studies on photo-crosslinkable polyphosphoramide esters containing divanillylidene cycloalkanone...

Polymer International 40 (1996) 287-293

Studies on Photo-crosslinkable Polyphosphoramide Esters Containing

Divan i I lyl idene Cycloal kanone Units

P. Kannan* & S. C. Murugavel

Department of Chemistry, Anna University, Madras 600 025, India

(Received 30 October 1995; revised version received 28 February 1996; accepted 18 March 1996)

Abstract: Two series of photo-crosslinkable polyphosphoramide esters were synthesized from 2,5-bis(4-hydroxy-3-methoxy benzy1idene)cyclopentanone and 2,6-bis(4-hydroxy-3-methoxy benzy1idene)cyclohexanone with various N - arylphosphoramide dichlorides by interfacial polycondensation using hexadecyl trimethylammonium bromide as phase transfer catalyst at ambient temperature. The resulting polymers were characterized by inherent viscosity, Fourier trans- form infra-red, 'H and 13C nuclear magnetic resonance spectroscopy. The thermal stability of the polymers synthesized was evaluated by thermogravimetric analysis. These polymers were studied for their photo-chemical reactions. The divanillylidene cycloakanone groups in the polymer chain function as photo-active centres. Crosslinking proceeds via 2n + 2n cycloaddition reaction of the divanillylidene cycloalkanone moieties. The rate of crosslinking decreases with increase in the size of the cycloalkanone ring, while the thermal stability increases with increase in the size of the cycloalkanone ring.

Key words: polyphosphoramide esters, spectral studies, thermal stability, crosslinking studies.

I NTRO D U CTlON

Photo-crosslinkable polymers are currently attracting considerable attention in a wide variety of applications such as photo-resists to make integrated circuits, printing plates, printing inks, photo-curable coatings and energy exchange systems, because of their excellent curing characteristics, thermal stability and chemical resistance. '-' Polymers bearing photo-dimerizable stil- bazolium groups are potential candidates for immobil- izing enzyme^.^.^ Polymers containing pendent cinnamic ester groups have been used as photo-sensitive polymers.'*9 Photo-crosslinkable liquid crystalline polymers find applications in anisotropic network systems such as liquid crystalline (LC) elastomers, LC thermo- sets, non-linear optics and information storage

Ho wever, the inherent flammability of these polymers is a serious drawback for their wider

* To whom all correspondence should be addressed.

application. Incorporation of phosphorus, nitrogen and halogen segments into the polymer backbone may provide a permanent remedy for this problem. Organophosphorus polymers possess good thermal stability and flame retardancy.13-16 McGrath and coworkers studied soluble phosphine-oxide-containing polymers as flame retardant materials.' 7,18 In continua- tion of our research, the present work deals with a new class of polyphosphoramide esters containing photo- dimerizable cycloalkanone groups in the main chain of the polymers. Synthesis, characterization and photochemical reactions are the main focus of this study.

EXPERIMENTAL

Materials

Vanillin (Merck), hexadecyltrimethylammonium bromide and phosphorus oxychloride (Fluka) were used as received. Cyclohexanone, aniline, p-anisidine,

287 Polymer International 0959-8103/96/%09.00 0 1996 SCI. Printed in Great Britain

288 P. Kannan, S . C. Murugavel

-NH

1 ' I I ' C'C P-0-c

I I 1 I I I 1 4&0 3500 3000 2500 2000 I500 1000 500

Wavenumber ( cm-'

Fig. 1. FTIR spectrum of polymer I.

p-toluidine, p-chloroaniline, p-bromoaniline and solvents were purified by reported procedure^.'^ Cyclo- pentanone was prepared from adipic acid using barium hydroxide."

2,5- Bis (4 -hydroxy-3-methoxy benzylidene )cyclopen tanone (HM B CP)

A mixture of cyclopentanone (1 mmol) and 4-hydroxy- 3-methoxy benzaldehyde (vanillin, 2 mmol) was dissolved in 25ml of absolute ethanol with three drops of boron trifluoride diethyl etherate. The solution was refluxed for 4h, cooled to 0°C and the crystallized product was filtered off and washed with cold ethanol. Recrystallization from trichloroethane gave fine yellow crystals with more than 80% yield of the title monomer, m.p. 215°C (lit.21 214°C).

IR (KBr): 347Ocm-' (yGH), 1640cm-' ( Y ~ = ~ ) , 1576 cm-' (yC&. 'H nuclear magnetic resonance (NMR) (dimethylsulphoxide (DMSO-d6), tetra- methylsilane (TMS)): 3.1 6 (S, 4H, -CH2- jl to the keto group of cyclopentanone), 7.3 6 (S, 2H, -CH=), 6-8 6 (m, 6H, aromatic), 10.1 6 (S, 2H, -OH), 3-7 6 (S, 6H, -OCH,).

2,6- Bis (4 -hydroxy-3-methoxy benzylidene ) cyclohexanone (HM B CH )

This monomer was prepared in a similar manner to HMBCP. Recrystallization from a 3 :1 mixture of methanol/water gave fine yellowish green crystals of the title monomer with 82% yield, m.p. 179°C (lit.2' 179°C).

IR (KBr): 3370cm-' (yGH), 1635cm-' (yce0), 1577cm-' (rcZc). 'H NMR (DMSO-d, ; TMS): 1.6 6 (S, 2H, y io the keto group of cyclohexanone), 2.7 6 (S, 4H, jl to the keto group of cyclohexanone), 7.5 6 (S, 2H, -CH=), 6.8 6 (my 6H, aromatic), 3.8 6 (S, 6H, -OCH3),.10.2 6 (S, 2H, -OH).

N -Arylphosphoramide dichlorides

The N-arylphosphoramide dichlorides were prepared from phosphorus oxychloride and the corresponding anilines at room temperature by a procedure reported elsewhere. 22

Polymerization

All the polymers were prepared by interfacial polycondensation using a phase transfer catalyst, namely hexadecyltrimethylammonium bromide (HDTMAB).23 A typical procedure for the preparation of polymer I is as follows: 1 mmol of HMBCP was dissolved in 20ml of aqueous sodium hydroxide (1 N) solution containing HDTMAB (2wt% of the diol). The solution became wine red in colour. A 20ml sample of chloroform solution of N-phenylphosphoramide dichloride (1 mmol) was added to the aqueous solution with vigorous stirring at 20°C. The stirring was continued until the wine red colour of the solution changed to yellow ( N 10 min). The organic layer was separated and poured into n- hexane. The precipitated polymer was filtered off, purified by reprecipitation from the same solvents, and dried to constant weight in vacuo at 50°C (yield > 75%). Other polymers (11-X) were prepared in a similar manner. These polymers were soluble in chloroform, methylene chloride, acetone, tetrahydrofuran, dimethyl- formamide (DMF), dimethylacetamide and DMSO, and insoluble in benzene, toluene and pentane.

Characterization

The inherent viscosity of the polymers was measured in DMF (0-5gdl-') at 30°C using a suspended level Ubbelohde viscometer. The molecular weights (K and M, of the polymers were determined by gel permeation chromatography (GPC) (Waters 501), calibrated with

-

POLYMER INTERNATIONAL VOL. 40. NO. 4, 1996

Studies on photo-crosslinkable polymers 289

TABLE 1. Yield, viscosity and molecular weight of the polymers

Polymer n0.O Polymer structure Yield (%) qinhb Molecular weight" --

X Y M, M , MwIMn ~~ ~

PPE-I 0 H 86 0.48 4500 4600 1.02 PPE-I1 0 OCH, 80 0.43 4200 4400 1.04 PPE-Ill 0 CH3 85 0.50 4500 4800 1.06 PPE-IV 0 CI 75 0.47 4100 4200 1.02 PPE-V 0 Br 78 0.48 4900 5100 1.04 PPE-VI 1 H 80 0.45 4600 5100 1.10 PPE-VII 1 OCH, 78 0.42 4500 5000 1.11 PPE- Vlll 1 CH3 82 0.48 4800 5300 1.10 PPE-IX 1 CI 78 0.39 4600 4900 1.06 PPE-X 1 Br 76 0.42 4500 4900 1.08

PPE = polyphosphoramide ester ' C = 0.5 g/dl in DMF at 30°C " Determined by GPC

polystyrene standards. IR spectra of the polymers were recorded on a Brucker IFS 66V FTIR spectrophotometer using KBr pellets. 'H and 13C NMR spectra were recorded in DMSO-d, using TMS as the internal stan- dard on an EM-3990-90 MHz and JEOL GSX400 NMR spectrometer, respectively. The UV absorbance spectra were recorded on a Hitachi U2000 spectrophotometer. Thermogravimetric studies were performed on a Mettler TA3000 thermal analyser, in nitrogen atmosphere, at a heating rate of 20"Cmin-' with a sample weight of 3-5 mg. The differential scanning calorimetry traces were measured on a Perkin Elmer differential scanning calorimeter at a heating rate of 20"Cmin-' in nitrogen atmosphere.

Photochemical studies

The photolyses of the polymers in the form of films were done in a UV spectrophotometer. A thin film was cast on the outer surface of the quartz cuvette from chloroform solution. The film was irradiated for various intervals of time with the UV lamp kept at a distance of 10 cm from the sample. Subsequently, the irradiated film was subjected to spectral analysis.

RESULTS AND DISCUSSION

Divanillylidene cycloalkanones were prepared from vanillin and the corresponding cycloalkanones using

-0CHS

Aromatic pmtonr p - f ~ t h y l ~ ~ p r d m ~

3 -methylom protons

I c

I 1 I I 1 1 I

9 0 7 6 5 4 3 2 I 0 5 (ppm)

Fig. 2. 'H NMR spectrum of polymer VZ.



L 0

L I

I 1 I 1 200 IS0 100 so

td ppm)

Fig. 3. "C NMR spectrum of polymer VI.

boron trifluoride diethyl etherate as catalyst. N - Arylphosphoramide dichlorides were prepared from POC1, with the corresponding anilines at ambient temperature. All the polymers were obtained with >75% yield by interfacial polycondensation using a phase transfer catalyst at 20°C (Scheme 1). All the polymers were yellow in colour and powdery. The films were glossy yellow and brittle. The inherent viscosity (qinh) of the polymers determined in DMF at 30°C and the molecular weights obtained from GPC analysis are given in Table 1. The results reveal that these polymers are of low molecular weight. These data are in accord- ance with those for polyphosphate esters.24 This may be attributed to the low reactivity of phosphoramide

I

I I I I

200 400 800 800

Temperature ('C)

Fig. 4. Thermogravimetric traces of polymers I-Y.

dichlorides leading to side reactions such as hydro- l ~ s i s . ~ ~

The infrared spectrum of polymer I is shown in Fig. 1. The absorption band near 3078 cm- ' corresponds to N-H stretching, and those at 1640 and 1600cm-' correspond to C=O of cyclopentanone and exocyclic double bond stretchings, respectively. All the polymers showed absorption around 1320, 1300, 1180 and 960cm-' due to the P-N-C (aromatic), P=O and P-0-C (aromatic) stretchings of phosphoramide

These results support the formation of polyphosphoramide esters.

The representative 'H NMR spectrum of polymer V I is shown in Fig. 2. The aromatic protons of the main

I I 1 200 400 600 8

Trmprraturr ( T)

Fig. 5. Thermogravimetric traces of polymers VZ-X.

POLYMER INTERNATIONAL VOL. 40, NO. 4, 1996

Studies on photo-crosslinkable polymers

chain and pendent phenyl ring appear as a broad multi- plet in the region of 6.4 to 7.1 6. The N-H proton resonates as a singlet at 8.2 6. The signals corresponding to olefinic protons and methoxy protons appear at 7.5 6 and 3.7 6, respectively. The methylene protons of the cyclohexanone appear at 1.8 6 and 2.8 6, whereas the methylene protons of cyclopentanone appear at 3.1 6.12

The proton decoupled 13C NMR spectrum of polymer VZ is shown in Fig. 3. The aromatic carbons of the main and side chains are centred around 112-150 6. The resonance peaks at 133, 27 and 22 6 correspond to a, /? and y carbons of the cyclohexanone. The signal at 188 6 may be assigned to the keto carbon of the cyclo-

29 1

hexanone.28 The methoxy carbon resonates at 55 6. All the carbon resonances are indicated in the spectrum.

The thermal stability of the polymers was evaluated by thermogravimetric analysis (TGA) in nitrogen atmosphere. Figures 4 and 5 show the typical TGA traces of polymers I-X. The temperatures corresponding to 10% and 50% weight loss and char yield at 800°C are sum- marized in Table 2. In general, these classes of polymers are stable up to 290-330°C in nitrogen atmosphere and start degrading thereafter. A two-step degradation pattern was observed in all cases. The first step corre- sponded to formation of various small fragments, which subsequently became carbonized in the second step.

I I I 1 I 1 1

60 I10 160 210 260 310 360 410 460 Temperature ("C)

Fig. 6. DSC trace of polymer I .

TABLE 2. Thermal behaviour of polymers I-X

Polymer DSC Temperature ("C) Char yield at no. corresponding to 800°C (X)

T, ("C) T, ("C) 10% wt loss 50% wt loss

PPE-I PPE-I1 PPE-Ill PPE-IV PPE-V PPE-Vl PPE- Vll PPE- VIII PPE-IX PPE-X

84

80 87 87 80

-

- - 85 83

175 173 172 182 179 171 170 168 174 172

31 0 300 290 31 5 31 0 330 31 5 31 0 330 320

450 445 430 485 500 465 460 440 490 520

16 15 12 19 20 18 15 13 20 22



300 400 500 Wavelength (nm)

I

300 400 d0 Wavelength (nm 1

Fig. 7. Changes in the UV spectral characteristics of polymers I and VI at various intervals of time: (a) 0, (b) 5, (c) 10, (d) 15, (e) 20, (f) 25, (g) 30, (h) 35, (i) 40 min.

INNOOH CHCl3 HOTMAB 20.C l v

V

x = 0,I v = H, OCH3 CHI, CI Br

Scheme 1. Synthesis of polyphosphoramide esters.

Interestingly, the cyclohexanone polymers (VI-X) were more stable than the cyclopentanone-containing polymers (I-V). A similar trend was also observed in the char yield at 800°C. Such differences in the thermal stability and char yield may be attributed to the greater strain existing in the cyclopentanone moiety. In addi- tion, the halogen-containing polymers exhibited higher char yield than the non-halogenated polymers, this may be ascribed to the well known phosphorus-halogen syn- ergism which is the prevailing mechanism in the char formation. The substitution on the pendent phenyl ring affects the thermal stability in the following order Br > C1 > H > OCH, > CH, . The halogen-containing polymers exhibit higher thermal stability than the non- halogenated polymers. Obviously, the combination of nitrogen, phosphorus and halogen impart higher

thermal stability. Figure 6 shows a typical DSC ther- mogram of polymer I. The glass transition temperatures (Ts) of these polymers are in the range 80-87°C. The melting temperatures (T,s) are in the range 168-182°C. Besides the glass transition, the traces of the DSC thermo- grams of these polymers exhibit a broad exothermic peak around 420°C. This may be ascribed to the thermal cross-linking of the polymer b a c k b ~ n e . ~ ~ . ~ ~

The photo-crosslinking ability of the polymers was studied in a thin film by UV spectrophotometry. Figure 7 shows the changes in the UV spectral pattern during the photolyses of polymers I and V I at various intervals of time. The absorbance band at around 370nm corresponds to the n -, n* transition of the exocyclic double bond. During the successive irradiations, a decrease in the intensity of the absorbance was observed. This may be due to the photo-crosslinking of the polymer chains, which involves the 2n + 2n cycloaddition reaction of the exocyclic double bond leading to formation of cyclobutane rings." Figure 8 shows the relative rates of

r I I , I I I I

0 5 10 15 20 25 30 35 40 4

Time (min. )

I

Fig. 8. Dependence of photo-crosslinking rate on the irradiation time for polymers I and VZ.


Studies on photo-crosslinkable polymers 293

photo-crosslinking of polymers I and VI. The relative reactivity (Ao-A,)/Ao is plotted against the time of irradiation, where A , is the absorbance before irradiation and A, is the absorbance after irradiation for time t . The cyclopentanone-containing polymers undergo photo- crosslinking more rapidly than the cyclohexanone polymers. This may be due to the increase in the size of the cycloalkanone ring increasing the unfavourable geometry for 211 + 2.n cycloaddition.12 The substitution on the pendent phenyl ring does not cause a significant effect on the photolysis.

CONCLUSIONS

Two series of polyvanillylidene cycloalkanone phosphoramide esters have been synthesized and characterized spectroscopically. The inherent viscosity data reveal that these polymers are low molecular weight materials. Photochemical studies show that these polymers undergo efficient crosslinking under the influence of UV irradiation. Furthermore, the increase in the size of the cycloalkanone ring decreases their photoreacti- vity, whereas it increases their thermal stability. Finally, it may be concluded that these polymers can be utilized for developing photo-resist printing plates and other applications where thermal stability, flame retardancy and photo-crosslinking properties are essential require- ments.

ACKNOWLEDGEMENT

The author S. C. Murugavel is grateful to the Uni- versity Grants Commission, New Delhi, India, for financial assistance.

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studies on photo-crosslinkable polyphosphoramide esters containing divanillylidene cycloalkanone...

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