,67–78}

who concluded, variously, that it was caused by cross-linking

of polymer chains, hydrogen-bonding, or the reduction of

conjugation occurring upon polymerization.

• direct transesterification of methyl cyanoacrylate with a

diol to form the corresponding bis-cyanoacrylate⁵¹

• condensation of either methyl or ethyl cyanoacetate with

paraformaldehyde followed by transesterification of the

intermediate oligomer with a higher alkyl alcohol and sub-

sequent thermal depolymerisation⁵²

• oxidation of a phenylselenide precursor^53,54

• condensation of diethyl 2,4-dicyanoglutarate (from the

waste residue of large scale cyanoacrylate synthesis) with

paraformaldehyde and subsequent depolymerization^55,56

For this reason (the loss of the m(CBN) intensity in poly-cya-

noacrylate), an R group containing a second nitrile moiety

on the ester chain of the cyanoacrylate molecule was

selected. It was hoped that the band arising from this nitrile

group would be retained in the Fourier Transform Infrared

(FTIR) spectrum of the polymer and would, therefore, be

useful for the MIR spectral imaging of the treated finger-

prints. Other considerations for the R group are the availabil-

ity of starting materials and the predicted volatility of the

cyanoacrylate produced (since fingerprints are developed by

exposing the sample to vapors of cyanoacrylate monomer).

To maintain monomer volatility, the molecular weight of the

R group was kept low. With these criteria in mind, 4 target

compounds were chosen (see Fig. 2) namely 2-cyanoethyl 2-

cyanoacrylate (1b), 1-cyanoethyl 2-cyanoacrylate (1c), tri-

deuteromethyl 2-cyanoacrylate (1d), and pentadeuteroethyl

2-cyanoacrylate (1e).

Mid-Infrared Spectral Imaging

Mid-infrared (MIR) spectral imaging involves the simultane-

ous collection of thousands of mid-infrared spectra across a

sample using a focal plane array detector. This allows for the

generation of chemically specific spectral data, while main-

taining spatial information. Images can then be generated

based on spectral/chemical contrast between components

within a sample. The application of MIR spectral imaging for

the detection and enhancement of latent (untreated) finger-

prints has been demonstrated.^57–63Tahtouh et al. have also

reported the MIR spectral imaging of fingerprints on difficult

surfaces (such as Australian polymer banknotes) treated

with conventional (ethyl) cyanoacrylate. The aim of this

work was the identification, synthesis, and characterization

of modified cyanoacrylates, which may be used as reagents

for MIR spectral imaging of fingerprints.

This article discusses the synthesis and characterization of

these four monomers, which have successfully been applied

for the fuming of latent fingerprints and subsequent infrared

chemical imaging; details are presented elsewhere.⁷⁹

The four novel cyanoacrylate monomers targeted (1b–1e)

(see Fig. 2) retain the active vinyl moiety, and hence are

expected to react with fingerprint residue in the same

manner as conventional cyanoacrylates.

FIGURE 2 Structures of target cyanoacrylate monomers.

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ARTICLE

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EXPERIMENTAL

115 C. After 8 days (192 h) at 100–115 C, ꢂ14.0 mL water

(78% of theoretical) had been collected, and the rate of

water evolution had dropped significantly. The benzene was

removed by rotary evaporation followed by 2 h under

vacuum. Vacuum distillation yielded 100.2 g (72.6%) 2-cya-

noethyl cyanoacetate (2a). Bp. 154–156 ^ꢁC (1 mmHg). ¹H

NMR (300 MHz, CDCl₃, ppm): d 2.79, (t, J ¼ 6 Hz, 2H), 3.58

(s, 2H), 4.42 (t, J ¼ 6 Hz, 2H). ¹³C NMR (75 MHz, CDCl₃,

ppm): d 18.3, 25.1, 61.3, 113.2, 116.9, 163.3. MS: m/z 68

(100), 54 (35), 40 (29), 41 (22), 28 (19), 27 (11). IR (KBr,

thin film, cm^ꢀ1): 3489, 2978, 2950, 2257, 1754, 1470, 1453,

1419, 1392, 1341, 1180, 1029. Calculated for C₆H₆N₂O₂: C,

52.17; H, 4.38; N, 20.28. Found: C, 51.46; H, 4.47; N, 20.04.

Materials

Ethyl 2-cyanoacrylate was obtained as tubes of commercial

superglue from a retail outlet and was used without further

purification. All other materials and solvents were commer-

cially available and were used as received, unless otherwise

indicated. Drying of organic residues was performed using

AR grade anhydrous Na₂SO₄. Benzene, toluene, and p-xylene

were dried by storing over sodium wire. Where anhydrous

reaction conditions were needed, glassware was flame-dried

before use, and all reactions were performed under an

atmosphere of nitrogen unless otherwise stated. Reactions

were either stirred magnetically or using an IKA RW 20.n

overhead mechanical stirrer.

Anthracene/Ethyl 2-cyanoacrylate Adduct;

Ethyl 9,10-dihydro-9,10-endoethanoanthracene-11-

cyano-11-carboxylate (3a)

Glassware used for the deprotection of anthracene/alkyl 2-

cyanoacrylate adducts via the retro-Diels-Alder reaction was

treated with chromic acid cleaning solution and washed

thoroughly with deionised water before being oven dried at

100 ^ꢁC. The glassware was then treated with a solution of

chlorotrimethylsilane (5% in toluene) followed by several

Into a 1-L two-necked round bottom flask fitted with an

overhead mechanical stirrer and condenser with gas inlet

valve was charged anthracene (53.5 g, 0.300 mol), ethyl 2-

cyanoacrylate (41.3 g, 0.330 mol), and anhydrous benzene

(600 mL) saturated with a strong stream of SO₂(minimum 5

min). The mixture was heated under reflux for 20 h. The so-

lution was cooled, and the unreacted anthracene (6.7 g) was

filtered off. The benzene was then removed by rotary evapo-

ration before placing the flask under reduced pressure for

minimum of 2 h to ensure complete solvent removal. The

crude anthracene/ethyl 2-cyanoacrylate adduct (3a) weighed

78.2 g (86%) and was used without further purification for

ꢁ

washings with dry toluene and oven drying at 100 C.

Measurements

Infrared spectra were recorded on a Nicolet Magna IR 760

spectrophotometer or Digilab FTS 7000 IR spectrophoto-

meter. Liquid samples were placed directly between two

freshly pressed 13-mm KBr disks, and the spectra were

collected in transmission. Solids were prepared as KBr disks

and spectra were collected in transmission. All ¹H and ¹³C

Nuclear Magnetic Resonance (NMR) spectra were obtained

using a Bruker DRX NMR spectrometer operating at 300.13

MHz and 75.48 MHz, respectively, using either deuterated

chloroform (CDCl₃), deuterated acetone (acetone-d₆) or

deuterated dimethylsulfoxide (DMSO-d₆) as solvents. Gas

chromatography-mass spectrometry (GS-MS) was performed

on a Hewlett Packard 5890 GC coupled to a 5970 series

Mass Sensitive Detector. Elemental analyses were performed

1

the next step. H NMR (300 MHz, CDCl₃, ppm): d 1.26 (t, J ¼

7 Hz, 3H), 2.19 (dd, J₁¼ 13.2 Hz, J₂¼ 2.7 Hz, 1H), 2.80 (dd,

J₁¼ 13.2 Hz, J₂¼ 2.7 Hz, 1H), 4.17 (m, 2H), 4.42 (t, J_9,12a

¼

J_9,12b¼ 2.7 Hz, 1H), 4.87 (s, 1H), 7.10–7.50 (m, 8H). ¹³C

NMR (75 MHz, CDCl₃, ppm): d 14.0, 38.0, 43.2, 47.3, 51.8,

63.2, 119.8, 123.5, 123.8, 125.0, 125.8, 126.3, 126.6, 127.5,

127.6, 137.1, 138.0, 142.3, 143.0. IR (KBr disk, cm^ꢀ1): 3074,

3016, 2985, 2961, 2232, 1747, 1458, 1369, 1276, 1224,

1189, 1097, 1048, 755, 637, 579.

on

a Carlo Erba 1106 automatic analyser (Australian

Anthracene/2-Cyanoacrylic Acid Adduct;

9,10-Dihydro-9,10-endoethanoanthracene-11-

cyano-11-carboxylic Acid (4)

National University–Microanalytical Laboratory). Melting

points were obtained on a John Morris Electrothermal melt-

ing point apparatus and were corrected using benzoic acid

(Mp. 122 ^ꢁC) as reference. Simultaneous thermogravimetric

analysis (TGA) and differential thermal analysis (DTA) were

per_ꢁformed on a SDT 2960 module (TA instruments) with a

10 C/min heating rate.

Into a 500-mL round bottom flask equipped with an over-

head mechanical stirrer was charged 3a (crude) (78.2 g),

absolute ethanol (200 mL), and potassium hydroxide (25.2 g,

0.450 mol) in water (200 mL). The resulting suspension was

stirred at ambient temperature (25–30 ^ꢁC) for 3 h. Quench-

ing of the reaction in water (1 L) and filtration afforded 8.8

g anthracene. The orange-red filtrate was acidified to pH 2

with 10 M hydrochloric acid to precipitate the anthracene/2-

cyanoacrylic acid adduct (4) as a finely divided white solid.

The solid 4 was filtered and washed with water and air

dried at room temperature to yield 60.5 g (85.8%) of 4, mp.

201–204 ^ꢁC (ref. 46, 199–203 ^ꢁC). ¹H NMR (300 MHz,

acetone-d₆, ppm): d 2.21 (dd, J₁¼ 12.9 Hz, J₂¼ 2.7 Hz, 1H),

Intermediates and Monomers

2-Cyanoethyl Cyanoacetate (2a)

This reaction was performed in a 500-mL two-necked round

bottom flask fitted with magnetic stirrer, Dean-Stark reflux

trap with condenser and calcium chloride drying tube. Into

the round bottom flask was charged cyanoacetic acid

(85.0 g, 1.00 mol), 3-hydroxypropanenitrile (79.6 g, 1.12

mol), Amberlyst 15 cationic exchange resin (H^þform, 6.00 g)

as catalyst, and anhydrous benzene (300 mL þ 10 mL in the

Dean-Stark trap). The mixture was stirred to dissolve the

cyanoacetic acid and heated via an oil bath. The oil bath

temperature was slowly elevated over 1 h to a maximum of

2.84 (dd, J₁¼ 12.9 Hz, J₂¼ 2.7 Hz, 1H), 4.63 (t, J_9,12a

¼

J_9,12b¼ 2.7 Hz, 1H), 5.08 (s, 1H), 7.10–7.65 (m, 8H). ¹³C

NMR (75 MHz, acetone-d₆, ppm): d 39.0, 44.0, 48.1, 52.2,

121.0, 124.6, 124.7, 126.3, 126.7, 127.1, 127.2, 128.2, 138.8,

NOVEL ALKYL 2-CYANOACRYLATE MONOMERS, TAHTOUH, KALMAN, AND REEDY

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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA

140.0, 144.0, 144.6, 168.2. IR (KBr disk, cm^ꢀ1): 3070, 3024,

2972, 2880, 2810, 2631, 2245, 1718, 1464, 1405, 1281,

1267, 1171, 1112, 934, 763, 730, 638, 573, 517. Calculated

for C₁₈H₁₃NO₂: C, 78.53; H, 4.76; N, 5.09. Found: C, 78.55; H,

4.94; N, 5.18.

7.10–7.60 (m, 8H). ¹³C NMR (75 MHz, CDCl₃, ppm): d 38.7,

43.0, 52.0, 57.9, 117.7, 123.6, 123.9, 125.4, 126.0, 126.8,

126.9, 127.9, 128.0, 135.5, 136.6, 141.7, 142.7, 168.6. IR

(KBr disk, cm^ꢀ1): 3070, 3047, 2960, 2242, 1786, 1744,

1722, 1611, 1460, 1445, 1242, 1207, 1170, 1059, 96, 883,

803, 764, 725, 672, 633, 585. Calculated for C₁₈H₁₂ClNO: C,

73.60; H, 4.12; N, 4.77. Found: C, 75.09; H, 4.54; N, 5.24.

Anthracene/2-Cyanoacrylic Acid Potassium Salt

Adduct; Potassium 9,10-Dihydro-9,10-

endoethanoanthracene-11-cyano-11-carboxylate (5)

Into a 500-mL two-necked round bottom flask fitted with a

mechanical stirrer and dropping funnel was charged a

solution of 4 (27.5 g, 0.100 mol) in acetone (100 mL). The

solution was stirred and 40% w/v potassium hydroxide in

absolute methanol added till the pH was 9.5. The suspension

of white solid was stirred at room temperature for 3 h, fil-

tered, washed with 100 mL acetone, and air-dried at room

temperature to give 22.5 g (71.9%) of anthracene/2-cyano-

acrylic acid potassium salt adduct (5), mp. 252–256 ^ꢁC dec.

(ref. 46, 250–260 ^ꢁC). ¹H NMR (300 MHz, DMSO-d₆, ppm):

Anthracene/2-Cyanoethyl 2-cyanoacrylate Adduct; 2-

Cyanoethyl 9,10-Dihydro-9,10-endoethanoanthracene-

11-cyano-11-carboxylate (3b)

Into a 500-mL two-necked round bottom flask fitted with

magnetic stirrer bar, dropping funnel, and reflux condenser

with gas inlet tube was charged under nitrogen, 6 (29.5 g,

0.10 mol), triethylamine (12.1 g/16.7 mL, 0.12 mol), and

anhydrous benzene (150 mL). The solution was stirred

under nitrogen and warmed to 60 ^ꢁC. 3-Hydroxypropaneni-

trile (7.1 g, 0.10 mol) was added dropwise over 1 h. After

stirring at 60 ^ꢁC for 3 h, the suspension of triethylamine

hydrochloride was diluted with benzene (300 mL) and

extracted several times with saturated ammonium chloride

solution followed by saturated sodium chloride solution. The

benzene extract was collected and dried over anhydrous so-

dium sulfate and solvent-stripped under vacuum to yield

23.4 g (71.2%) of crude anthracene/2-cyanoethyl 2-cyanoa-

crylate adduct (3b). The crude product was recrystallized

from ethanol to yield 20.8 g (63.1%) 3b. Mp. 127–129 ^ꢁC.

¹H NMR (300 MHz, CDCl₃, ppm): d 2.26 (dd, J₁¼ 13.2 Hz,

J₂¼ 2.7 Hz, 1H), 2.59–2.76 (m, 2H), 2.84 (dd, J₁¼ 13.2 Hz,

d 1.82 (dd, J₁¼ 12.5 Hz, J₂¼ 2.9 Hz, 1H), 2.75 (dd, J₁

¼

12.5 Hz, J₂¼ 2.5 Hz, 1H), 4.33 (t, J_9,12a¼ J_9,12b¼ 2.6 Hz,

1H), 4.77 (s, 1H), 6.95–7.40 (m, 8H). ¹³C NMR (DMSO-d6): d

38.8, 43.3, 48.9, 51.4, 122.9, 123.4, 125.0, 125.3, 125.4,

125.5, 125.8, 125.8, 126.0, 140.3, 142.0, 143.7, 144.2, 165.6.

IR (KBr disk, cm^ꢀ1): 3471, 3073, 3028, 2961, 2938, 2862,

2238, 1615, 1458, 1352, 897, 793, 754, 642, 579, 519, 478.

Calculated for C₁₈H₁₂KNO₂: C, 68.98; H, 3.86; N, 4.47. Found:

C, 68.20; H, 3.97; N, 4.51.

Anthracene/2-Cyanoacryloyl Chloride Adduct;

9,10-Dihydro-9,10-endoethanoanthracene-11-cyano-11-

carbonyl Chloride (6)

J₂¼ 2.7 Hz, 1H), 4.10–4.40 (m, 2H), 4.48 (t, J_9,12a¼ J_9,12b

¼

2.7 Hz, 1H), 4.96 (s, 1H), 7.10–7.60 (m, 8H). ¹³C NMR (75

MHz, CDCl₃, ppm): d 17.7, 37.9, 42.9, 47.2, 51.7, 61.0, 116.4,

119.1, 123.5, 124.0, 124.9, 125.9, 126.5, 126.7, 127.6,

127.7, 136.7, 137.5, 142.1, 142.7, 166.4. IR (KBr disk, cm^ꢀ1):

3071, 3045, 3024, 2970, 2944, 2870, 2248, 2242, 1753,

1461, 1412, 1387, 1336, 1278, 1234, 1198, 1111, 1055,

1023, 1000, 768, 636, 586, 567. Calculated for C₂₁H₁₆N₂O₂:

C, 76.81; H, 4.91; N, 8.31. Found: C, 76.67; H, 4.94; N, 8.32.

A suspension of 4 (41.3 g, 0.15 mol) in anhydrous benzene

(150 mL) was stirred under nitrogen in a 500-mL three-

necked round bottom flask fitted with overhead mechanical

stirrer, dropping funnel, and reflux condenser. The reaction

was catalyzed with a few drops of pyridine. Thionyl chloride

(21.4 g/13.1 mL, 0.18 mol) was added dropwise via the

dropping funnel over a period of 20 min. The resulting

mixture was stirred under nitrogen for 1 h at 60 ^ꢁC. An

additional 5.95 g/3.6 mL (0.05 mol) thionyl chloride was

added and stirring continued for 2 h. Further portions of thi-

onyl chloride (2 mL) were added as required to ensure com-

plete reaction. As the desired product is soluble in warm

benzene while the starting material (4) is not, the reactions

completion was signified by the disappearance o_ꢁf the

suspension of 4. This typically required stirring at 60 C for

about 4 h. Heating to temperatures exceeding 60 ^ꢁC led to

the appearance of anthracene in the final product due to a

retro Diels-Alder side-reaction. The warm solution was

filtered and allowed to cool. The white needles were col-

lected and recrystallized from hot benzene to yield 19.8 g of

anthracene/2-cyanoacryloyl chloride adduct (6). A second

crop (8.1 g) could be obtained by solvent-stripping the

mother liquor and recrystallizing the residue from hot

benzene. Total yield 27.9 g (63.3%). Mp. 81–84 ^ꢁC (ref. 46,

Anthracene/1-Cyanoethyl 2-cyanoacrylate Adduct;

1-Cyanoethyl 9,10-Dihydro-9,10-endoethanoanthracene-

11-cyano-11-carboxylate (3c)

Into a 500-mL two-necked round bottom flask fitted with

magnetic stirrer bar, dropping funnel, and reflux condenser

with gas inlet tube was charged under nitrogen, 6 (29.5 g,

0.10 mol), triethylamine (12.1 g/16.7 mL, 0.12 mol), and an-

hydrous benzene (150 mL). The solution was stirred under

ꢁ

nitrogen and warmed to 60 C, and 2-hydroxypropanenitrile

(7.1 g, 0.10 mol) was added dropwise over 40 min. After

stirring at 60 ^ꢁC for 3 h, the suspension of triethylamine

hydrochloride was diluted with benzene (300 mL) and

extracted several times with saturated ammonium chloride

solution followed by saturated sodium chloride solution. The

benzene extract was collected and dried over anhydrous so-

dium sulfate and solvent-stripped under vacuum to yield

22.9 g (69.5%) of crude anthracene/1-cyanoethyl 2-cyano-

acrylate adduct (3c). The crude product was recrystallized

from ethanol to yield a total of 18.2 g (55.3%) 3c in two

crops. This product was a mixture of stereoisomers. The

1

ꢁ

82–85 C). H NMR (300 MHz, CDCl₃, ppm): d 2.28 (dd, J₁¼

13.5 Hz, J₂¼ 2.7 Hz, 1H), 2.75 (dd, J₁¼ 13.5 Hz, J₂¼ 2.7

Hz, 1H), 4.45 (t, J_9,12a¼ J_9,12b¼ 2.7 Hz, 1H), 5.06 (s, 1H),

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WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

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NMR spectra clearly show two distinct groups of resonances,

resulting from two diastereoisomeric pairs of enantio-

mers with a ratio of ꢂ53% enantiomeric pair a and 47%

enantiomeric pair b (see NMR section below for detailed

discussion).

charged under nitrogen, 4 (29.5 g, 0.10 mol), and triethyl-

amine (12.1 g/16.7 mL, 0.12 mol) in anhydrous benzene

(150 mL). Ethanol-d₆(5.21 g/5.84 mL, 0.10 mol) was added

dropwise over 10 min. After stirring at 60 ^ꢁC for 3 h, the

suspension of triethylamine hydrochloride was diluted with

benzene (300 mL), filtered, and extracted several times with

saturated ammonium chloride solution followed by saturated

NaCl solution. The benzene extract was collected and dried

over anhydrous sodium sulfate and solvent-stripped under

vacuum to yield 19.48 g (63.2%) crude anthracene/penta-

deuteroethyl 2-cyanoacrylate adduct (3e). The crude product

was recrystallized from ethanol to yield 17.7 g (57.5%) 3e.

Mp. 126–127 ^ꢁC. ¹H NMR (300 MHz, CDCl₃, ppm): d 2.24

(dd, J₁¼ 12.9 Hz, J₂¼ 2.7 Hz, 1H), 2.84 (dd, J₁¼ 12.9 Hz,

J₂¼ 2.7 Hz, 1H), 4.46 (t, J_9,12a¼ J_9,12b¼ 2.7 Hz, 1H), 4.91

(s, 1H), 7.10–7.60 (m, 8H). ¹³C NMR (75 MHz, CDCl₃, ppm):

d 13.0 (sept-J_CD¼ 19 Hz), 37.9, 43.1, 47.3, 51.7, 62.4 (quin–

J_CD¼ 23 Hz), 119.8, 123.5, 123.8, 124.4, 125.8, 126.3, 126.6,

127.4, 127.5, 137.1, 137.9, 142.3, 142.9, 166.7. IR (KBr disk,

cm^ꢀ1): 3075, 3026, 2959, 2871, 2269, 2233, 2184, 1955,

1916, 1745, 1460, 1332, 1281, 1254, 1237, 1192, 1174,

1115, 1082, 1043, 953, 765, 752, 637, 583, 513. Calculated

for C₂₀H₁₂D₅NO₂: C, 77.89; H/D 5.56; N, 4.54. Found: C,

78.20; H/D, 5.56; N, 4.88.

Enantiomeric Pair a. ¹H NMR (300 MHz, CDCl₃, ppm): d 1.67

(d, J ¼ 6.9 Hz, 3H), 2.25 (dd, J₁¼ 13.2 Hz, J₂¼ 2.7 Hz, 1H), 2.83

(dd, J₁¼ 13.2 Hz, J₂¼ 2.7 Hz, 1H), 4.49 (t, J_9,12a¼ J_9,12b¼ 2.7

Hz, 1H), 4.95 (s, 1H), 5.32 (q, J ¼ 6.9 Hz, 1H), 7.10–7.60 (m,

8H). ¹³C NMR (75 MHz, CDCl₃, ppm): d 18.5, 37.9, 42.9, 47.1,

51.8, 59.4, 116.4, 118.7, 123.6, 123.9, 125.3, 125.9, 126.7,

126.8, 127.8, 127.9, 136.2, 136.2, 141.8, 142.7, 165.4.

Enantiomeric Pair b. ¹H NMR (300 MHz, CDCl₃, ppm): d

1.67 (d, J ¼ 6.9 Hz, 3H), 2.30 (dd, J₁¼ 13.2 Hz, J₂¼ 2.7 Hz,

1H), 2.80 (dd, J₁¼ 13.2 Hz, J₂¼ 2.7 Hz, 1H), 4.49 (t, J_9,12a

¼

J_9,12b¼ 2.7 Hz, 1H), 4.89 (s, ¹H), 5.27 (q, J ¼ 6.9 Hz, 1H),

7.10–7.60 (m, 8H). ¹³C NMR (75 MHz, CDCl₃, ppm): d 18.6,

38.6, 42.9, 47.2, 51.7, 59.6, 116.0, 118.5, 123.6, 124.1, 124.9,

125.9, 126.5, 126.8, 127.8, 127.9, 136.5, 137.3, 142.1, 142.6,

165.5. IR (KBr disk, cm^ꢀ1): 3073, 3044, 3020, 2955, 2875,

2239, 1977, 1931, 1825, 1762, 1461, 1381, 1329, 1305,

1274, 1247, 1221, 1185, 1092, 1046, 1026, 953, 881, 768,

636, 586, 517. Calculated for C₂₁H₁₆N₂O₂: C, 76.81; H, 4.91;

N, 8.31. Found: C, 76.48; H, 4.97; N, 8.35.

2-Cyanoethyl 2-Cyanoacrylate (1b)

Anthracene/Trideuteromethyl 2-Cyanoacrylate Adduct;

Trideuteromethyl 9,10-Dihydro-9,10-endoethanoanthra-

cene-11-cyano-11-carboxylate (3d)

Into a 500-mL two-necked round bottom flask fitted with a

small condenser, gas inlet valve, rubber septum, and a mag-

netic stirrer was charged under SO₂, 3b (32.8 g, 0.10 mol),

powdered maleic anhydride (9.81 g, 0.10 mol), phosphorus

pentoxide (ꢂ1 g; based on 10 g/mole monomer), hydroqui-

none (ꢂ15 mg, 1000 ppm based on theoretical yield of

monomer), and anhydrous p-xylene (300 mL) inhibited with

a strong stream of SO₂(minimum 5 min). SO₂(polymeriza-

tion inhibitor) was introduced into the system via an inlet

tube on top of the condenser and the reaction mixture was

brought to a gentle reflux. The reaction was monitored by

NMR of regularly withdrawn 100-lL aliquots in CDCl₃. Once

NMR indicated the reaction’s completion (45 hours), the mix-

ture was allowed to cool to room temperature and the by-

product, anthracene/maleic anhydride adduct (7), was fil-

tered off. The mixture was solvent-stripped at 35–40 ^ꢁC

under reduced pressure. The crude product was dissolved in

anhydrous benzene (inhibited with SO₂), cooled, and filtered

to remove any residual crystalline 7. The residual xylene

was removed by repeated addition and evacuation of anhy-

drous benzene (inhibited with SO₂) to yield crude monomer,

which was stabilized with P₂O₅and hydroquinone and kept

in benzene solution (100 mL) under SO₂until just before

use. The purity of the crude monomer, 6.44 g (42.9%), was

found by NMR to be ꢂ80%, with 3b and 7 being the only

Into a 500-mL two-necked round bottom flask fitted magnetic

stirrer bar, dropping funnel, and reflux condenser was charged

under nitrogen, 6 (29.5 g, 0.10 mol), and triethylamine (12.1 g/

16.7 mL, 0.12 mol) in anhydrous benzene (150 mL). Methanol-

d₄(3.61 g/ 4.06 mL, 0.10 mol) was added dropwise over

10 min. After stirring at 60 ^ꢁC for 3 h, the suspension of triethyl-

amine hydrochloride was diluted with benzene (300 mL) and

extracted several times with saturated ammonium chloride solu-

tion followed by saturated NaCl solution. The benzene extract

was collected and dried over anhydrous sodium sulfate and

solvent-stripped under vacuum to yield 20.81 g (71.3%) crude

anthracene/trideuteromethyl 2-cyanoacrylate adduct (3d). The

crude product was recrystallized from ethanol to yield 17.73 g

1

ꢁ

(60.7%) 3d in three crops. Mp. 97–99 C. H NMR (300 MHz,

CDCl₃, ppm): d 2.24 (dd, J₁¼ 13.2 Hz, J₂¼ 2.7 Hz, 1H), 2.83 (dd,

J₁¼ 13.2 Hz, J₂¼ 2.7 Hz, 1H), 4.46 (t, J_9,12a¼ J_9,12b¼ 2.7 Hz,

1

1H), 4.91 (s, H), 7.10–7.60 (m, 8H). ¹³C NMR (75 MHz, CDCl₃,

ppm): d 38.1, 43.2, 47.3, 51.8, 53.2 (sept–J_CD¼ 22.3 Hz), 119.7,

123.6, 124.0, 124.9, 125.9, 126.5, 126.7, 127.6, 127.6,.137.2,

137.9, 142.3, 143.0, 167.4. IR (KBr disk, cm^ꢀ1): 3077, 3032,

2961, 2869, 2265, 2236, 2178, 2118, 2077, 1957, 1917, 1751,

1458, 1332, 1284, 1255, 1198, 1173, 1112, 1078, 963, 857, 748,

637, 584, 517. Calculated for C₁₉H₁₂D₃NO₂: C, 78.06; H/D, 5.17;

N, 4.79. Found: C, 77.81; H/D, 5.21; N, 4.68.

1

contaminants. H NMR (300 MHz, CDCl₃, ppm): d 2.83 (t, J ¼

6.3 Hz, 2H), 4.50 (t, J ¼ 6.3 Hz, 2H), 6.73 (s, 1H), 7.13 (s,

1H). ¹³C NMR (75 MHz, CDCl₃, ppm): d 17.7, 60.6, 113.8,

115.6, 116.1, 144.5, 159.8.

Anthracene/Pentadeuteroethyl 2-cyanoacrylate Adduct;

Pentadeuteroethyl 9,10-dihydro-9,10-endoethanoanthra-

cene-11-cyano-11-carboxylate (3e)

1-Cyanoethyl 2-Cyanoacrylate (1c)

Into a 500-mL two-necked round bottom flask fitted mag-

netic stirrer bar, dropping funnel, and reflux condenser was

Into a 500-mL two-necked round bottom flask fitted with a

small condenser, gas inlet valve, rubber septum, and a

NOVEL ALKYL 2-CYANOACRYLATE MONOMERS, TAHTOUH, KALMAN, AND REEDY

261

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA

magnetic stirrer was charged under SO₂, 3c (16.4 g, 0.05

mol), powdered maleic anhydride (5.88 g, 0.06 mol), and

anhydrous p-xylene (300 mL) inhibited with a strong stream

of SO₂(minimum 5 min). SO₂(polymerization inhibitor) was

introduced into the system via an inlet tube on top of the

condenser and the reaction mixture was brought to a gentle

reflux. The reaction was monitored by NMR of regularly

withdrawn 100-lL aliquots in CDCl₃. Once NMR indicated

the reaction’s completion (71.5 h), the mixture was allowed

to cool to room temperature and the by-product, 7, was fil-

tered off. The mixture was solvent-stripped at 35–40 ^ꢁC

under reduced pressure. The crude product was dissolved in

anhydrous benzene (inhibited with SO₂), cooled, and filtered

to remove any residual crystalline 7. The residual xylene

was removed by repeated addition and evacuation of anhy-

drous benzene (inhibited with SO₂) to yield crude monomer,

which was kept in benzene solution (100 mL) under SO₂

until just prior to use. The crude monomer (4.57 g) was

found by NMR to be a mixture of ꢂ50% w/w 1c and 50%

w/w 3c. Based on this, the yield was taken to be ꢂ2.3 g

(ꢂ31%) 1c. ¹H NMR (300 MHz, CDCl₃, ppm): d 1.78 (d, J ¼

6.9 Hz, 3H), 5.50 (q, J ¼ 6.6 Hz, 1H), 6.56 (s, 1H), 7.06 (s,

1H). ¹³C NMR (75 MHz, CDCl₃, ppm): d 17.9, 59.1, 113.3,

114.5, 116.2, 144.8, 158.7.

introduced into the system via an inlet tube on top of the

condenser, and the reaction mixture was brought to a gentle

reflux. The reaction was monitored by NMR of regularly

withdrawn 100-lL aliquots in CDCl₃. Once NMR indicated

the reaction’s completion (97.5 h), the mixture was allowed

to cool to room temperature and the by-product, 7, was

filtered off. The mixture was solvent-stripped at 35–40 ^ꢁC

under reduced pressure. The crude product was dissolved in

anhydrous benzene (inhibited with SO₂), cooled, and filtered

to remove any residual crystalline 7. The residual xylene

was removed by repeated addition and evacuation of anhy-

drous benzene (inhibited with SO₂) to yield crude monomer,

which was kept in benzene solution (100 mL) under SO₂

until just before use. Some product was lost during removal

of residual solvent due to the volatility of monomer. The

crude monomer (14.07 g) was found by NMR to be a mix-

ture of ꢂ50% w/w 1e and 50% w/w 3e. Based on this, the

1

yield was taken to be ꢂ7 g (ꢂ54%) 1e. H NMR (300 MHz,

CDCl₃, ppm): d 6.61 (s, 1H), 7.07 (s, 1H). ¹³C NMR (75 MHz,

CDCl₃, ppm): d 12.9 (sept-J_CD¼ 19 Hz), 62.4 (quin–J_CD¼ 23

Hz), 114.4, 116.8, 142.9, 160.4.

RESULTS AND DISCUSSION

Approaches to the Synthesis of Novel Cyanoacrylates

As discussed above, the most common route for cyano-

acrylate synthesis is via the Knoevenagel condensation of a

cyanoacetate with formaldehyde followed by thermal depoly-

merization. At the outset of this work, attempts were made

to isolate 2-cyanoethyl 2-cyanoacrylate (1b) via this route.⁸⁰

The precursor compound, 2-cyanoethyl cyanoacetate (2a),

was successfully synthesized in high yield (ꢂ73%) via the

acid-catalysed esterification of cyanoacetic acid and 3-

hydroxypropanenitrile. Although this compound has been

previously reported,^81,82the methods outlined by these

authors proved unsatisfactory in our hands. An alternative

procedure, using benzene and Amberlyst 15 cation exchange

resin, proved successful. The crude product wa_ꢁs distilled at

reduced pressure and yielded 2a (b.p. 154–156 C/1 mmHg)

of sufficient purity at 73% theoretical yield. This product

was characterized by its ¹H and ¹³C NMR spectra as well as

IR, MS and elemental analysis data, which all supported the

proposed structure.

Trideuteromethyl 2-Cyanoacrylate (1d)

Into a 500-mL two-necked round bottom flask fitted with a

small condenser, gas inlet valve, rubber septum, and a mag-

netic stirrer was charged under SO₂, 3d (15.0 g, 0.051 mol),

powdered maleic anhydride (6.05 g, 0.062 mol), and anhy-

drous p-xylene (300 mL) inhibited with a strong stream of

SO₂(minimum 5 min). SO₂(polymerization inhibitor) was

introduced into the system via an inlet tube on top of the

condenser, and the reaction mixture was brought to a gentle

reflux. The reaction was monitored by NMR of regularly

withdrawn 100-lL aliquots in CDCl₃. Once NMR indicated

the reaction’s completion (48 h), the mixture was allowed to

cool to room temperature and the by-product, 7, was filtered

off. The mixture was solvent-stripped at 35–40 ^ꢁC under

reduced pressure. The crude product was dissolved in anhy-

drous benzene (inhibited with SO₂), cooled, and filtered to

remove any residual crystalline 7. The residual xylene was

removed by repeated addition and evacuation of anhydrous

benzene (inhibited with SO₂) to yield crude monomer, which

was kept in benzene solution (100 mL) under SO₂until just

before use. The crude monomer (5.55 g) was found by NMR

to be a mixture of ꢂ70% w/w 1d and 30% w/w 3d with

small traces of 7. Based on this, the yield was taken to be

ꢂ3.89 g (ꢂ66.3%) 1d. ¹H NMR (300 MHz, CDCl₃, ppm): d

7.04 (s, 1H), 6.60 (s, 1H). ¹³C NMR (75 MHz, CDCl₃, ppm): d

52.8 (sept-J_CD¼ 22.6 Hz), 114.4, 116.8,143.5, 161.0.

The next step in this synthetic route involved the Knoevena-

gel condensation of 2a with paraformaldehyde to form

oligo-2-cyanoethyl 2-cyanoacrylate (8). Despite difficulties

involved with the viscous oligomeric product,

8 was

obtained in crude form. Several attempts to depolymerize

the oligomeric product in the presence of P₂O₅and hydro-

quinone polymerization inhibitors failed. The mixture was

observed to boil vigorously and gradually darken and no

evidence of distilling monomer was observed. Eventually, the

reaction mixture blackened and charred completely. No

monomeric product was observed in the analysis of the

distillate, either by GC-MS or ¹H NMR. Both analyses of the

distillate collected during the attempted depolymerization

indicated the presence of significant amounts of 2a and 3-

hydroxypropanenitrile. Both these compounds were absent

Pentadeuteroethyl 2-Cyanoacrylate (1e)

Into a 500-mL two-necked round bottom flask fitted with a

small condenser, gas inlet valve, rubber septum, and a mag-

netic stirrer was charged under SO₂, 3e (30.8 g, 0.10 mol),

powdered maleic anhydride (10.79 g, 0.11 mol), and anhy-

drous p-xylene (350 mL) inhibited with a strong stream of

SO₂(minimum 5 min). SO₂(polymerization inhibitor) was

262

WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE

SCHEME 1 Simultaneous Knoeve-

nagel condensation and Diels-Alder

‘‘protection’’ yields an anthracene/

alkyl 2-cyanoacrylate adduct. Retro-

grade Diels-Alder (‘‘deprotection’’)

of anthracene/alkyl 2-cyanoacrylate

adduct yields monomeric alkyl 2-

cyanoacrylate.

from the analysis of the crude oligomer and so it follows

that they have been produced during the attempted depoly-

merization process. The exact mechanism for the formation

of these by-products is not clear.

route that was first described in a patent by Ray and Doran

in 1969.⁴⁰This method involves the synthesis of monomeric

2-cyanoacrylates (1) by heating a mixture of the correspond-

ing ester of cyanoacetic acid (2), formaldehyde, anthracene,

and a basic catalyst in an inert, nonaqueous organic solvent

to give the Diels-Alder adduct of the monomeric ester and

anthracene (3). The monomeric cyanoacrylate can then be

displaced from the anthracene adduct by heating the adduct

alone or in the presence of a more reactive dienophile

such as maleic anhydride.⁴⁰The later approach allows the

liberated anthracene to be trapped as the stable anthracene/

maleic anhydride adduct (7), and the monomer can be

isolated in an anhydrous environment. The overall process is

shown in Scheme 1.

The difficulties encountered here have also been reported by

numerous groups. For example, Denchev et al. report difficul-

ties in depolymerizing oilgo-allyloxyethyl cyanoacrylate even

when it is heated to temperatures in excess of 300 ^ꢁC.¹

Similarly, Kotzev et al. were unable to isolate the following

cyanoacrylate monomers: 2-methyl-2-propenyl, 2-butenyl

and 1-propyl-2-propenyl⁸³and Vijayalakshmi et al. report

difficulties in obtaining tert-butyl and undecyl cyano-

acrylate.³⁸Using a molar excess of the cyanoacetate in the

condensation step, Denchev et al. were able to obtain a more

easily depolymerizable product, but the yields were still

comparatively low.¹

This method avoids the formation of an oligomeric inter-

mediate and, therefore, does not require a depolymerization

step. Furthermore, the anthracene adduct precursors are

stable crystalline compounds at room temperature. They can,

therefore, be isolated in reasonable purity by recrystalliza-

tion. Ray and Doran used this method for the synthesis of

monofunctional cyanoacrylates such as ethyl cyanoacrylate.⁴⁰

Giral et al. describe a modification of this method that uses

acidic catalysts for the synthesis of fluorinated cyano-

acrylates.⁴⁸As mentioned above, however, acidic catalysts

fail to effect the Knoevenagel condensation of 2a with

paraformaldehyde.

Because of the difficulties encountered in the attempted

synthesis via the common route, which involves a depoly-

merization step, alternative procedures for the synthesis of

cyanoacrylates were investigated.

An alternative preparative method involving the use of acid

catalysts (e.g., piperdine hydrochloride and glacial acetic

acid) for the condensation reaction between alkyl cyanoace-

tates and formaldehyde^35–38also proved unsuccessful in our

hands.

The majority of the remaining alternative approaches for the

synthesis of cyanoacrylates are either poorly documented,⁵¹

involve the use of extremely reactive intermediates (such as

2-cyanoacryloyl chloride),⁵⁰involve a complex multistep syn-

thesis^53,54and/or still involve a depolymerisation step.^52,56

The exception is the Diels-Alder protection and deprotection

Ray and Doran’s protection/deprotection method is amena-

ble to much smaller quantities than those normally used

for the Knoevenagel condensation/depolymerization method.

This was demonstrated by De Keyser et al., who used

this method to synthesize ꢂ300 mg of isobutyl [3-¹⁴C]

cyanoacrylate.⁴⁵

NOVEL ALKYL 2-CYANOACRYLATE MONOMERS, TAHTOUH, KALMAN, AND REEDY

263

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA

The anthracene adduct of 2-cyanoethyl 2-cyanoacrylate (3b)

was successfully synthesized from equimolar quantities of

2-cyanoethyl cyanoacetate (2a), paraformaldehyde, and an-

thracene in refluxing benzene in the presence of piperidine

catalyst, and the water formed was azeotropically removed.

The product was isolated as a crystalline solid that was

recrystallized from methanol. The overall yield for the reac-

tion was quite poor (ꢂ14%). Furthermore, the product was

often contaminated with residual amounts of anthracene,

which were difficult to remove by recrystallization alone.

Small quantities of anthracene could be removed by sublima-

tion at reduced pressure using sublimation apparatus fitted

with a cold finger. This, however, was difficult and time-con-

suming with larger samples. Changes to the solvent system

or method of addition or reagents gave similar results at

best. Using a 2.5–3 molar excess of anthracene led to

improvements of yield to a maximum of ꢂ30%. The product

obtained from these reactions, however, was still contami-

nated with significant amounts of unreacted anthracene.

cene/2-cyanoacrylic acid potassium salt (5) with a dihalide

(or haloalkane). Alternatively, anthracene/2-cyanoacryloyl

chloride adduct (6), which may be formed by treating 4 with

thionyl chloride in the presence of pyridine, can be treated

with a dihalide (or haloalkane) in the presence of triethyl-

amine. Kronenthal and Schipper report the direct esteri-

fication of 4 with a haloalkane (or dihalide in the case of

bis-adducts) in the presence of triethylamine.⁴³

This general approach has also been used by other authors

for the synthesis of thiocyanoacrylates,⁴¹carbalkoxyalkyl

2-cyanoacrylates,⁴³and cyanoacrylate-capped polyiso-

butylenes.⁴⁷Warrener has also reported

a number of

anthracene/2-cyanoacrylate adducts, but the corresponding

cyanoacrylate monomers were not isolated.⁴⁴

Using the method reported by Buck, 3a was synthesized by

reaction of 1a (in the form of commercial superglue) with

anthracene in refluxing benzene under SO₂. The product 3a

was used in its crude form for the next step—saponification

with potassium hydroxide (KOH) in a mixture of ethanol and

water at room temperature. At this stage, a convenient puri-

fication step became apparent and involved simple filtration

of the insoluble residual anthracene and (poly) ethyl 2-cya-

noacrylate. Subsequent acidification to pH 2 yielded 4 as a

white crystalline powder in high purity and yield. Adjusting

the pH of a solution of 4 in acetone with methanolic potas-

sium hydroxide (40% w/v) to pH 9.5 and subsequent filtra-

tion yielded 5. Alternatively, 6 could be isolated by treatment

of 4 with thionyl chloride (SOCl₂) in benzene in the presence

of pyridine. All products were characterized by melting

point, FTIR, ¹H and ¹³C NMR, and elemental analysis. All

data corresponded to those previously reported (where

available) and were consistent with the proposed structures.

6 was found to react with atmospheric moisture, reverting

back to 4. For this reason, it was prepared immediately

before use. During the conversion from 4 to 6, the reaction

was heated in a water bath that was not allowed to exceed

60 ^ꢁC. Heating above this temperature was found to cause

deprotection of the anthracene adducts, producing anthra-

cene that was difficult to remove from the product. Further-

more, because of the extreme reactivity of SOCl₂used in the

conversion, strictly anhydrous conditions were necessary

and fresh SOCl₂gave the best results. Scheme 2 shows a

summary of these reactions and the average yield of 4, 5,

and 6. These three compounds were produced in very high

yield and high purity and provide convenient precursors for

the synthesis of anthracene-protected cyanoacrylate esters

via esterification.

2-Cyanoethyl 2-cyanoacrylate (1b) was isolated by treatment

of 3b (produced by this method) with maleic anhydride in

p-xylene under strictly anhydrous conditions. This included

pretreatment of glassware with a silanating agent and using

anhydrous p-xylene, which was inhibited with a strong

stream of sulfur dioxide (SO₂). The reaction was also con-

ducted under SO₂in the presence of other polymerization

inhibitors including phosphorus pentoxide (P₂O₅) and hydro-

quinone. The reaction was monitored via NMR of regularly

withdrawn 100-lL aliquots.

Despite the success of isolating 1b via this method, the prod-

uct was not pure and only a limited amount was obtained

due to the difficulty of obtaining the precursor anthracene

adduct in a high yield (and purity). An alternative method

for the synthesis of the precursor (3b) was, therefore,

investigated.

In a slight modification of the Ray and Doran method, Buck

demonstrated that the protection/deprotection approach is

useful for the synthesis of difunctional or bis(2-cyano-

acrylate) monomers.⁴⁶This involves the Diels-Alder reaction

of either isobutyl 2-cyanoacrylate (1f) or ethyl 2-cyanoacry-

late (1a) with anthracene to form the corresponding anthra-

cene adduct (anthracene adduct of isobutyl 2-cyanoacylate

(3f) or 3a). Saponification of this adduct yielded the anthra-

cene/2-cyanoacrylic acid adduct (4). Further conversion to

the alkali metal salt or acid chloride derivative followed by

esterification reactions with an organic dihalide or glycol

gave the bis-anthracene adduct precursors to the bis(2-cya-

noacrylate) monomers. Heating the bis-adducts with excess

maleic anhydride in refluxing xylene effected the same retro-

grade diene scission described above, which, in this case,

yielded the bis(2-cyanoacrylate) monomers in up to 80%

overall yields.⁴⁶

Having successfully synthesized these three precursors,

attempts were made to synthesize the anthracene/2-cya-

noethyl 2-cyanoacrylate adduct (3b) via esterification. As

mentioned above, there are three main pathways (from each

of the three precursors) for the esterification reaction. A

number of attempts to synthesize 3b from either 4 or 5

failed. Following a method described by Buck, esterification

with 6 was attempted wherein a solution of freshly prepared

6 in dry DMF was added over 30 min to a stirred solution of

Note that the esterification step can be achieved in a number

of ways and can produce monofunctional or difunctional

products depending on the reagents used. Buck describes

the treatment of the alkai metal salt of 4 such as the anthra-

264

WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE

SCHEME 2 Synthesis of 4, 5, and

6 and average percentage yields

obtained.

3-hydroxypropanenitrile with an excess of triethylamine in

dry DMF at room temperature. The suspension of triethyl-

amine hydrochloride was poured into water, and the orange

precipitate was extracted with benzene. 3b was successfully

synthesized via this method with yields of ꢂ35% prior to

recrystallization. Yields were increased to ꢂ75% (crude)/

ꢂ60% (after recrystallization) by conducting the reaction in

anhydrous benzene.

noethyl 2-cyanoacrylate (1c), trideuteromethyl 2-cyanoacry-

late (1d), and pentadeuteroethyl 2-cyanoacrylate (1e)—see

Figure 2.

As noted previously, the isolation of monomeric cyano-

acrylate from its anthracene adduct was conducted in sila-

nated and oven-dried glassware. The reaction medium was

anhydrous p-xylene, which had been inhibited with a strong

stream of SO₂, and the reaction was conducted under SO₂.

The presence of other polymerization inhibitors such as

phosphorus pentoxide (P₂O₅) and hydroquinone did not

improve results, and hence, these were eliminated from sub-

sequent reactions. The reaction was monitored via NMR of

regularly withdrawn 100-lL aliquots. The reaction was typi-

cally allowed to proceed for 48–72 h, at which point the

level of monomer being produced had reached a maximum,

or signs of polymerization and loss of product were

observed. Monomeric samples were contaminated with vary-

ing amounts of their anthracene adduct and (to a lesser

extent) the by-product, 7. Estimations of monomer purity

were made based on the ¹H NMR spectra. Because of the

relatively small quantities of product and the high probabil-

ity of polymerization, no attempts were made to distil or

otherwise purify the monomers. The impurities present

generally did not interfere with the use of these monomers

for the fuming of latent fingerprints. This is discussed in

detail in a concurrent publication.⁷⁹

The method described here has a number of advantages

over the Ray and Doran simultaneous Knoevenagel conden-

sation/Diels-Alder protection method previously used for the

synthesis of 3b. First, the overall yield for the reaction was

higher (25–30% over four steps). Second, the purity of the

final product is much higher. This is due to the purity of

the precursor 4, which, in turn, is due to the filtration of the

aqueous solution prior to acidification. This is very effective

at removing the major contaminant, anthracene, from 4. The

formation of anthracene is avoided in the reactions that

follow due to the mild reaction conditions used. Lastly, and

perhaps most importantly, the new method allows for the

convenient synthesis of other anthracene/alkyl 2-cyano-

acrylate adducts (and hence other cyanoacrylate monomers)

by simply changing the alcohol used in the reaction with 6.

The overall reaction sequence is shown in Scheme 3.

Using this reaction scheme, four novel anthracene/alkyl 2-cya-

noacrylate adducts were prepared. These include anthracene/

2-cyanoethyl 2-cyanoacrylate (3b), anthracene/1-cyanoethyl 2-

cyanoacrylate (3c), anthracene/trideuteromethyl 2-cyanoacry-

late (3d), and anthracene/pentadeuteroethyl 2-cyanoacrylate

(3e)—see Figure 3.

Analysis and Characterization of Novel Compounds

Thermal Analysis of Anthracene/Alkyl

2-Cyanoacrylate Adducts

As mentioned above, the anthracene adducts of alkyl 2-

cyanoacrylates thermally decompose in a retrograde Diels-

Alder reaction to yield anthracene and cyanoacrylate

From these adducts, the following novel alkyl 2-cyanoacry-

lates were isolated: 2-cyanoethyl 2-cyanoacrylate (1b), 1-cya-

NOVEL ALKYL 2-CYANOACRYLATE MONOMERS, TAHTOUH, KALMAN, AND REEDY

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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA

SCHEME 3 Overall reaction sequ-

ence for synthesis of novel alkyl 2-

cyanoacrylates.

monomer. Giral et al. have reported using this thermal degra-

dation alone to isolate the desired monomeric product.⁴⁸

This method involves the use of glassware specially designed

to trap the liberated anthracene on its inside walls by

condensation while the monomeric vapors are distilled. More

commonly, however, a more reactive dienophile (such as

maleic anhydride) is introduced to trap the liberated anthra-

cene as a nonvolatile adduct.

thermal analysis of the four novel anthracene adducts

synthesized (blue trace DTA, green trace TGA, brown trace

derivative of TGA).

The first endotherm in the DTA traces for each of the four

adducts examined represents melting, and accordingly, there

is no corresponding mass loss in the TGA traces. The melting

points for 3b, 3d, and 3e are sharp (i.e., occur over a narrow

Furthermore, a patent by Warrener and Yong discusses the

possibility of using anthracene/alkyl 2-cyanoacrylate adducts

as solid precursors for the fuming of latent fingerprints.⁴⁴To

investigate methods of recovering the desired monomeric

cyanoacrylate from the corresponding anthracene adduct,

and to evaluate the potential use of these adducts for direct

fuming of latent fingerprints, the thermal decomposition of

all four novel anthracene/cyanoacrylate adducts was studied.

Simultaneous TGA and DTA were performed on a SDT 2960

module (TA instruments) with a 10 ^ꢁC/min heating rate

from ambient conditions to 600 ^ꢁC. Figures 4–7 show the

FIGURE 3 Structures of novel anthracene/alkyl 2-cyanoacrylate

adducts.

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ARTICLE

FIGURE 4 TGA-DTA curves for

anthracene/2-cyanoethyl 2-cyano-

acrylate adduct (3b). [Color fig-

ure can be viewed in the online

issue, which is available at

wileyonlinelibrary.com.]

temperature interval) and correspond well with that

obtained using conventional melting point apparatus. In con-

trast, the first endotherm in the DTA trace for 3c shows

melting over a wide range. This can be explained by the fact

that this compound is actually a mixture of stereoisomers

(see discussion below).

identity of the particular cyanoacrylate evolved, but many

factors need to be considered. First, it is likely that two fac-

tors determine the maximum decomposition temperature (i)

the strength of the dienophile (monomer), with stronger

dienophiles requiring higher temperatures; (ii) the volatility

of liberated monomer, with less volatile monomers requiring

higher temperatures. It is difficult to ascertain which factor

is more critical, and it is likely that both are indeed signifi-

cant. Furthermore, the sample size and viscosity, which in

turn can affect diffusion kinetics, may also be factors to con-

sider. Further work is needed to draw any meaningful con-

clusions regarding the relative decomposition temperatures

The second endotherm in the DTA traces for all four adducts

corresponds to thermal decomposition. It is clear from the

TGA traces that mass loss commences at temperatures

slightly above the melting point, with maxima ranging

between 220 and 265 ^ꢁC. It is tempting to suggest that the

temperature of maximum decomposition is a function of the

FIGURE 5 TGA-DTA curves for

anthracene/1-cyanoethyl 2-cyano-

acrylate adduct (3c). [Color figure

can be viewed in the online

issue, which is available at

wileyonlinelibrary.com.]

NOVEL ALKYL 2-CYANOACRYLATE MONOMERS, TAHTOUH, KALMAN, AND REEDY

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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA

FIGURE 6 TGA-DTA curves for an-

thracene/trideuteromethyl 2-cyano-

acrylate adduct (3d). [Color figure

can be viewed in the online

issue, which is available at

wileyonlinelibrary.com.]

of these compounds. Nevertheless, the information gained

during the thermal analysis of these compounds is useful for

the isolation of the monomeric product and also became of

importance for the fuming of latent fingerprints using the

anthracene adduct itself or the isolated (but crude) mono-

mer (see relevant discussion elsewhere⁷⁹). Table 1 below

summarizes the melting points and the temperature at which

thermal decomposition is at a maximum.

these compounds show a multiplet owing to the aromatic

protons (ArH₁–ArH₈), a triplet owing to C9-H, a singlet

owing to C10-H, and a doublet of doublets for each of the

magnetically inequivalent protons on C12 (labelled H_aand

H_b). Table 2 summarizes the H NMR data obtained for these

compounds, with the exception of 3c, which is discussed in

1

detail below.

Anthracene/1-cyanoethyl 2-cyanoacrylate (3c) contains two

chiral centres, which give rise to four possible stereoisomers

(see Fig. 9).

NMR Spectroscopy

Throughout this project, a number of Diels-Alder anthracene

adducts were synthesized. The general structure for these

compounds is shown in Figure 8. The ¹H NMR spectra of

The NMR spectra for 3c clearly show two distinct groups of

resonances, resulting from the two diastereoisomeric pairs

FIGURE 7 TGA-DTA curves for

anthracene/pentadeuteroethyl 2-

cyanoacrylate adduct (3e). [Color

figure can be viewed in the

online issue, which is available

at wileyonlinelibrary.com.]

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ARTICLE

TABLE 1 Summary of Thermal Analysis Data for Anthracene/

Alkyl 2-Cyanoacrylate Adducts

Melting Point

Temperature

via Conventional of Maximum

Melting

Point via

Compound TGA (^ꢁC)

Melting Point

Apparatus

(^ꢁC)

Thermal

Decomposition

(^ꢁC)

3b

3c

3d

3e

136.1

127–129

264.0

241.2

222.3

233.8

124.1 (broad) n/a

101.0

131.5

97–99

126–127

of enantiomers (referred to as enantiomeric pair a and enan-

tiomeric pair b). Note that one of these pairs is the (R,R) and

(S,S) pair, while the other is the (R,S) and (S,R) pair (as

shown in Fig. 9) but it is not possible to distinguish which

pair is which. In the ¹H NMR spectrum for the two disas-

tereoisomeric pairs of enantiomers, the resonances for the

following proton environments are not resolved at 300 MHz:

CH₃(doublet at 1.67 ppm), C9-H (triplet at 4.49 ppm),

ArH₁–ArH₈(multiplet from 7.10–7.60 ppm). The resonance

for the protons C12-H_aand C12-H_b, however, appear as dou-

blets of doublets at 2.25 and 2.83 ppm for enantiomeric pair

a and at 2.30 and 2.80 ppm for enantiomeric pair b. These

resonances for the enantiomeric pairs overlap, but clari-

fication of the individual resonances was obtained from a

sample with a higher proportion of enantiomeric pair b (see

discussion below and the corresponding spectra). The singlet

peaks at 4.95 for enantiomeric pair a and at 4.89 ppm for

enantiomeric pair b, assigned to the C10-H proton, proved

useful for determining the relative amount of each pair of

compounds. Finally, the resonances for the CHCN protons

appear as two partially overlapped quartets centred at 5.32

and 5.27 ppm for enantiomeric pair a and b, respectively.

The ¹³C NMR spectrum also showed two distinct groups of

resonances appearing mostly in pairs with slight intensity

differences.

FIGURE 8 General structure for Diels-Alder anthracene adducts.

[Color figure can be viewed in the online issue, which is available

at wileyonlinelibrary.com.]

talline compound was recovered from the mother liquor

after cooling in a refrigerator overnight. The ¹H and ¹³C

NMR spectra for this crop (shown in Figs. 10 and 11, respec-

tively) show a significant change in the ratio between enan-

tiomeric pair a and enantiomeric pair b (calculated to be

ꢂ29% enantiomeric pair a and 71% enantiomeric pair b).

This indicates that enantiomeric pair a is more soluble and

hence remains in solution, whereas enantiomeric pair b

crystallizes. This difference in solubility could be utilized to

separate these two diastereoisomeric pairs of enantiomers

by fractional crystallization, but this was not attempted.

Chromatographic separation is also feasible but was not

investigated further as the same racemic monomeric product

(1-cyanoethyl 2-cyanoacrylate (1c)) would result from each

diastereomeric racemate. Nevertheless, the isolation of this

small crop with a significantly higher proportion of enantio-

meric pair b allowed for confirmation of the spectral assign-

ment of the ¹H NMR spectra and unambiguous assignment

of the ¹³C NMR resonances for each enantiomeric pair,

based on the significant intensity differences (note intensity

differences in pairs of peaks in Fig. 11).

The ¹H NMR spectra of the deuterated compounds anthra-

cene/trideuteromethyl 2-cyanoacrylate adduct (3d) and

anthracene/pentadeuteroethyl 2-cyanoacrylate adduct (3e)

display resonances for the protons indicated in Table 2 only

(i.e., no resonances are observed for the deuterium atoms of

Upon recrystallization of crude 3c, the majority of the prod-

uct oiled out of solution as a viscous mass, which solidified

upon recovery and drying. The ratio of the enantiomeric

pairs was determined to be ꢂ53% enantiomeric pair a: 47%

enantiomeric pair b. A second (smaller) crop of white crys-

TABLE 2 Summary of ¹H NMR Data for Several Anthracene Adducts

ArH₁–ArH₈

C9-H

C10-H

Adduct

R

Solvent

(m)

(t)

(s)

C12-H_a(dd)

C12-H_b(dd)

6

ACl

CDCl₃

7.10–7.60

7.10–7.65

6.95–7.40

7.10–7.50

7.10–7.60

4.45 (J ¼ 2.7 Hz)

4.63 (J ¼ 2.7 Hz)

4.33 (J ¼ 2.6 Hz)

4.42 (J ¼ 2.7 Hz)

4.48 (J ¼ 2.7 Hz)

4.46 (J ¼ 2.7 Hz)

5.06

5.08

4.77

4.87

4.96

4.91

2.28 (J ¼ 2.7, 13.5 Hz)

2.21 (J ¼ 2.7, 12.9 Hz)

1.82 (J ¼ 2.9, 12.5 Hz)

2.19 (J ¼ 2.7, 13.2 Hz)

2.26 (J ¼ 2.7, 13.2 Hz)

2.24 (J ¼2.7, 13.2 Hz)

2.24 (J ¼ 2.7, 12.9 Hz)

2.75 (J ¼ 2.7, 13.5 Hz)

2.84 (J ¼ 2.7, 12.9 Hz)

2.75 (J ¼ 2.5, 12.5 Hz)

2.80 (J ¼ 2.7, 13.2 Hz)

2.84 (J ¼ 2.7, 13.2 Hz)

2.83 (J ¼ 2.7, 13.2 Hz)

2.84 (J ¼ 2.7, 12.9 Hz)

4

AOH

acetone-d₆

DMSO-d₆

CDCl₃

5

AO^ꢀK^þ

AOCH₂CH₃

AOCH₂CH₂CN

AOCD₃

3a

3b

3d

3e

CDCl₃

AOCD₂CD₃

CDCl₃

NOVEL ALKYL 2-CYANOACRYLATE MONOMERS, TAHTOUH, KALMAN, AND REEDY

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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA

FIGURE 9 The four stereoisomers of anthracene/1-cyanoethyl 2-cyanoacrylate adduct (3c).

the CD₃/CD₂CD₃chains). Although the absence of these

resonances indicates that these groups are in fact deuterated,

confirmation of the presence of the deuterium atoms was

possible from the ¹³C-²H heteronuclear coupling observed in

the ¹³C NMR spectra (see Figs. 12 and 13).

Note that the chemical shifts for all these resonances

also correspond well to the resonances of the analogous

non-deuterated compounds.

FTIR Spectroscopy

The FTIR spectra of all compounds synthesized were

collected in transmission mode as KBr disks for solids or as

thin films between two freshly pressed 13-mm KBr disks for

liquids. All spectra showed the expected vibrational bands

for the functional groups present. Evidence of the reaction

of the acid chloride–anthracene/2-cyanoacryloyl chloride

adduct (6) with moisture to form the corresponding carb-

oxylic acid–anthracene/2-cyanoacylic acid adduct (4) was

The ¹³C NMR spectra of the corresponding monomers, tri-

deuteromethyl 2-cyanoacrylate (1d) and pentadeutero-

ethyl 2-cyanoacrylate (1e), also show the expected ¹³C-²H

heteronuclear coupling. Table 3 summarizes the chemical

shifts, multiplicities, and coupling constants for the four

novel deuterated compounds synthesized during this

project.

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ARTICLE

FIGURE 10 ¹H NMR spectrum of 3c—crop 2—resonances from residual ethanol marked with ꢃ. [Color figure can be viewed in the

online issue, which is available at wileyonlinelibrary.com.]

provided by FTIR analysis. The FTIR spectrum of freshly pre-

pared 6 is shown in Figure 14 below. It shows a peak at

1786 cm^ꢀ1for the carbonyl (C¼¼O) stretch of the acid chlo-

ride, which is in the expected region. There is also, however,

a smaller peak at 1722 cm^ꢀ1, which indicates that, even in

this fresh sample, some 4 is present. Figure 15 shows three

overlaid spectra of the same product (6) taken at various

times after storage in a sealed vessel (exact time periods not

FIGURE 11 ¹³C NMR spectrum of 3c—crop 2. [Color figure can be viewed in the online issue, which is available at

wileyonlinelibrary.com.]

NOVEL ALKYL 2-CYANOACRYLATE MONOMERS, TAHTOUH, KALMAN, AND REEDY

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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA

FIGURE 12 ¹³C NMR spectrum of anthracene/trideuteromethyl 2-cyanoacrylate adduct (3d) showing septet (J_CD¼ 22.3 Hz) for CD₃

group at 53.2 ppm. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

recorded). The conversion from 6 to 4 is demonstrated by

the appearance of a peak at ꢂ1720 cm^ꢀ1corresponding to

the C¼¼O stretch of the carboxylic acid (4) (see Fig. 16 for

reference spectrum of pure 4). In fact, it is possible to use

FTIR to monitor the conversion by collecting an infrared

spectrum of the freshly prepared 6 at regular time intervals.

FIGURE 13 ¹³C NMR spectrum of anthracene/pentadeuteroethyl 2-cyanoacrylate adduct (3e) showing quintet (J_CD¼ 23 Hz) for CD₂

group at 62.4 ppm and septet (J_CD¼ 19 Hz) for CD₃group at 13.0 ppm. [Color figure can be viewed in the online issue, which is

available at wileyonlinelibrary.com.]

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ARTICLE

TABLE 3 Summary of Chemical Shift, Multiplicity, and

C₁₈H₁₂ClNO, with the calculated values being: C, 73.60; H,

Coupling Constants for Novel Deuterated Compounds

4.12; N, 4.77 and the found: C, 75.09; H, 4.54; N, 5.24.

This is due to the conversion of this acid chloride to the

corresponding carboxylic acid (4) upon reaction with mois-

ture as already noted.

Chemical

Shift

Coupling

constant,

Compound Assignment (ppm)

Multiplicity J_CD(Hz)

It is important to note that elemental analysis was not

performed on any of the monomeric cyanoacrylate samples.

As discussed above, because of the relatively small quantities

of product and the high probability of polymerization, no

attempts were made to distil or otherwise purify the mono-

mers. The purity was, therefore, not sufficient for elemental

analysis. Furthermore, the reactivity of the monomers would

necessitate the addition of stabilizers, which would also

interfere with elemental analysis.

3d

3e

CD₃

CD₂

CD₃

CD₂

CD₃

53.2

62.4

13.0

52.8

62.4

12.9

septet

quintet

septet

quintet

septet

22.3

23

19

1d

1e

22.6

23

19

Although these kinetic experiments were not conducted, the

conversion was found to begin almost immediately upon

exposure to atmospheric moisture at room temperature. This

highlighted the need to freshly prepare 6 for immediate use.

Mid-Infrared Imaging of Fingerprints with

a Novel Cyanoacrylate

Latent fingerprints on an Australian $5 polymer banknote

treated with monomer 1c were successfully imaged using

MIR spectral imaging (see Fig. 17). The detection and

imaging of fingerprints on polymer banknotes is notoriously

difficult using conventional fingerprint development and

imaging techniques. It is impossible to see the developed

fingerprint in the white light image in Figure 17, but the

same print is clearly visible in the MIR spectral image.

Elemental Micro Analysis

C, H, and N elemental analysis is still considered an essential

part of product characterization and proof of product purity.

During this project,

a number of known and novel

compounds were analyzed on a Carlo Erba 1106 automatic

analyser (Australian National University—Microanalytical

Laboratory). The results of this analysis are reported in the

experimental section, with the majority of analyses falling

within 0.4% of the calculated values.

This image was formed using the intensity of the carbonyl

absorption band in poly-1c, because even with the additional

nitrile group in 1c, the m(CBN) intensity remains too low for

practical application to imaging. Further discussion and

The elemental analysis for the anthracene/2-cyanoacryloyl

chloride adduct (6) varied from that calculated for

FIGURE 14 FTIR spectrum of anthracene/2-cyanoacryloyl chloride adduct (6).

NOVEL ALKYL 2-CYANOACRYLATE MONOMERS, TAHTOUH, KALMAN, AND REEDY

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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA

FIGURE 15 Overlay of three FTIR spectra of 6 showing gradual conversion to 4.

examples of fingerprints successfully developed using the

novel cyanoacrylate monomers described here can be found

in a concurrent publication.⁷⁹

The low intensity of the nitrile bands in the MIR spectra of

the polymerized novel cyanoacrylates (as seen in developed

fingerprints) is disappointing, but can be used as the

FIGURE 16 FTIR spectrum of anthracene/2-cyanoacylic acid adduct (4).

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ARTICLE

An important observation can be made using the data of

Tomlinson et al.,⁷⁵which appears to show isosbestic points

in regions of the MIR and NIR spectra of the polymerizing

cyanoacrylate. We have repeated the MIR experiment and

can confirm the clear presence of isosbestic points in spectra

(including the nitrile-stretching region) taken during the po-

lymerization of a thin film of ethyl cyanoacrylate.⁸⁰This

implies that there are only two species involved (the mono-

mer and the polymer) and tends to rule out polymerization

followed by cross-linking, as suggested elsewhere.⁷⁵

In the MIR spectra of poly-1b and poly-1c,⁷⁹an additional

m(CBN) band is observed as expected (often partially over-

lapped with the original m(CBN) band), but like the original

(a) band, it is broad and of insufficient intensity for useful

imaging. In hindsight, the lack of conjugation to this new

nitrile group in the cyanoacrylate structure means that it

was always unlikely to have solved the imaging problem.

However, in poly-1c the new nitrile group has been shown

to shift the frequency of the cyanoacrylate carbonyl stretch-

ing vibration, enabling fingerprint contrast to be achieved on

certain difficult backgrounds.⁷⁹

CONCLUSIONS

The common route for the synthesis of alkyl 2-cyanoacry-

lates (i.e., Knoevenagel condensation of alkyl cyanoacetates

with formaldehyde followed by thermal depolymerization)

was found to be unsatisfactory for the synthesis of small

quantities of the targeted novel cyanoacrylates. A Diels-Alder

protection/deprotection route involving the use of anthra-

cene to protect (and later deprotect) the reactive vinyl group

proved much more successful. In total four novel cyanoacry-

lates with potential as reagents for the MIR spectral imaging

of fingerprints on difficult surfaces were prepared. These

were 2-cyanoethyl 2-cyanoacrylate (1b), 1-cyanoethyl 2-cya-

noacrylate (1c), trideuteromethyl 2-cyanoacrylate (1d), and

pentadeuteroethyl 2-cyanoacrylate (1e).

FIGURE 17 Fingerprint treated with 1-cyanoethyl 2-cyano-

acrylate (1c) on Australian $5 polymer banknote. White light

photograph with area imaged using MIR spectral imaging indi-

cated. Monochrome representation of MIR spectral image

(inset) formed using sixth derivative data at 1713 cm^ꢀ1

.

inspiration for a discussion about the MIR nitrile intensities

of monomeric and polymeric cyanoacrylates. When the poly-

merization of ethyl cyanoacrylate is followed by infrared or

Raman spectroscopy, the key observations are that the CBN

stretching band loses intensity, broadens, and moves to

higher frequencies. These trends occur rapidly at first, then

more slowly.^67,68,75,80A careful examination of the literature

reveals that there is no direct evidence for cross-linking or

hydrogen-bonding of nitrile groups in polymeric cyanoacry-

late; rather, the reduction of conjugation upon polymeriza-

tion can explain most of the observations that have been

made. It is consistent with (i) the increase in the stretching

frequency (since the CBN bond order increases once this

bond is no longer conjugated to the rest of the monomer);

and (ii) the decrease in m(CBN) infrared intensity, since the

bond is less polar once conjugation is removed.⁸⁴The broad-

ening of the peak is probably due to the greater number of

local environments experienced by nitrile groups in the

polymer (and the presence of residual monomer), and the

variable rate of the reaction can be attributed to the increase

in the viscosity of the cyanoacrylate as polymerization

continues.

The identities of the four novel monomers and their inter-

mediates were confirmed using a variety of analytical techni-

1

ques including H and ¹³C NMR spectroscopy, FTIR spectros-

copy, and elemental analysis. The loss of conjugation to the

a-nitrile group during polymerization, and the lack of conju-

gation to the new nitrile group in poly-1b and poly-1c has

been identified as the cause of m(CBN) intensities that are

too low for practical MIR imaging applications, but success-

ful MIR imaging of fingerprints using the novel cyanoacry-

lates has been achieved using other spectral bands.⁷⁹

MT gratefully acknowledges the assistance of Dr Ronald G.

Shimmon.

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