Radiat. Phys. Chem. Vol. 34, No. 5, pp. 829-837, 1989 Int. J. Radiat. Appl. Instrum., Part C Printed in Great Britain. All rights reserved

0146-5724/89 $3.00+0.00 Copyright © 1989 Pergamon Press plc

GAMMA-RADIATION INDUCED POLYMERIZATION OF METHYL METHACRYLATE IN ALIPHATIC

HYDROCARBONS: KINETICS A N D EVIDENCE FOR INCORPORATION OF HYDROCARBON IN THE

POLYMER CHAIN

HARI MOHAN and R. M. IYER

Chemistry Division, Bhabha Atomic Research Centre, Trombay, Bombay 400-085, India

(Received 4 November 1988)

Abstract---On 7-radiolysis, the rate of polymerization of methyl methacrylate in hydrocarbon solvents is observed to decrease. It is explained by hydrocarbon entry into the polymer chains. The hydrocarbon entry into the polymer chains is observed to take place at later stages of polymerization and increases with hydrocarbon chain length. The extent of hydrocarbon entry into the polymer chains is estimated by NMR and GLC analysis. It is observed to be equal to ~ 12% corresponding to ~97 hexadecane molecules in each polymer chain. The IR, DSC, MW determination and radiation effects on the polymer showed evidences for hydrocarbon entry into the polymer. It is explained by chain transfer from the growing polymer radical to the hydrocarbon molecules. The chain transfer constant is determined to be equal to 1 x 10 -2.

1. INTRODUCTION

y-Radiation induced polymerization of neat methyl methacrylate has been studied by several workers. The rate of polymerization has been found to vary as the square root of the intensity of radiation which provides specific confirmation for the bimolecular nature of the termination processes (Flory, 1967; Odian, 1970; Okamura et al., 1956; Seitzer and Tobolsky, 1955). However, radiation induced poly- merization in solvents would differ from bulk poly- merization as the solvent also would contribute to initiator concentration and affect the rate of initiation, polymerization and other physical and mechanical properties. Besides, polymerization is in- fluenced by the polar nature of the solvent, solubility of the polymer, viscosity and chain transfer processes providing useful information on the polymerization kinetics and energy transfer processes (Ballantine et al., 1956; Chapiro, 1962; Panajkar and Rao, 1983; Williams, 1968). Radiation induced polymerization of methyl methacrylate in aromatic hydrocarbons and chlorinated hydrocarbons have been reported (Manion and Burton, 1952; Mark et al., 1968).

In order to understand the high degree of cross- linking observed on y-irradiation of polyethylene in presence of acrylic acid, radiation induced polymer- ization of acrylic acid and methacrylic acid in presence of n-alkanes as model compounds was investigated by Charlesby et al. (Campbell and Charlesby, 1973; Charlesby and Fydelor, 1972). It was shown that growing polymer radical can transfer its activity to the hydrocarbon solvent molecule. By

using ~4C labelled hexadecane, each polymer molecule was observed to contain a large number of hexa- decane molecules. The present work deals with the y-radiation induced polymerization of methyl methacrylate in linear aliphatic hydrocarbon solvents with emphasis on evaluating the extent of hydro- carbon incorporation into the polymer backbone, possible sites of entry and various physical properties of the polymer formed in aliphatic hydrocarbons.

2. EXPERIMENTAL

Sample preparation

Commerical grade MMA was distilled under vacuum, inhibitor removed by aqueous sodium hydroxide solution and dried over anhydrous sodium sulphate. All other chemicals used were of AR grade purity. 4 ml of the sample solution was taken in a standard joint tube, flushed with argon gas for 20 rain, closed and irradiated in a 6°Co source at a dose rate of 0.25 Mrad per hour. The amount of the polymer formed was determined gravimetrically using chloroform as a solvent, precipitating with methanol and drying in a vacuum oven at 333 K.

Physical analysis of the polymer

Infrared spectra of polymer samples were recorded on Perkin-Elmer 577 grating spectrometer. Differen- tial scanning calorimetric analysis of polymers (20 mg) was recorded on Perkin-Elmer 1B model. The samples were heated at a rate of 10K per min and sensitivity was kept at 5 mCal s -k Gas

829

830 HARt MOHAN and R. M. IYER

e , ,

,,, 5

o a.

0 I 0 50 I00

T I M E ( M I N )

Fig. 1. Percentage polymer formed on 7-irradiation (0.25 Mradh- ' ) of MMA + hexane. (A) 0; (A) 0.48; ( 0 ) 0.91; (O) 1.54; (×) 2.56; (1 ) 3.84; (~) 5.12 and

( 0 ) 6.42 mol dm-3 of hexane.

chromatographic analysis were carried out using a 1.7 m column (internal diameter = 0.5 cm) of 10% Carbowax-400 on chromosorb W and flame ionization detector.

3. RESULTS AND DISCUSSION

IO m

N

B

ho.~ ~- x J

0 0 50 I 00

TIME (MIN)

Fig. 2. Percentage polymer formed on 7-radiation (0.25 Mrad h ~) of MMA + dodecane. (A) 0.28; (A) 0.55; ( 0 ) 0.88; (©) 1.46; (x ) 2.20; ( 1 ) 2 .93 and

(IS]) 3.68 mol dm -3 of dodecane.

deactivation) is inconsequential in the process, the

rate of solution polymerization can be written as:

gp 1~2 Rp=~/2I ' [qbmM +q~sS]l.'2M. (1)

The various terms of this equation have their usual meanings (Chapiro, 1962). The plot o f Rp vs I ~/2

Figures 1-3 show the percentage conversion of monom er with y-dose for different concentrat ions of

M M A in hexane, dodecane and hexadecane respect- ively. For comparis ion purposes, percentage conver- sion of neat M M A is also shown in Fig. 1 under identical experimental conditions. It can be seen from the figures that (a) percentage conversion decreases with decrease in M M A concentrat ion, with linear rate o f conversion in the initial stages of polymerizat ion for all the composi t ions of M M A , (b) auto acceler- at ion at high percentage conversion and high concen- tration of M M A . F r o m the linear por t ion of these figures, the rate of polymerizat ion (Rp) was deter- mined, which was observed to decrease with decrease in M M A concentra t ion (Tables 1-3). On y-irradi- at ion of m o n o m e r solution, both the monomer and the solvent would decompose to give radicals and ions, which depending upon their reactivity, concen- trat ion and life times may be able to initiate the polymerization. I f energy transfer (sensitization and

IO

N

5

0 0 50 I 00

TIME (MIN)

Fig. 3. Percentage polymer formed on 7-irradiation (0.25Mradh-l) of MMA+hexadecane. (A) 0.21; (A) 0.43; (0 ) 0.68; (O) 1.14; (×) 1.71; (1 ) 2.27 and

(I-1) 2.86 mol dm -3 of hexadecane.

Table 1. Polymerization of MMA in hexane at 300 K. Dose rate = 0.25 Mrad h

Cone. of MMA Cone. of hexane Sample (tool dm -3) (mol dm -3) Rp × 104 R i x 107

no. M S (moldm-as i) GR (moldm-3s i) v x 10 2 M.W. x 10 5

1 9,35 -- 2.18 10.9 7.32 2.98 2.88 2 8.76 0.48 1.68 - - - - - - 2.88 3 8.18 0.91 1.32 -- - - - - 2.78 4 7.48 1.54 0.90 8.70 5.83 1.54 2.68 5 6.24 2.56 0.54 -- - - 2.58 6 4.67 3.84 0.33 7.10 4.75 0.69 2.53 7 3.12 5.12 0.18 - - - - - - 2.40 8 1.52 6.42 0.10 6.30 4.24 0.23 2.33

Polymerization of methyl methacrylate

Table 2. Polymerization of MMA in dodecane at 300 K. Dose rate = 0.25 Mrad h-

831

Conc. o f M M A Conc. of dodecane Sample (mol dm -3) (mol dm -3) Rp x 104 Ri × 107

no. M S ( m o l d m - J s -1) G R ( m o l d m - 3 s - I ) v x 10 -2 M.W. x 10 -3

1 9.35 - - 2.18 10.9 7.32 2.98 2.88 2 8.76 0.28 1.82 - - - - - - 2.88 3 8.18 0.55 1.48 - - - - - - 2.68 4 7.48 0.88 1.11 9.0 6.26 1.77 - - 5 6.24 1.46 0.83 - - - - - - 2.63 6 4.67 2.20 0.50 8.3 5.25 0.95 2.53 7 3.12 2.93 0.27 - - - - - - 2.48 8 1.52 3.68 0.14 6.9 4.58 0.30 2.38

Table 3. Polymerizat ion of M M A in hexadecane at 300 K. Dose rate = 0.25 M r a d h -I

Cone. of M M A Cone. o f hexadeeane Sample (mol dm -3) (mol dm -3) Rp × 10 4 R i x 10 7

no. M S ( m o l d m 3s i) GR ( m o l d m - 3 s - I ) v x 10 -2 M.W. x 10 -3

1 9.35 - - 2.18 10.87 7.32 2.98 2.88 2 8.76 0.21 2.15 - - - - - - 2.88 3 8.18 0.43 1.63 - - - - - - 2.80 4 7.48 0.68 1.41 9.70 6.53 2.16 2.75 5 6.24 1.14 1.05 - - - - - - 2.68 6 4.67 1.71 0.67 8.50 5.66 1.13 2.63 7 3.12 2.27 0.34 - - - - - - 2.58 8 1.52 2.86 0.22 7.30 4.92 0.44 2.48

[dpmM + (~sS]l/2M would be a straight line. Figure 4 shows the non-linear variation of Rp with I ~/2 [C~mM + dp, S]]/2M in hexane, dodecane and hexade- cane, indicating that these solvents are not acting as inert solvents. The radical yields as determined by induction period method using benzoquinone as a radical scavenger also do not decrease linearly (Fig. 5) (Kumar et al., 1970). This may be one of the reasons for the non-linear variation of Rp with II/2[c~sM + C~sS]J/EM since Rp is directly proportional to the square root of radical concentration. This type of behavior (Figs 4, 5) is also observed on radiation induced polymerization of MMA and styrene in aromatic hydrocarbons. This has been explained on the basis of energy transfer mechanism because

aromatic hydrocarbons act as energy sinks (Spinks and Woods, 1964). But in the present case where linear hydrocarbons were used, it is not likely that decrease in Rp is via a energy transfer mechanism. The second possibility is that the hydrocarbons are acting as diluents in the initial stages of polymerization and later on, they are consumed in the polymerization process itself. Rate of initiation (R~) was calculated by the use of following relationship:

R. = GR Dose rate ' 100 x ~ ; (2)

where N is the Avogadro number and GR is the radical yield. These values are shown in Tables 1-3. The kinetic chain length (v) which is defined as the

% X

,a, n.

2 . 0

1.0

o . o

o/. °. ,4.

I 0 5 10

S] I / 2 "rl/2 [(~rn M -I- ~s M X t0 3

Fig. 4. Plot of rate of polymerization of MMA vs P/2(~bmM + c~S)I/2M in (O) hexane; (A) dodecane and

(O) hexadecane.

~. A o ' - - t . . ~

0 I 0 5 I0

CONC. OF SOLVENT (tool dm - 5 )

Fig. 5. Plot of radical yield as a function of solvent concentration (O) hexane; (A) dodecane and

(©) hexadecane.

t.9

832 HARIMOHAN and R. M. IYER

>- I -

Z bJ I-- Z

bJ >

--I W

5 . 6 0

4 . 0

1 . 8 5 1 . 5 0

3 . 0 2 . 0

d" ( ~m )

b

I.O O .O 2 9 . 8

~ j ~ c j

2 0 0 150 I 0 0 5 0 0

o r (l~l~m)

Fig. 6. NMR(IH) spectra in CDCI 3 of (a) PMMA and (b) P(MMA.hexadccane).NMR(13C) spectra of (c) PMMA and (d) P(MMA.hexadecane).

number of monomer units consumed per initiating species is determined from the ratio of Rp/R, The values are shown in Tables 1-3.

Search for hydrocarbon incorporation in the polymer chain

The extent of hydrocarbon, if any, which is chemi- cally bonded to the polymer chains was determined by the following methods.

(a) NMR analysis. After dissolving the polymer in chloroform and precipitating it with methanol, it was dried in a vacuum oven at 333 K. This polymer was dissolved in CDC13 and analysed by NMR (Varian FT-80A). The NMR (IH) spectra of PMMA (Fig. 6a) showed peaks at 3.60, 1.85 and 0.84 ppm. This spec- tra matched with the reported spectra of PMMA (Slonim and Lyubimov, 1970). The peak at 1.50 ppm was due to moisture in CDCI 3 and was present in all the samples. Figure 6b shows the NMR spectra of polymer obtained on ~,-irradiation (1.8 Mrad) of MMA in presence of hexadecane (80: 20). The differ- ence in the NMR spectra of PMMA and those prepared in hydrocarbon solvents was in the intensity of peaks at 1.27 and 0.84 ppm, being lower in neat

PMMA. The NMR spectra of linear chain hydrocar- bons show peaks at 1.27 and 0.84 ppm, the intensity of 1.27 ppm peak being ~ 13 times higher than that of 0.84 ppm peak. These peaks are identified as due to CH z and CH 3 respectively of aliphatic hydrocar- bons (Pouchert, 1983; Szymanski and Yelin, 1968). Therefore, the peaks observed at 1.27 and 0.84 ppm in the polymer samples obtained on 7-irradiation of MMA in hydrocarbon solvents can be assigned to the presence of hydrocarbon in the polymer chains. In order to have quantitative data, NMR spectra of different polymers were recorded with same gain, concentration, number of scans and finally nor- malised with OCH 3 peak (3.60ppm), which was nearly same in all the cases. The relative peak heights are shown in Table 4. It can be seen that intensity of 1.27 and 0.84 ppm peaks increased with increasing chain length of hydrocarbons, the increase in the intensity of 1.27 ppm peak being appreciable. On comparing 1.27 ppm peak of standard hexadecane with the polymer prepared from 80:20 composition of MMA:hexadecane (75% conversion), results showed that ~ 11.9% of hexadecane is present in the polymer. For the same composition and percentage

Table 4. Relative height of different peaks of ~H NMR spectra of different polymers

Sample Position of peaks (ppm) Percentage of hydrocarbon no. Sample 3,60 1.27 0,84 in the polymer ~ 1 PMMA 1.00 0.08 0.30 -- 2 P(MMA.hexane) 1.00 0.17 0.31 1.3 3 P(MMA. dodecane) 1,00 0.35 0.35 3.8 4 P(MMA. hexadecane) 1,00 0.92 0.47 11,9

"For 80 : 20 composition of MMA :hydrocarbon.

Polymerization of methyl methacrylate 833

conversion the amount of hexane and dodecane present in the polymers was estimated to be ~ 1.3 and -,, 3.8% respectively.

Figure 6c shows J3C NMR spectra of PMMA. The N M R spectra (Fig. 6d) of P(MMA.hexadecane) was similar to that of PMMA except for an additional peak at 29.8 ppm. Standard hexadecane gave peaks at 13.74 ppm --CH3(1, 16); 22.41 ppm --CH2(2, 15); 31.69 ppm --CH2(3, 14); and 29.42 ppm --CH2(4-13), the intensity of 29.42 ppm peak being

5 times higher than the others (Breitmayer et al., 1979). The peak observed in the polymer at 29.8 ppm is therefore assigned to the presence of hexadecane in the polymer chains. The higher intensity ( ~ 5 times) of this peak in standard hexadecane may be the reason for observing only 29.8ppm peak in P(MMA.hexadecane) sample. On comparing the 29.8 ppm peak of standard hexadecane with that of the polymer obtained on 7-irradiation (75% con- version) of MMA + hexadecane (80"20), the results showed that ~ 11.6% of hexadecane is present in the polymer, which is in agreement with ]H NMR data.

(b) GLC analysis. The amount of hydrocarbon which is chemically bonded to the polymer was indirectly estimated by gas chromatography on the assumption that the fraction which could not be extracted from the polymer must be bonded to the polymer. The data showed that a small but measurable fraction of hydrocarbon has bonded to the polymer chain. This fraction increased with increasing chain length and concentration of the hydrocarbon. For a 80:20 composition of MMA + hexadecane, polymer obtained after ~ 75% conversion showed that ~ l l . l % of hexadecane is present in the polymer. For same composition and percentage conversion, the amount of hexane and dodecane in the polymer was estimated to be equal to ~2.4 and ~4.6% respectively.

(c) IR analysis. Figures 7a and 7b show the infrared spectra of PMMA and P(MMA.hexadecane) respect- ively. The characteristic differences in the spectra of P(MMA.hexadecane) are in the appearance of a band

at 660 cm -~ and disappearance of 1630cm -~ band. The infrared spectra of aliphatic hydrocarbons show peak at 720cm -~ (Puchert, 1981). Tentatively it is concluded that the peak observed at 660 cm- ~ in case of P(MMA.hexadecane) is due to the presence of hexadecane in the polymer chains. This peak was prominent only at high polymer conversion ( > 40%) and its intensity was found to increase with increase in percentage polymer conversion and chain length of the hydrocarbon. The peak at 1630cm -] has been identified as due to aliphatic unsaturation (Szyman- ski, 1964). From the present results it appears that the unsaturation in the polymer vanishes when the poly- merization is carried out in presence of hexadecane. It is worth mentioning that since the infrared spectra was recorded for the thin film (20 mg cm-~), which was dried under vacuum at 333 K, the chances of physical trapping of the hydrocarbon in the polymer film are negligible.

Evidences for incorporation of hydrocarbons in polymer chains

The results suggest that a fraction of the hydro- carbon solvent is chemically bonded to the polymer chain. In such a case, polymer formed in hydro- carbon solvents would have different physical and radiation chemical properties as compared to PMMA. Following experiments were carried out to study such variations.

(a) DSC analysis. The various physical transfor- mations which may take place on heating the polymer are crystallization and mobility of polymer chains which gives rise to glass transition and melting. Figure 8 show differential scanning calorimetric spectra of PMMA. Four transitions at 350, 400, 430 and 475 K were observed. The transition at 400 K is assigned to Tg of PMMA (Manche and Carroll, 1972). This transition remained unchanged for poly- mer samples prepared in hydrocarbon solvents. The transition at 430 and 475 K, which are due to crystal- ization and melting were not observed in polymers prepared in hydrocarbon solvents indicating perhaps

I0O

W 0 Z

I-

l- i 50-

Z

ri-

P

g

(o)

..... (b)

I

8

, I i I

, s I It l

!L: | I Sl

4 0 0 0 3 0 0 0 2 0 0 0 IO00

WAVENUMBER (Cm - I )

Fig. 7. Infrared spectra of (a) PMMA and (b) P(MMA.hexadecane).

200

834 HARI MOHAN and R. M. IYER

3 5 0

l 4 0 0 475

i 4150 i 505 305 TEMPERATURE (K)

Fig. 8. DSC spectra of PMMA.

that the polymer is amorphous in nature due to higher branching (Mark et al., 1968). The transition at 350 K is usually assumed to be due to polymer chain mobility. This was observed at 350, 340 and 331 K for polymers obtained on polymerization of MMA in hexane, dodecane and hexadecane respect- ively. It is seen that this transition takes place at lower temperature as the chain length of the hydrocarbon solvent is increased indicating thereby that the in- creased branching in the polymer may result in lower- ing the temperature of polymer chain mobility. These results suggest that physical nature of the polymer formed in hydrocarbon solvents is different from that of PMMA, which we conclude is due to entry of hydrocarbon in the polymer chains of PMMA.

(b) M. W. determination. The viscosity average molecular weight (M,,) of the polymer was deter- mined from intrinsic viscosity measurements in ben- zene at 303 _ 0.1 K using the following relationship (Fox et al., 1962).

log r / + 4.28 log ML, 0.76 (3)

The viscosity of the polymer solution (r/$p) was measured at several concentrations (C). The intrinsic viscosity (r/) was determined from the plot of ~lsp/C vs C at C = 0. The molecular weight of the polymer prepared in hydrocarbon solvents is lower than that of PMMA (Table 1-3) and this difference increases as the fraction of hydrocarbon is increased. This suggests that the propagation of polymer radical

is prevented in presence of hydrocarbon. This may be due to chain transfer from growing polymer radical to hydrocarbon producing a polymer of lower molecular weight.

(c) Radiation effects on the polymer. (1) Table 5 shows the gaseous yields produced on y-irradiation (7.68 Mrad) of PMMA and polymers prepared in hydrocarbon solvents. On y-irradiation, PMMA would undergo decomposition giving rise to H2, CO, CO 2 and CH 4 whereas polymers prepared in hydro- carbon solvents would in addition give rise to H2 and gaseous hydrocarbons also if hydrocarbon is present in the polymer.G(H2) and G(hydrocarbon) yields were found to be slightly higher for P(MMA.hexadecane) as compared to PMMA while G(CO) and G(CO2) yields were comparable. Higher yields of H 2 and CH4 observed on ?-irradiation of P(MMA.hexadecane) also suggests the presence of hexadecane in the polymer. In case of P(MMA.hexane) and P(MMA.dodecane), yields of H2 and CH 4 were comparable to PMMA, indicating that the fraction of hexane and dodecane entry into PMMA is very small in conformity with N M R and GC data. (2) In addition to the formation of CO, CO2, H2 and CH 4 chain scission (GS) and cross- linking (GX) also take place on y-irradiation of PMMA and these can be determined by the following relationship (Dole, 1973).

1 1 G S - GX I - - dose; (4)

M, M ° 100 U

where N is Avogadro number, M, and M ° are the molecular weights after and before v-irradiation. Pure PMMA is a degrading type of polymer and for low conversions, GX is equal to zero. In such a case, slope of the line obtained on plotting 1/M, vs dose would give GS/100N. On the assumption that poly- mers prepared in hydrocarbon solvents may undergo crosslinking if hydrocarbons are present in the poly- mer chains, using the same value of GS for polymers prepared in hydrocarbon solvents as obtained for PMMA, GX for polymers prepared in hydrocarbon solvents was determined and is shown in Table 6. It is observed that GX increases with increasing chain length of the hydrocarbon and with percentage

Table 5. Yields of gaseous products produced on 7"irradiation (300 K) of polymers

Sample no. Polymer sample GCO GCO2 GCH4 GH2 GC2H6 GC3H8

1 PMMA 0.35 0.18 0.15 0.14 0.002 - - 2 P(MMA. hexane) 0.35 0.18 0.16 0.14 0.002 - - 3 P(MMA.dodecane) 0.35 0.18 0.15 0.14 0.003 0.001 4 P(MMA.hexadecane) 0.35 0.18 0.23 0.18 0.003 0.001

Table 6. Yield of crosslinking produced on ?-irradiation of polymers, GS = 1.64, temperature = 303 K

GX

Sample no. Polymer sample conver. = 30% conver. = 70%

| PMMA 0 0. I 2 P(MMA.hexane) 0.10 0.06 3 P(MMA.dodecane) 0.16 0.30 4 P(MMA. hexadecane) 0.27 0.38

Polymerization of methyl methacrylate 835

polymerization, which is expected due to higher amount of hydrocarbon incorporation in the polymer chains for higher chain length of the hydrocarbon and higher percentage polymerization. It may be concluded that the polymers prepared in hydro- carbon solvents undergo crosslinking, the extent of crosslinking being directly related to the chain length and the amount of the incorporated hydrocarbon.

Mechanism for hydrocarbon entry in the polymer chains

The results presented above conclusively prove that hydrocarbon gets chemically bonded to the polymer chains during polymerization, which can happen due to following ways:

(a) Unsaturation in the hydrocarbon, y-Irradiation of hydrocarbon produces unsaturation and these species may enter the polymer chains through the growing polymer radical and continue the polymer- ization.

CH3

CH~-----CH--CH2,~,R + , '"CH2--~" I

COOCH 3

CH31 ~ H3

-'+ ,vVCHE--C C" (5)

I ~H2 I,,~R H 3 COOC

For a typical 80: 20 composition of MMA: hexadec- ane, and taking G value for unsaturation equal to 2.5 (Pancini et aL, 1970), y-irradiation (1.8 Mrad) would result in the incorporation of < 0.1% of hexadecane in the polymer. This is a very small fraction compared to the observed value of ~ 12%.

(b) Unsaturation in the polymer. The infrared stud- ies suggest that the unsaturation produced in PMMA is reduced when the polymerization is carried out in presence of hydrocarbon solvent. The growing poly- mer radical would terminate either by addition reac- tion or by disproportionation in which saturated and unsaturated polymers are produced.

CH3 CH3

'¢'~CH2--~' + ~CH2--~' ~OOCH 3 ~OOCH 3

CH3 CH 3

--+,'~,ACH2--~H + ,v,~CH~---~ OOCH3 IOOCH

(6)

The hydrocarbon radicals produced on 7-irradiation may enter into polymer chains through these unsatur- ations.

CH 3 R CH 3

+ R' ~ CH---~'

[ ~OOCH3 COOCH 3

(7)

For each unsaturation one hydrocarbon molecule enters into the polymer chain. The calculation shows that for the observed 12% hexadecane incorporation, each polymer chain would contain as much as 97 hexadecane molecules.

(c) Reaction of hydrocarbon radicals. The hydrocar- bon radicals produced on v-irradiation may directly add onto MMA double bond

CH 3 CH 3

R' + CH~---~ ~ -* R - - C H 2 - - C [ (8)

I ~COOCH3 COOCH 3

and start a chain or add onto PMMA radicals formed when H, CH3 o r COOCH 3 are detached by 7-rays as H2, CH4, CO2 etc are formed. For a typical 80:20 composition of MMA:hexadecane and taking G(radical) = 10, 7-irradiation (1.8 Mrad) would result in the incorporation of ~ 0.1% of hexadecane in the polymer.

From these discussions, it is clear that the observed entry of hydrocarbon in the polymers can not be accounted by these processes. At best these factors may contribute to a small fraction of hydrocarbon bonding to the polymer.

Thermal and photopolymerization of MMA in hydrocarbon solvents was carried out to understand the mechanism of hydrocarbon entry into the poly- mer chains and also to eliminate the entry of hydro- carbon into the polymer chains by hydrocarbon radicals and unsaturation in the hydrocarbon pro- duced during 7-irradiation. Since hexadecane does not decompose on heating to moderate temperatures (335 K) and does not absorb ultraviolet (253.7 nm) radiation, it was felt that thermal and photopolymer- ization of MMA + hexadecane should result in negli- gible entry of hexadecane in the polymer if the entry is through hydrocarbon radicals and unsaturation in the hydrocarbon. Table 7 shows that 6.3 and 5.8% of hexadecane is bonded to the polymer obtained on thermal and photopolymerization. Figure 9a and b show the percentage polymerization of MMA and MMA+hexadecane (80:20) respectively on heating at 335 K. The rate of thermal polymeriz- ation was observed to reduce from 4.9 x 10 -6 to

Table 7. Polymerization of MMA and MMA+ hexadecane by various methods Rate of polymerization (moldm -3 s -~) Percentage hexadecane in polymer

Sample no. Polymer sample Ther. Photo. ~,-irr. Ther. Photo. 7-irr. 1 PMMA 4.9 x 10 -6 7.3 x 10 -5 2.2 x 10 -4 -- - - - - 2 P(MMA.hexadecane) 5.2 x l0 -7 1.2 x l0 -5 1.3 x 104 6.3 5.8 l l.9

836 HARI MOHAN and R. M. IYER

~00

o {~

5O 52 0 ~

T I M E {HOURS)

Fig. 9. Percentage polymerization of (a) MMA; (b) MMA + hexadecane (80:20) on heating at 335 K, (c) MMA and (d) MMA+hexadecane (80:20) on photolysis

(253.7 nm) at 303 K.

0.52 x 10 - 6 mol dm -3 s -1 in presence of hexadecane (Table 7). The rate of photopolymerization (253.7 nm, 303 K) was also observed to decrease from 7.3 × 10 -5 to 1.2 x 10-Smoldm 3s-1 in presence of hexadecane (Fig. 9c, 9d, Table 7). The observed entry of hexadecane in the polymer chains could be due to chain transfer from growing polymer radical to hy- drocarbon (Rao and Rao, 1985).

CH3 CH3 I I

~.4:;H2--C' + RH--~,~CHz--CH' + R' I I

COOCH 3 COOCH3 (9)

The hydrocarbon radical may now enter into the polymer chains either by initiating polymerization or by adding onto the unsaturation in the polymer chains as discussed in the previous section. Since the hydrocarbon radicals may not have the same reactiv- ity as that of monomer radicals, the lower rates of polymerization in presence of hydrocarbon can be understood.

The chain transfer from growing polymer radical to hydrocarbon would also be operative on 7- irradiation of MMA in presence of hydrocarbon solvents. This process could lower the molecular weight of the polymer formed in hydrocarbon sol- vents (Tables 1-3). Assuming the degree of polymer- ization (DP) to be equal to kinetic chain length (v), the chain transfer constant (Cs) could be determined from the following relationship:

1 1 S D P . - DP~o + Cs--~ ; (10)

where DP, and DPo are the degree of polymerization in presence and absence of solvent, S and M are the concentrations of solvent and monomer respectively. The chain transfer constant (C,) is determined on plotting 1/DP, vs S/M and is equal to 1.03 x 10 -2 (Fig. 10). This was observed to be nearly same for hexane, dodecane and hexadecane. This is expected due to almost equal bond energy of C---C and C- -H bonds in these hydrocarbon solvents. The chain transfer constant for neat MMA has been determined

5

_o o

c n C)

~" 2 - -

I

0 I I I I 0 I 2 3 4 5

$ / M

Fig. 10. Plot of 1/DP, vs S/M for various solvents, (O) hexane; (O) dodecane and (A) hexadecane.

to be 1 × 10 -5, (9) which is much lower than that estimated in presence of hydrocarbon. Thus from these studies, it appears that chain transfer from growing polymer radical to hydrocarbon may be the main source of hydrocarbon entry into the polymer chains.

Our results are based on extensive investigations on the estimation of hydrocarbon entry into the polymer chains by various methods, which show that as much as 12% of hexadecane is bonded to the polymer chains, which works out to be about 97 hexadecane molecules per polymer chain. The consequence of such hydrocarbon incorporation on physico- mechanical and thermal properties of modified PMMA would be worth studying from the appli- cations point of view. Some insight with this aspect has been discussed in this paper.

4. C O N C L U S I O N S

The rate of polymerization of MMA is reduced in presence of linear chain hydrocarbons. The hydrocar- bons do not act as inert solvents and take part in chemical bonding to the polymer. The results suggest that hydrocarbon enters into the polymer combi- nation at the later stages of polymerization and this fraction increases with increasing chain length of the hydrocarbon, concentration of hydrocarbon and per- centage conversion. About 97 hexadecane molecules are present in each polymer chain.

Acknowledgements--Sincere thanks are due to Dr K. N. Rao for his keen interest and critical advise. Thanks are also due to Dr M. H. Rao for helpful discussions and to Dr V. K. Jain for his help in recording NMR spectra.

REFERENCES

Ballantine D. S., Glines A., Metz D. J., Behr J . , Mesrobian R. B. and Restaino A. J. (1956) J. Polym. Sci. 19, 219.

Breitmaier E., Haas G. and Voelter W. (1979) Atlas of Carbon-13 NMR Data, p. 1038. Heyden, London.

Polymerization of

Campbell D. and Charlesby A. (1973) Eur. Polym. 3. 9, 301. Chapiro A. (1962) Radiation Chemistry of Polymeric Sys-

tems, p. 250. Interscience Publishers, New York. Charlesby A. and Fydelor P. J. (1972) Int. J. Radiat. Phys.

Chem. 4, 107. Dole M. (1973) The Radiation Chemistry of Macromolecules

(Edited by Dole M.), Vol. II. p. 97. Academic Press, New York.

Flory P. J. (1967) Principles of Polymer Chemistry, p. 106. Cornell University Press, New York.

Fox T. G., Kinsinger J. B., Mason H. F. and Schuele E. M. (1962) Polymer 3, 71.

Kumar V., Rao K. N. and Shankar J. (1970) Radiat. Eft. 5, 227.

Manche E. P. and Carroll B. (1972) Physical Methods in Macromolecules (Edited by Carroll B.), p. 262. Marcel Dekker, New York.

Manion J. P. and Burton M. (1952) J. Phys. Chem. 56, 560. Mark H. F., Gaylord N. G. and Bikales N. M. (1968)

Encyclopedia of Polymer Science and Technology, Vol. 1, p. 267. Wiley, New York.

Odian G. (1970) Principles of Polymerization, p. 269. McGraw Hill Book Company, New York.

Okamura S., Yamashita T. and Higashimura T. (1956) Bull. Chem. Soc. Japan 29, 647.

methyl methacrylate 837

Panajkar M. S. and Rao K. N. (1983) Radiat. Phys. Chem. 21, 419.

Pancini E., Santaro V. and Spadaccini G. (1970) Int. J. Radiat. Phys. Chem. 2, 1471.

Pouchert C. J. (1981) The Aldrich Library of Infrared Spectra, p. 4. The Aldrich Chemical Co., Wisconsin.

Pouchert C. J. (1983) The Aldrich Library of NMR Spectra, Vol. 1, p. I 1. The Aldrich Chemical Co., Wisconsin.

Rao M. H. and Rao K. N. (1985) Radiat. Phys. Chem. 26, 669.

Seitzer W. H. and Tobolsky A. V. (1955) J. Am. Chem. Soc. 77, 2687.

Slonim I. Ya. and Lyubimov A. N. (1970) The NMR of Polymers, p. 208. Plenum Press, New York.

Spinks J. W. T. and Woods R. J. (1964) An Introduction to Radiation Chemistry, p. 374. Wiley, New York.

Szymanski H. A. (1964) Interpreted Infrared Spectra, Vol. I, p. 32. Plenum Press, New York.

Szymanski H. A. and Yelin R. E. (1968) NMR Band Hand Book, p. 133. Plenum Press, New York.

Williams F. (1968) Fundamental Processes in Radiation Chemistry (Edited by Ausloos P.), p. 515. Interscience, New York.