Macromol. Chem. Phys. 2001, 202, 943–948 943

Studies on the Copolymerisation of N-Arylmaleimides

with Alkyl(meth)acrylate

Vishal Anand, Veena Choudhary*

Centre for Polymer Science and Engineering, Indian Institute of Technology, Delhi, Hauz Khas, New Delhi – 110016, India

Introduction

Polymers are increasingly used in outdoor applications

where environmental conditions (i.e. temperature, humid-

ity, solar radiation etc.) influence their performance. One

of the application area of polymers is solar technology

where polymers find applications as covers (glazing),

thin films, honey combs and housing for flat plate collec-

tors, optical lenses for concentrating collectors, insula-

tion, piping, adhesives and sealant etc.[1–3] The main rea-

son for the increasing use of polymers in solar energy

conservation systems is their lightweight, ease of process-

ing and design flexibility. Good optical properties,

mechanical properties (high tensile strength, impact

strength), high softening point, durability (retention of

optical and mechanical properties, abrasion resistance,

collection of little dirt) are some of the requirements for

polymers in such applications.

In our earlier papers, we reported the copolymerisation

of N-substituted arylmaleimides with methyl methacryl-

ate.[4–10] It was observed that the nature of the substituent

and their position affected the copolymerisation behavior

and softening temperature. On the other hand a decrease

in toughness was observed upon the incorporation of such

rigid monomers. The present studies were undertaken

with the aim to investigate systematically the copolymer-

isation of N-phenylmaleimide and N-tolylmaleimide with

butyl acrylate. The effect of the incorporation of such

comonomers on the thermal behavior was also evaluated.

Tercopolymers of MMA, N-arylmaleimides and butyl

acrylate were also prepared. The effect of feed composi-

tion on molecular, structural and thermal characterisation

was also investigated.

Experimental Part

Materials

Maleic anhydride (Loba Chemie) was purified by distilla-tion. p-Toludine (s.d. fine chem.), methanol (s.d. finechem.), chloroform (s.d. fine chem.) and silica gel for col-umn chromatography (CDH) were used as supplied. Acetone

Full Paper: The paper describes the synthesis, characteri-sation and polymerisation of N-phenylmaleimide (NPM)and N-tolylmaleimide (NTM) with butyl acrylate. Eightcopolymer samples were obtained by varying the molefraction of N-arylmaleimide in the initial feed from 0.2 to0.7. Structural and molecular characterisation of the sam-ples was done using 1H NMR spectroscopy and intrinsicviscosity measurement. Copolymer composition wasdetermined by taking the ratio of intensities of signals dueto 1OCH2 (butyl acrylate) at d = 4.0 l 0.1 ppm and aro-matic proton at d = 7.1–7.4 ppm of NPM/NTM. The reac-tivity ratio of NPM: butyl acrylate and NTM: butyl acry-late were found to be r1 = 2.49 l 0.01:r2 = 2.83 l 0.03 andr1 = 0.48 l 0.04 :r2 = 1.75 l 0.04, respectively. NTMshowed much less reactivity as compared to NPM. Ther-mal stability of the copolymers was evaluated by record-ing TG/DTG traces in nitrogen atmosphere. Tercopoly-mers were also prepared by taking 0.3/0.5 and 0.4/0.4mole fraction of MMA/NPM or NTM. The mole fractionof butyl acrylate in all these tercopolymers was kept con-stant at 0.2. Structural, molecular and thermal characteri-sation was also carried out.

Macromol. Chem. Phys. 2001, 202, No. 6 i WILEY-VCH Verlag GmbH, D-69451 Weinheim 2001 1022-1352/2001/0603–0943$17.50+.50/0

Plots of mole fraction of butyl acrylate in feed (M1) vs. molefraction of butyl acrylate in copolymers (m1) .

944 V. Anand, V. Choudhary

(Qualigens) was dried by storage over anhydrous potassiumcarbonate overnight followed by distillation. N,N-Dimethyl-formamide (DMF) (CDH) was dried over phosphorus pent-oxide overnight. Aniline (s.d. fine chem.) was purified bydistillation under reduced pressure. Toluene was dried byusing sodium wire.

Tetrahydrofuran (THF) (CDH) was dried over metallicsodium overnight, refluxed with benzophenone and then dis-tilled. 2,29-Azoisobutyronitrile (AIBN) was recrystallisedusing chloroform and then dried.

Methyl methacrylate (MMA) (Merck) was washed withdilute sodium hydroxide to remove the inhibitor, followed bydistilled water. It was dried over anhydrous sodium sulfateovernight and distilled under reduced pressure.

Preparation of N-Phenylmaleimide and N-p-Tolylmaleimide

N-phenylmaleimide (NPM)/N-p-tolylmaleimide (NTM) mo-nomers were synthesised by reacting aniline/p-toludine withmaleic anhydride using the procedure reported elsewhere.[11]

The reaction scheme for the preparation of monomers isshown below:

Letters within parenthesis represent the designation of themonomers.

Procedure

Maleic anhydride (1 M) was placed in a three-necked roundbottom flask and 300 ml of toluene were added. Freshly dis-tilled aniline (1 M) dissolved in toluene (100 ml) was addeddropwise under constant stirring. The entire setup was placedin a cold water bath. After complete addition, the reactionmixture was stirred for 3 h followed by the addition of200 ml of DMF and 2 ml conc. H2SO4. The mixture wasrefluxed for another 4 h and the water was continuouslyremoved using a Dean-Stark set up. At the end of the reac-tion, the resulting solution was chilled to obtain light yellow,needle-like crystals of NPM. Similar procedure was followedfor NTM. Yield was about 60–65%. The monomers werefurther purified by passing through the silica gel column.

Preparation of Polymers

The homopolymerisation of NPM and NTM monomers wascarried out in THF and that of butyl acrylate in bulk, usingAIBN as an initiator at 608C under nitrogen atmosphere.

All the copolymers were prepared using a typical solutionpolymerisation reaction. 20% (w/w) solution of monomers inTHF was placed in a three-necked round bottom flaskequipped with a reflux condenser, a CaCl2 drying tube and anitrogen gas inlet tube. The whole assembly was placed in athermostatted oil bath and stirred with a magnetic stirrer.Nitrogen was passed through the reaction mixture and tem-perature was raised to 608C. The polymerisation wasinitiated by adding 1% AIBN (w/w) as an initiator. The reac-tion was terminated at low conversion (f15%) by pouringthe contents of the flask into a large excess of methanol. Theprecipitated polymer was washed repeatedly with hot metha-nol to remove unreacted monomer and was dried in vacuumoven. The polymer was purified by dissolving in chloroformand reprecipitating using methanol as non-solvent. The poly-mer was separated by filtration and dried in a vacuum ovenat 508C.

Tercopolymer samples were prepared by taking butylacrylate, MMA and NPM/NTM in the initial feed. The molefraction of butyl acrylate in the tercopolymer was kept con-stant at 0.2 and that of MMA/N-arylmaleimides was varied.

Homopolymers of NPM, NTM and butyl acrylate havebeen designated as PNPM, PNTM and PBA, respectively.Copolymers have been designated by adding prefix P to themonomer designation of maleimides followed by a numeri-cal suffix indicating the mole fraction of maleimides multi-plied by 10. For example, a copolymer obtained by taking0.2 mole fraction of NPM has been designated as PNPM2.Tercopolymers have been designated by adding the prefix Tto the monomer designation of the maleimides followed by anumerical suffix indicating the mole fraction of maleimidesmultiplied by 10. For example, a tercopolymer obtained bytaking 0.3 mole fraction of NPM, 0.5 and 0.2 mole fractionof MMA and butyl acrylate respectively has been designatedas TNPM3.

Characterisation of Monomers and Homo/co/tercopolymers

Structural characterisation was done by 1H NMR and IRspectroscopic techniques. 1H NMR spectra of the monomers/polymers were recorded on a Bruker Spectrospin DPX 300spectrometer using CDCl3 as solvent and tetramethylsilane asan internal standard. FTIR spectra of the monomers wererecorded in thin film using a Biorad Digilab FTS-40 FTIRspectrophotometer.

Molecular characterisation of the polymers was done byintrinsic viscosity measurements. The intrinsic viscosity [g]was measured in chloroform at 30 l 0.18C using Ubbelhodesuspension level viscometer.

A DuPont 2100 thermal analyser having a 910 DSC mod-ule and 951 TG module was used for the thermal characteri-sation of monomers and polymers. DSC scans were recordedin static air atmosphere at a heating rate of 108C/min byusing 5 l 1 mg of samples.

The thermal stability of the copolymers and tercopolymerswas determined by recording TG/DTG traces in nitrogenatmosphere (flow rate = 60 cm3/min). A heating rate of108C/min and the sample size of 10 l 1 mg was used in eachexperiment.

Studies on the Copolymerisation of N-Arylmaleimides... 945

Results and Discussion

Characterisation of Monomers

NPM and NTM showed a melting transition at 918C and

1528C, respectively. In the FTIR spectra of these mono-

mers, characteristic peaks due to AC2O of imide at 1710

and 1740 cm–1, AC2Ca stretching at 1620 cm–1, C1H

stretching vibration of vinylic and aromatic groups at

3040 cm–1 and C1H bending of olefinic bond at 1290

and 700 cm–1 were observed. In case of NTM the charac-

teristic absorption due to para-substituted phenyl ring

was observed at 830 cm–1.1H NMR spectra of NPM and NTM monomers are

shown in Figure 1. Integration was used to calculate the

number of protons. The proton resonance signals due to

aryl group were observed at d = 7.0–7.3 ppm while the

olefinic protons appeared as singlet at d = 6.8 ppm. In

case of NTM a resonance signal at d = 2.38 ppm was also

observed due to the methyl group.

Characterisation of Homopolymers and Copolymers

1H NMR spectra of the copolymers are shown in Figure

2. Resonance signals due to 1OCH2 protons of butyl

acrylate and aromatic protons of N-arylmaleimides were

observed at d = 4.0 l 0.1 ppm and d = 7.1–7.4 ppm,

respectively. The other characteristic proton resonance

signal due to 1CH2 group of butyl acrylate were

observed at d = 1.8–2.0 ppm. The intensity of the signal

due to aromatic protons increased with increasing N-aryl-

maleimide content in copolymers.

The copolymer composition was therefore determined

by taking the ratio of the intensity of the resonance sig-

nals due to the aromatic and 1OCH2 protons. The results

of the copolymer composition thus determined are sum-

marised in Table 1. A plot of M1 (mole fraction of butyl

acrylate in the feed) vs. m1 (mole fraction of butyl acry-

late in copolymers) is shown in Figure 3. In the copoly-

merisations of butyl acrylate with N-arylmaleimides, an

increase in M1 did not show a linear increase in m1.

The reactivity ratios of the monomers were calculated

from the knowledge of copolymer composition using

Fineman-Ross and Kelen Tudos methods. The values of

r1 (N-arylmaleimide comonomer) and r2 (butyl acrylate)

are given in Table 2. A higher reactivity ratio of butyl

Figure 1. 1H NMR spectra of (a) NPM and (b) NTM.

Figure 2. 1H NMR spectra of (a) PNTM2 and (b) PNTM3.

Table 1. Composition of butyl acrylate and N-arylmaleimidecopolymers and intrinsic viscosity at 30.0 l 0.1 8C in CHCl3 ofhomopolymers and copolymers.

Sampledesignation

Mole fraction of N-arylmaleimide ½g�dL=g

feed copolymer

PNPM2 0.2 0.117 0.212PNPM3 0.3 0.224 0.184PNPM5 0.5 0.507 0.110PNPM7 0.7 0.756 0.098PNPM 1.0 – 0.080

PNTM2 0.2 0.119 0.200PNTM3 0.3 0.205 0.124PNTM5 0.5 0.449 0.110PNTM7 0.7 0.560 0.091PNTM 1.0 – 0.060PBA 0.0 – 0.264

946 V. Anand, V. Choudhary

acrylate was observed as compared to NTM whereas

NPM showed a marginal difference.

From the plots of gsp/C vs. C for PNPM and PNTM

copolymers, intrinsic viscosity [g] was obtained as an

intercept. The values of intrinsic viscosity [g] are given in

Table 1. Intrinsic viscosity is a measure of the hydrody-

namic volume and depends on the molecular weight, the

size of the polymer coil in a given solution and also on

the composition of a copolymer. In butyl acrylate/N-aryl-

maleimide copolymers, an increase in the N-arylmale-

imide content resulted in a decrease in the [g] values.

DSC scans of the samples recorded in the temperature

range of 50–3008C showed a sharp endothermic peak in

the temperature range of 200–2408C, which may be due

to the degradation of samples.

TG/DTG traces of copolymers having varying mole

fractions of comonomer were recorded to study the effect

of copolymer composition on the thermal behavior of

copolymers. The relative thermal stability was deter-

mined by comparing the following temperatures.

Ti = initial decomposition temperature,

Te = extrapolated initial decomposition temperature,

Tmax = temperature of maximum rate of weight loss,

Tf = final decomposition temperature and% char

at 5008C.

Homopolymers and copolymers showed single step

degradation except the PNTM homopolymer where two

step degradation was observed. Ti and Te increased initi-

ally upon incorporation of up to 0.3 mole fraction of N-

arylmaleimides. Further increase of maleimide content in

the copolymers resulted in a significant decrease in Ti, Te,

Tmax and Tf. All these temperatures were also low for the

homopolymers i.e. PNPM and PNTM. The lower thermal

stability of the homopolymers compared to the copoly-

mers could be attributed to the formation of low molecu-

lar weight materials. Intrinsic viscosity also showed a

decrease with increasing amount of maleimide in the

copolymers. Degradation at lower temperature could be

attributed to the increased number of end groups in low

molecular weight polymers that act as initiating sites for

degradation. End group initiated degradation has been

reported in case of PMMA in the temperature range of

250–3358C.[12–15] The results of thermal stability are

summarised in Table 3. Typical TG/DTG traces for

PNTM copolymers are shown in Figure 4.

Characterisation of Tercopolymers

1H NMR spectra of the tercopolymers are shown in Figure

5. In the 1H NMR spectra of tercopolymers, resonance sig-

nal due to 1OCH2 protons of butyl acrylate, 1OCH3 pro-

tons of MMA and aromatic protons of N-arylmaleimides

were observed at d = 4.0 l 0.1 ppm, d = 3.6 l 0.01 ppm and

d = 7.1–7.4 ppm, respectively. The other characteristic

proton resonance signals were also observed at d = 0.8–

1.2 ppm due to 1CH3 group of MMA and d = 1.8–

2.0 ppm due to ACH2 group of butyl acrylate. From the

intensities of the peaks it is clear that the tercopolymers

are richer in MMA and show very small amount of butyl

acrylate.

The feed composition of N-arylmaleimides/MMA and

the sample designation is shown in Table 4. In all the terco-

polymers, the mole fraction of butyl acrylate was taken as

0.2. The values of [g] are given in Table 4. Intrinsic viscos-

ity of the tercopolymers was found to decrease with the

increasing amount of NPM/NTM in the initial feed.

DSC scans of tercopolymers having varying mole frac-

tions of comonomers were recorded to study the effect of

Table 2. Reactivity ratios of butyl acrylate (r2) and comono-mers (r1).

Method NPM : butyl acrylate NTM : butyl acrylater1 r2 r1 r2

Fineman-Ross 2.475 2.801 0.476 1.785Kelen Tudos 2.499 2.860 0.484 1.710Avg. 2.487 2.831 0.480 1.748

Figure 3. Plots of mole fraction of butyl acrylate in feed (M1)vs. mole fraction of butyl acrylate in copolymers (m1) .

Table 3. Results of TG/DTG traces of NPM/NTM and butylacrylate copolymers.

Sample Ti

�C

Te

�C

Tmax

�C

Tf

�C

% char at5008C

PBA 254 376 416 439 4.2PNPM2 287 380 412 431 8.0PNPM3 300 385 417 437 9.2PNPM5 186 285 332 350 10.7PNPM7 180 277 320 331 8.9PNPM 176 264 305 319 6.2

PNTM2 316 384 414 434 9.0PNTM3 325 390 418 434 13.1PNTM5 205 294 341 355 8.5PNTM7 203 280 330 347 8.5PNTM 198 260

525313555

339570

28.612.5

Studies on the Copolymerisation of N-Arylmaleimides... 947

copolymer composition on the glass transition tempera-

ture of the copolymers (Figure 6). In our earlier papers,

we reported the formation of random copolymers when a

mixture of MMA and NPM or NTM is polymerized.[4–7]

Present studies also show the formation of random copo-

lymers when NPM or NTM is copolymerised with butyl

acrylate. The reactivity ratio of MMA:NTM,

MMA:NPM, and MMA:BA have been reported as

0.93:0.47, 1.02:0.183 and 1.8 :0.37, respectively.[7, 16–18]

From the above reactivity ratios it can be concluded that

a mixture of MMA, BA, and NPM/NTM can form a mix-

ture of random copolymers. The presence of two

endotherms in DSC scans could be due to the presence of

a mixture of two copolymers or oligomers. A shift in the

glass transition temperature was also observed as a func-

tion of tercopolymer composition (Table 5).

Single step degradation was observed in all the sam-

ples. Ti, Te, Tmax and Tf were higher in the tercopolymers

Figure 4. TGA scans of (a) PNTM2, (b) PNTM3, (c) PNTM5, and (d) PNTM7.

Figure 5. 1H NMR spectra of (a) TNPM3 and (b) TNPM4.

Table 4. Intrinsic viscosity at 30.0 l 0.1 8C in CHCl3 of terco-polymers.

Sampledesignation

Mole fraction of comonomersin feed

½g�dL=g

N-arylmaleimide MMA

TNPM3 0.3 0.5 0.216TNPM4 0.4 0.4 0.060TNTM3 0.3 0.5 0.208TNTM4 0.4 0.4 0.054

948 V. Anand, V. Choudhary

having higher amounts of MMA in the initial feed. On

the other hand char yield was lowered. The results are

summarised in Table 5. Pure PMMA prepared by free

radical polymerisation showed a three-step degrada-

tion.[12–15] The first step in the temperature range of 186–

2508C is attributed to H1H linkage, the second step due

to chain end initiated degradation was in the temperature

range of 250–3588C and the third step in the temperature

range of 358–4058C was due to random chain scission.

Incorporation of small amounts of maleimides and butyl

acrylate in PMMA backbone hinders the formation of

weak linkages.

Acknowledgement: Financial assistance provided by Councilof Scientific & Industrial Research (CSIR) to one of the authors(Vishal Anand) is gratefully acknowledged.

Received: November 2, 1999Revised: August 29, 2000

[1] A. Blaga, Solar Energy 1978, 21, 331.[2] K. A. Geddes, R. D. Deanim, Chemtech 1982, 739; Chem.

Abstr. 1983, 98, 19383x.[3] W. F. Carrol, P. Schissel, “Polymers in Solar Energy Utili-

sation”, ACS Symp. Ser. 1983, 220, 3.[4] Renu Bharel, Veena Choudhary, I. K. Varma, Proc. 9th

National Symposium on Thermal Analysis, P. V. Ravin-dran, S. R. Bhardwaj, M. Sunderraman, S. R. Dharwadkar,Eds., Nov. 1993, 266.

[5] Renu Bharel, Veena Choudhary, I. K. Varma, J. Appl.Polym. Sci. 1993, 49, 31.

[6] Renu Bharel, Veena Choudhary, I. K. Varma, PolymerScience and Recent Advances, Vol. 1, I. S. Bhardwaj, Ed.,Allied publishers Ltd. 1994, 22.

[7] Renu Bharel, Veena Choudhary, I. K. Varma, J. Appl.Polym. Sci. 1994, 54, 2165.

[8] Veena Choudhary, Alpana Mishra, J. Appl. Polym. Sci.1996, 62, 707.

[9] Alpana Mishra, Veena Choudhary, Abstract of IUPACMacro Seoul ’96, “36th IUPAC International Symposiumon Macromolecules”, Korea, August 4–9, 1996, 708.

[10] Alpana Mishra, T. J. M. Sinha, Veena Choudhary, J. Appl.Polym. Sci. 1998, 68, 527.

[11] GSFC(PU), Indian Patent 177/BOM/1992 Baroda.[12] T. Kashiwagi, A. Inaba, J. E. Brown, K. Hatada, T.

Kitayama, E. Masuda, Macromolecules 1986, 19, 2160.[13] T. Hirata, T. Kashiwagi, J. E. Brown, Macromolecules

1985, 18, 1410.[14] T. Kashiwagi, T. Hirata, J. E. Brown, Macromolecules

1985, 18, 131.[15] L. E. Manring, D. Y. Sogah, G. M. Cohen, Macromolecules

1989, 22, 4652.[16] J. M. Barrales-Rienda, J. I. Gonzalez De La Campa, J.

Gonzalez Ramos, J. Macromol. Sci., Chem. 1977, A11,267.

[17] L. Choudhary, D. S. Varma, I. K. Varma, F. W. Wang, J.Thermal Anal. 1993, 39, 633.

[18] N. Grassie, B. J. D. Torrance, J. D. Fortune, J. D. Gemmell,Polymer 1965, 6, 653.

Table 5. Results of TG/DTG traces of MMA, NPM/NTM andbutyl acrylate tercopolymers.

Sample Ti

�C

Te

�C

Tmax

�C

Tf

�C

% char at500 8C

TNTM3 287 384 411 430 3.18TNTM4 264 378 403 421 9.81TNPM3 251 367 399 417 4.04TNPM4 245 369 407 427 5.41

Figure 6. DSC scans of (a) TNTM4, (b) TNTM3, (c) TNPM3,and (d) TNPM4.