A Novel Synthesis of Poly(styrene peroxide) withControlled Peroxy Linkages at Room Temperature

Dedicated to Professor K. Kishore, I.I.Sc., Banglore, India, in whose collaboration we initiated this work and who left us for hisheavenly abode in February 1999

R. P. Singh,* Shrojal M. Desai, S. Sivaram, K. Kishore

National Chemical Laboratory, Pune 441, 008, IndiaFax: 911-20-5893232; E-mail: singh@poly.ncl.res.in

Keywords: degradation; polystyrene peroxide; sequence analysis; synthesis

Introduction

Polymeric peroxides,[1–4] although long known, have only

recently received considerable attention as an important

class of compounds.[5–8] Vinyl polyperoxides have gener-

ated much interest because of their potential applications

for use as auto-combustible polymeric fuels,[9] and

initiators and curatives.[10] Poly(styrene peroxides) (PSPs)

are used as initiators in preference to the conventional

peroxides for the synthesis of novel polymers like

interpenetrating networks and comb-polymers.[11] Because

of their controlled combustibility by degradation,[8] PSP

can also be used as a unique auto-combustible polymer fuel.

In view of the potential use of PSP as an initiator and

auto-combustible polymeric fuel, we present a unique

report where PSP, a copolymer of styrene and oxygen with a

low concentration of peroxy linkages, is synthesized at

room temperature using a 5,10,15,20-tetraphenyl-21H,

23H-porphinecobalt(II) pyridine complex [CoTPP(Py)]

(a reversible oxygen carrier). The synthesis of [CoTPP(Py)]

is reported elsewhere.[12] The microstructure and composi-

tion of PSP has been confirmed by FT-IR and NMR

spectroscopies. A suitable mechanism for the mode of

action of the [CoTPP(Py)] complex as a reversible oxygen

carrier has also been presented. Researchers have exten-

sively studied the thermal decomposition of equimolar PSP

but we have made a first ever attempt to study the thermal

decomposition of synthesized copolyperoxides of polystyr-

ene (having a low concentration of peroxy linkages) using

thermogravimetry and differential scanning calorimetry.

Full Paper: The present report provides an elegant techni-que for the oxidative polymerization of vinyl monomersat room temperature using a 5,10,15,20-tetraphenyl-21H,23H-porphine cobalt(II) pyridine complex [CoTPP(Py)](a reversible oxygen carrier). The content of peroxy linkagesin the poly(styrene peroxide) (PSP) copolymer is controlledby varying the reaction time. The presence of peroxy linkagesin the PSP copolymer has been substantiated using Fouriertransform infrared spectroscopy (FT-IR), gel permeationchromatographic (GPC) techniques and nuclear magneticresonance (NMR) spectroscopy. The sequence of the peroxylinkages in PSP is determined from an in-depth study of high-resolution 1H and 13C NMR spectra. Moreover, we haveattempted to study the thermal degradation of PSP having alow concentration of peroxy linkages, using differentialscanning calorimetry (DSC) and thermogravimetric analysis(TGA). The measured dissociation energy of PSP was foundto be 211� 12 kJ �mol�1. The synthesis of the PSP copoly-mer containing active peroxy segments as demonstrated here,could help in understanding the mechanism of the generationand degradation of peroxides instinctively formed during thepolymer processing.

FT-IR spectrum of PSP R-3.

Macromol. Chem. Phys. 2002, 203, 2163–2169 2163

Macromol. Chem. Phys. 2002, 203, No. 15 � WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2002 1022-1352/2002/1510–2163$17.50þ.50/0

Experimental Part

Styrene was freed from initiators by washing with 5% NaOHsolution followed by washing with distilled water, dried overanhydrous sodium sulfate and then distilled under reducedpressure. Distilled styrene (25 ml) containing a reversibleoxygen (O2) carrier, i.e., 0.2 mg of a 5,10,15,20-tetraphenyl-21H,23H-porphinecobalt(II) pyridine complex [CoTPP(Py)],was added to a 100-ml measuring flask and exposed to air. Thereaction was carried out for different time intervals at roomtemperature (20 8C) to control the content of the peroxides.During the initial polymerization, the peroxide content is highbecause the O2 carrier and polystyrene segments start to format the same time that the O2 in [CoTPP(Py)] gets depleted byforming the corresponding peroxy radical. With a longerreaction period (> 12 h), the transport of the oxygen getscurtailed because of the lower mobility of the growingpolyalkyl peroxide chains (in the reaction medium). In absenceof O2, the peroxide content gets depleted in the PSP. Hence, it ispossible to prepare samples containing different concentra-tions of peroxide linkages in the chain by controlling the timeof the reaction. The polymer was precipitated in methanol,purified by repeated precipitation from chloroform and wasdried under vacuum at ambient temperature to constant weight.The time of reaction for sample R-1, R-2 and R-3 was 1, 2 and6 h, respectively.

The molecular weight of PSP was obtained using WATERSALC/GPC 244 instrument with tetrahydrofuran as a mobilephase at 25 8C, using polystyrene standards. 1H NMR and 13CNMR spectra were recorded at ambient temperature in CDCl3containing tetramethylsilane as an internal standard on aBRUKER AC 200 and DRX-500 FT-NMR instrument at200 MHz and 500 MHz, respectively. FT-IR spectra wererecorded on a PERKIN–ELMER 16 PC Fourier transforminfrared spectrophotometer. The thermogravimetric analysis(TGA) and differential scanning calorimetry (DSC) were per-formed on a PERKIN–ELMER DSC-7 instrument at heatingrates of 10 8C �min�1 in nitrogen flux.

Results and Discussion

The polymerization of styrene in the presence of

[CoTPP(Py)] was an instantaneous reaction without any

induction period and we obtained the copolyperoxide[13] of

the type:

The mechanism of this novel co-polymerization is given

by Equation (2)–(6):

The reaction of Co(III)TPP(Py)� with O2 generates

Co(III)TPP(Py)�OO.

which in turn reacts with the alkyl

radical (R.

) and converts it into a peroxy radical (ROO.

):

The R.

can also be converted to ROO.

by its direct

reaction with O2:

The deoxygenated Co(II)TPP(Py) produced in this

process moves to the top of the reaction mixture surface

and absorbs O2 from the air to regenerate Co(III)TPP-

(Py)�OO.

and the reaction with R.

continues. This shows

that [Co(II)TPP(Py)] acts as an oxygen carrier and main-

tains a continuous supply of O2 to form the polystyrene

peroxide.

As the PSP chains grow, the mobility of R.

, compared to

[Co(II)TPP(Py)], is curtailed for its reaction with O2 and

hence, it is most likely that [Co(II)TPP(Py)] acts as a sole

source of oxygen supply for the oxidative polymerization.

The two peroxy radicals couple together, liberate oxygen

and terminate the reaction.

Thus, the present investigation provides a new method

for the oxidative polymerization of styrene and can be

termed ‘‘oxygen-transfer polymerization’’. The oxygen

concentration decreases towards the bottom of the flask and

the alkyl radicals are compelled to react with the monomers

to form the homopolymer.[13] The [Co(II)TPP(Py)] is

completely regenerated after the polymerization.

The peroxy content in PSP is elegantly controlled by the

reaction time which significantly influences both the mole-

cular weight and molecular-weight distribution of the resul-

ting polymer.[14] Gel permeation chromatography (GPC)

traces for the three polystyrene samples containing

different quantities of peroxide linkages (R-1, R-2, and R-

3) are shown in Figure 1 and results are summarized in

Table 1. The molecular weight decreases with an increasing

peroxide content, while the polydispersity index (PDI)

increases.

The percentage of peroxide linkages in PSP was estima-

ted using 1H NMR (Figure 2) spectra and are presented in

Table 1. In neat polystyrene, CH and CH2 protons appear

at 1.63 and 1.42 ppm whereas in neat PSP they appear at

2164 R. P. Singh, S. M. Desai, S. Sivaram, K. Kishore

5.3 and 4.0 ppm, respectively. By comparing the area (A) of

(CHþCH2) protons at 1.5 ppm in polystyrene segments

with that of CH2 protons at 4.0 ppm in peroxide segments,

one can calculate the percentage of peroxide linkages:

The 1H NMR spectrum of PSP samples R-1, R-2, and R-3

are shown in Figure 2. In the polystyrene homopolymer, it is

difficult to separate out the CH and CH2 proton signals as

they have merged together. However, in PSP all the four

signals are seen clearly and their intensities are dependent

upon the proportion of the peroxide linkages present in the

polystyrene sample. Figure 3 shows the 500-MHz 1H NMR

spectrum of R-3 with peaks assigned on the basis of its

comparison with equimolar copolymer, atactic polystyrene

and styrene rich polyperoxide reported in the literature.[15]

The 1H NMR spectrum of R-3 closely resembles that of

styrene-rich polyperoxide and the probable co-monomer

sequences are assigned in the spectrum.

In the first report of its kind Cais and Bovey[15] have

studied the microstructure and molecular dynamics of PSP

Figure 1. GPC traces of PSP containing different concentrationsof peroxy linkages. R-1, R-2, and R-3 indicate the samplescontaining 2.2, 4.4, and 9.0 mol-% of peroxide in the PSP sample,respectively. It is clearly seen that the polydispersity increaseswith an increase in the peroxy content.

Table 1. Molecular weight of poly(styrene peroxide) samples.

Molecularweight

R-12.2 mol-% of

peroxide

R-24.4 mol-% of

peroxide

R-39.0 mol-% of

peroxide

Reaction yield 0.5 gm 1.14 gm 3.57 gmMn 4.68� 104 3.39� 104 1.92� 104

Mw 1.14� 105 1.02� 105 0.82� 105

Mz 1.78� 105 1.69� 105 1.42� 105

Mv 1.06� 105 0.93� 105 0.74� 105

PDI a) 2.44 3.00 4.26[Z] 52.7 48.1 40.7

a) Polydispersity index.

ð7Þ

Figure 2. 1H NMR (at 200 MHz) spectra of all the three samplesof PSP. By comparing the area (A) of the CHþCH2 proton signalsat 1.5 ppm in the polystyrene segments with those of the CH2

proton signals at 4.0 ppm in the peroxide segments, it is possible tocalculate the percentage of the peroxy linkages present.

A Novel Synthesis of Poly(styrene peroxide) with Controlled Peroxy Linkages . . . 2165

by 13C NMR spectroscopy. Using the same formula and

explanation, we have attempted to postulate the micro-

structure and hence the monomer sequences in the vitiated,

oxidized PSP copolymer. A cursory examination of the 13C

NMR spectra of the copolyperoxide shows that the Ca, Cb

and aromatic quaternary carbon resonance regions are more

suitable for the analysis of co-monomer sequences in the

chain. The 13C NMR spectrum of the styrene-rich copol-

ymer shown in Figure 4 and the 13C-Dept (distortionless

enhancement by polarization transfer) NMR spectrum in

Figure 5 exhibits quaternary-, a-, and b-carbon resonances,

which are identical in chemical shift to those observed for

the equimolar copolymer and can, therefore, be assigned to

alternating sequences of styrene (St) and peroxide (O–O)

units. Small peaks for the quaternary and backbone carbons

are also observed, with chemical shifts identical to those in

polystyrene homopolymer, indicating that the polymer is a

styrene rich polyperoxide. The styrene rich polyperoxide

can, therefore, be regarded as a triad, consisting of styrene

sequences and peroxide. Comparing the backbone reso-

nance in the styrene-rich polyperoxide[15] to that in our

case, the most probable assignment of the resonances at

75.6 and 82.4 ppm (Figure 4 and 5) are b(O2StO2) and

a(O2StO2), respectively.

Thus, it is reasonable to assign the low-field group of

resonances (75 to 85 ppm) to the backbone carbons directly

bonded to oxygen i.e., a-carbons in the triad sequences

derived from the St–O–O– (StO2) dyad and b-carbons in

triads derived from the –O–O–St (O2St) dyad. Since, in our

case, the backbone carbon resonances at 65.6, 70.8, 74.1,

79.7, 80.2, 81.4, 84.8, and 85.5 ppm are absent, the pos-

sibility of b(OStO2), b(OStSt), b(OStO), b(O2StSt),

a(O2StO), a(OStO), a(StStO2), and a(OStO2) triads clearly

become unobvious. Additional information to substantiate

the above assignment of the triad monomer sequence comes

from the quaternary carbon resonance (Figure 4b) where

the peak centered at 137.4 ppm corresponds to O2StO2 and

the one at 145.16 corresponds to an StStSt triad. The area of

Figure 3. 1H NMR (at 500 MHz) spectrum of sample R-3representing the copolymer sequence. The 1H NMR spectrum ofPSP shown here closely resembles that of styrene rich polyperoxide, and the probable co-monomer sequences are assigned inthe spectrum.

Figure 4. 13C NMR (at 500 MHz) spectrum of neat polystyrene (a), styrene rich poly peroxide R-3 (b), andequimolar polystyrene peroxide (c).

2166 R. P. Singh, S. M. Desai, S. Sivaram, K. Kishore

the O2StO2 peak is almost half of that expected from the

analysis of the backbone resonances, whereas the area of

the lower field quaternary resonance is in reasonable agree-

ment. This discrepancy is because of a differential satura-

tion[15] of O2StO2. Because of this differential saturation

effect, it was not possible to make quantitative use of the

quaternary carbon resonance. The detailed study of 13C

NMR spectrum of the styrene-rich polyperoxide revealed

that the sensitivity of carbons to their environment is in the

increasing order C2,3,4<C1<Ca<Cb. Following the

treatments of Ito and Yamashita[16] and Pyun,[17] assuming

that the polymer chains are of sufficient length, we calcula-

ted the unconditional probabilities of triad occurrence

(determined by peak areas from the 1H and 13C NMR

spectra) and also calculated unconditional and conditional

probabilities using the second-order Markovian description

of copolymerization (Lewis–Mayo model).[18] The values

obtained from these statistical studies revealed the highest

probability of O2StO2 triad sequences.

Alike in homopolymers, the molecular weight and the

end groups in copolyperoxides are also controlled by the

chain-transfer reactions.[19] Mayo and Miller[20] have repor-

ted that in poly(a-methylstyrene peroxide), synthesized at

1 atmosphere of oxygen, the chains begin with HOO–

CH2C(CH3)(Ph)– or O CHC(CH3)(Ph)– groups and end

with –CH C(CH3)(Ph)– groups. Thus, the major terminat-

ing group in PSP is the unsaturated chain end. This is clearly

seen in the 1H NMR spectrum (Figure 3) of PSP. The peak at

4.9 ppm corresponds to a –OOCH C(H)(Ph)– proton. In

the 13C NMR spectrum (Figure 4), the signal corresponding

to such an unsaturated carbon falls in the region of the

aromatic carbon signals and hence is not detectable. In the

FT-IR spectrum of PSP (Figure 6), we observed a broad

hump between 3 400 and 3 600 cm�1, which indicates

the presence of terminal hydroperoxide. A sharp peak at

1 025 cm�1 is characteristic of a peroxide linkage. A strong

absorption at 1 690 cm�1 implies the presence of a carbonyl

species of the type [–PhC(H) O]/[–PhCOOH] and the

presence of a sharp peak at 1 600 cm�1 is attributable to the

C C stretching vibration.

Figure 7 shows the isothermal thermogravimetric (TG)

thermograms of PSP samples as a function of time. The

polyperoxides, under isothermal conditions, show contin-

uous weight loss with time without any step, which sug-

gested the degradation was devoid of any side reactions.

During the decomposition of equimolar PSP, on average

one O–O and one C–C bond are broken, while two C–O

bonds are converted into C O bonds. Since the decom-

position of O–O and C–O bonds depends on the number of

Figure 5. 13C-Dept NMR (at 500 MHz) spectrum. Thisspectrum supports our assignments of the CH and CH2 carbonatoms attached to the oxygen atom of the peroxide and thequaternary carbon in the benzene ring.

Figure 6. FT-IR spectrum of PSP R-3.

Scheme 1.

A Novel Synthesis of Poly(styrene peroxide) with Controlled Peroxy Linkages . . . 2167

peroxy linkages present in the backbone, the exothermicity

of the reaction is significantly less than that of equimolar

PSP. It is known that PSP decomposes exothermically to

give benzaldehyde and formaldehyde as major products,[21]

but in this case styrene and hydroxy-terminated derivatives

of polystyrene are also expected to evolve, as the concen-

tration of peroxides is much less. This hypothesis is

supported by the proposed mechanism (Scheme 1).

Thermal degradation involves O–O bond dissociation as

the rate-controlling step. This was verified by calculating

the O–O energy (Ed) from the thermal data. From the

dynamic TG runs, temperatures corresponding to 50%

degradation (T1/2) were obtained and Ed was evaluated by

using the empirical relationship:[22]

T1=2 ¼ 5:75Ed þ 185 ð8Þ

Small differences in Ed values, (Table 2) are because of

the varying peroxide contents. The Ed values correspond to

the O–O bond dissociation energies of the polyperoxides

and are quite comparable with the Ea (activation energy)

obtained from Fuoss’s relationship.[23] This reassures that

the activation barrier in the thermal decomposition of

polyperoxide is certainly the O–O bond dissociation

energy. Obviously, the higher the dissociation energy, the

more stable would be the polyperoxide, thus sample R-1 is

thermally more stable compared to other polyperoxides.

Thevalues ofEa andEd for equimolar PSP obtained from the

literature[24] are 159.94 and 175.8 kJ �mol�1, respectively,

which are significantly lower than those obtained in the

present case (222.4 and 227.2, respectively).

The exothermic DSC thermograms of PSP shown in

Figure 8 are smooth and reaffirm that poly(styrene peroxide)

degrades without any side reactions. Table 3 shows the data

of the enthalpy of exothermic degradation of PSP samples

containing different concentrations of peroxide linkages.

The DSC thermograms for the pure PS exhibit an endo-

therm at 450 8C which is characteristic of its decomposi-

tion, whereas PSP shows exothermic decomposition around

110 8C. It has also been reported[5] that equimolar PSP

explodes at �100 8C. However, we did not observe this in

our case, indicating that the PSP samples contain a low

concentration of peroxy linkages. The enthalpy of decom-

position (DHd) calculated by different methods for

equimolar PSP[25] gives a mean value of�217.7 kJ �mol�1,

which is significantly higher than that obtained in the

present case. Thus, the lower DHd values further substan-

tiate the presence of a lower concentration of peroxy

linkages in the PSP backbone. The Kissinger plot[26] of ln k

vs. T �1 from the DSC thermogram is given in Figure 9. TheTable 2. Thermal degradation data of poly(styrene peroxide)sfrom TG thermograms.

PSPsample

IDT a) T1/2 Tmax IPDT b) Ea Ed

8C 8C 8C 8C kJ �mol�1 kJ �mol�1

R-1 122.5 159.3 182.5 198 222.41 227.27R-2 102.4 156.8 171.2 188 186.78 207.50R-3 96.7 150.0 168.3 186 177.65 199.46

a) Initial degradation temperature.b) Integral procedure degradation temperature.

Figure 7. The isothermal TG thermograms of PSP samples as afunction of time. The polyperoxides under the isothermalconditions show continuous weight loss with time, suggestingthat the degradation is devoid of any side reaction.

Figure 8. The exothermic DSC thermograms of PSP. Theexothermic DSC thermograms of PSP shown here are smoothand support our claim that poly(styrene peroxide) degradeswithout any side reactions.

Table 3. Thermal degradation data of PSP from DSCthermograms.

PSPsample

Tm DH DS Ea

8C J � g�1 � 10�2 J � g�1 kJ �mol�1

R-1 150.83 �30.49 �20.21 221.24R-2 147.70 �32.91 �22.28 191.63R-3 143.70 �102.79 �71.53 176.94

2168 R. P. Singh, S. M. Desai, S. Sivaram, K. Kishore

slope of the plot gives the activation energy, Ea, and the

order of reaction. The process of decomposition was found

to follow first-order kinetics. TheEa value was found to vary

with the peroxy content of the sample but was comparable

to the O–O bond dissociation energy.[27] Although the onset

temperature did not show much varitation, the enthalpy of

degradation (DHd) changes significantly with the peroxy

content in the backbone. This indicates that the greater the

number of peroxy linkages in the polystyrene, the more

exothermic the sample.

Conclusion

The present report provides an elegant technique for the

oxidative polymerization of vinyl monomers at room tem-

perature. The [Co(II)TPP(Py)] acts as an efficient reversible

oxygen carrier to synthesize polystyrene peroxide with

controlled peroxy linkages. The content of the peroxy

linkages in the copolymer backbone can be effectively

controlled by varying the reaction time. The sequence of the

peroxy linkages in the PSP, as revealed by 1H and 13C NMR

studies is O2StO2. The termination of polymerization is

controlled by chain-transfer reactions and usually results in

carbonyl and vinylic species at the chain ends, as is evident

from FT-IR and 1H NMR spectra. The continuous weight

loss with time, in the isothermal TG thermograms, suggests

the degradation to be devoid of any side reactions. The

smooth exothermic DSC thermograms are complementary

with the TG results. Thus, the synthesis of polystyrene

containing active peroxy segments demonstrated here, may

help in better understanding the preparation and application

of such a copolymer for specialty applications.

Acknowledgement: The authors thank C.S.I.R., New Delhi forits financial assistance through a grant No. [80(0019)/EMR-II] tothis project. The authors also thank the Central NMR Facility,N.C.L., Pune, for its extensive help.

Received: December 25, 2001Revised: April 12, 2002Accepted: May 2, 2002

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Figure 9. Kissinger plot of ln k vs. T�1 for R-1, R-2 and R-3obtained from the DSC thermograms taken at 10 8C �min�1.

A Novel Synthesis of Poly(styrene peroxide) with Controlled Peroxy Linkages . . . 2169