Kinetic study of vinyl polymerization with a new oligo azo peroxidic initiator

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PII: soo14-3057(!Mpo181-4

KINETIC

STUDY OF VINYL POLYMERIZATION

WITH A NEW OLIGO AZ0 PEROXIDIC

INITIATOR

C. VOLGA,’ B. HAZER*‘** and 0. TORUL’

‘Karadeniz Technical University, Department of Chemistry, Trabzon 61080, Turkey 2TlfBITAK-Marmara Research Center, Department of Chemistry, P.O. Box 21 Gebze 41470

Kocaeli, Turkey

(Received I8 December 1995; accepted in final form 13 February 1996)

Abatraet-Gligo (4,4’-azobis-cyanopentanoyl-5-peroxy-2,5-dimethyl-n-hexyl~roxide), Lu-Ab, was synthesized by the interfacial condensation of 4,4’-azobis-(4cyanopntanoyl chloride) with 2,5-dimethyl-2,5- dihydroperoxyhexane in the presence of a base. The molecular weight of the initiator was determined to be 1586 g/mol by gel permeation chromatography (GPC). Oligo-azoperoxide, Lu-Ab, was characterized by infrared and nuclear magnetic resonance spectroscopies.

Styrene and methyl methacrylate were separately polymerized with Lu-Ab at 60°C by free radical low conversion polymerization in order to calculate kinetic parameters. In these units needed connection overall rate constants (k) and k& were found to be 1.86 x lo-’ L mol-’ s-’ and 1.22 x 10’ L-’ mol s for styrene; 3.73 x IO-’ L mol-’ s-’ and 0.21 x 10) L-’ mol s for methyl methacrylate, respectively. Lu-Ab is an initiator without chain transfer to initiator, like azobisisobutyronitrile (AIBN) but is only half as effective as AIBN. 0 1997 Elsevier Science Ltd

INTRODUCMON

Oligoperoxides contain more than one peroxy group. Vinyl polymerization with oligoperoxide in limited polymerization conditions yields active polymer having undecomposed peroxy groups in the polymer backbone [l-7]. These active polymers can initiate the polymerization of a vinyl monomer to give block copolymers [8]. Azo-peroxidic initiators are also used in block copolymer synthesis since the thermal stabilities of the azo and peroxide groups are different [%12]. Several azo-peroxidic initiators are based on the capping of 4,4’-azobis-cyanovaleryl chloride or isobutyronitrile-2-azo-3’-isovaleryl chloride [8] with benzoyl or acetyl hydroperoxide [lo] or tert-butyl hydroperoxide [ll-121. In the first stage of the polymerization, azo-peroxidic initiators can produce free. radicals by decomposition of either azo or peroxy groups in the presence of a vinyl monomer, and active polymers are formed. Active polymers, in our recent work, were used in reaction with polybutadiene to yield graft copolymer [ 131.

the polymer samples. DSC thermograms of the polymers were taken on a Du Pont 910 Differential Scanning Calorimeter at a heating rate lOC/min. GPC chro- matograms were taken on a Shimadzu GPC Instrument including CR-4A chromatopac computer and printer, CTOdA column furnace, RID-6A detector and LC-9A liquid pump. THF was used as eluent at a flow rate of 0.75 mlimin. Calibration standards 250.000. 90.000 and 50,000 gjmol of low dispersity which were purchased from Polyscience.

Materials

Materials used and suppliers are listed as follows: 2,5-Dimethyl-2,5-dihydroperoxyhexane (Luperox 2,5- 2,5) was a white crystalline product of Lucidol Division, Penwalt Crop., Buffalo, N.Y. It was recrystallized from CCL&

4,4’-azobis+l-cyanopentanoic acid): Merck A.G., Phophorus pentachloride: Merck A.G.,

AIBN: Merck A.G.

Styrene and methyl methacrylate monomers from (FLUKA A.G.) were freed from inhibitor by washing with 10% NaOH solution and water, and then by vacuum distillation from CaH2.

This work refers to kinetic investigations of styrene and methyl methacrylate polymerizations initiated by the oligo-azoperoxidic initiator [ 141 which yields active polymers which can lead to graft copolymers by a free radical mechanism.

Insrrumentation

EXPERIMENTAL

A 200 MHz Bruker-AC 200L NMR and a Nicolet 510P FT-IR spectrometers were used for recording the spectra of

Synthesis by 4,4’-azobis-(4-cyanopentanoyl chloride). A slurry of 5 g of 4,4’-azobis-(4-cyanopentanoic acid) in 60 mL of benzene was kept at room temperature for 45 min. The slurry treated with 4.2 g of phophorus pentachloride [I 5, 161 added over a period of 45 min. Stirring was continued for 45 min at 30-35°C. The solution was then evaporated and 3 mL of chloroform were added. 4,4’-azobis-(4-cyanopentanoyl chloride) was precipitated in petroleum ether, filtered and washed with two 10 mL portions of petroleum ether, and three 10 mL portions of petroleum ether/n-hexane (1:3). The pale yellow solid (m.p. 97°C) was dried in vacuum at room temperature and stored in the refrigerator. The yield was 7.5 g.

*Present address: Zonguldak Karaelmas University, Depart- Synthesis of oligo (4,4’-azobis-cyanopentanoyl-5-peroxy- ment of Chemistry, Zonguldak 67100, Turkey. 2,5-dimethyl-n-hexyl peroxide), (Lu-Ab). 3.90 g of Luperox

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908 C. Volga et at. 2,5-2,5 in 30% NaOH solution were stirred at 0°C for I hr [5]. 6.96 g of 4,4’-azobis-(4cyanopentanoyl chloride) in 40 mL of ether was added to this mixture at 0-5°C. The reaction mixture was filtered and the ether and water phases were separated. The water phase was washed with 30 mL portions of ether and added to the ether phase. The ether phase was washed with distilled water and dried over anhydrous NaSOd. The solvent was evaporated and the white crystals were dried in vacuum at room temperature. Molecular weight of Lu-Ab was found to be 1586 g/mol (by GPC). Peroxygen content was 8.2% (by iodometric analyses) [18]. Theoretical value is 15% IR, NMR and TGA curves of Lu-Ab are shown in Figs 1, 2 and 3, respectively.

For polymerization of styrene (or methyl methacrylate) with Lu-Ab, monomer and Lu-Ab were charged separately in a Pyrex tube; NZ was purged through a needle into the tube. The tightly capped tube with a rubber septum was put in an oil bath at 60°C. This temperature was sufficient to activate the azo group in the oligo-azoperoxidic initiator. After the required period of polymerization, the tube was quickly cooled and the contents coagulated into methanol. The block copolymer was filtered off and dried in vacuum at room temperature.

Table 1 shows the conditions and the results for the polymerization of styrene with Lu-Ab at WC; polymer yield was kept under 16%.

I I I I I I I I 1 1 1 I I 1 I 1

3.8 3.6 3.4 3.2 3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 Fig. 1. ‘H-NMR spectrum of Lu-Ab.

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Kinetic study of vinyl polymerization 909

0 **“c~H2-cH2~:=N-~H*-cH2-c~o**

HO’ H3 H3 ‘OH

4,4’-azobis-(4-cyanopentanoic acid)

I

PC15 or WC12) 0 CN

CN

‘C-CHz-CH2-

t!

L

0 -N=N - Cl’ -CH*-CH2-C+ f!H3 t&H3 ‘Cl

4,4’-azobis-(4-cyanopentanoyl

chloride) CH3 CH3 HO0 -C -C h -CHz-CHz -0OH H3 A H3

I

2,5-dimethyL2,Sdihydroperoxy hexane

(Luperox-2,5-25)

Oligo (4,4’-azobis-cyanopentanoyl-5-peroxy-2,5-dime~yl-n-he~l peroxide) (Lu-Ab)

Scheme 1. RESULTS AND DISCUSSION

Synthesis and characterization of oligo-azoperoxide initiator (Lu-Ab)

A new oligo-azoperoxide initiator (Lu-Ab) was synthesized by the chain extension reaction of 4,4’-azobis-(4-cyanopentanoyl chloride) and 2,5- dimethyl-2,5-dihydroperoxy hexane. Oligo-azoperox-

ide was characterized by IR and ‘H-NMR

spectroscopy. Figures 1 and 2 show NMR and

FT-IR spectrum of Lu-Ab, respectively. The

‘H-NMR spectrum contains characteristic bands of Luperox 2,5-2,5 and azobis-cyanopentanoyl groups as indicated in Fig. 1: (d,,): 1.21 (CH,-C-N=), 1.28 (CH,-C-GG), 1.30 (CH&-N=), 1.70 (-CH&), 2.50 (-CHm).

FT-IR spectrum of Lu-Ab in Fig. 2 showed

characteristic absorptions at 3200 cm-’ (G-H), 2300 cm’ (CkN), 1780 cm-’ (c---O) and 1580 cm-’ (N=N).

IU, of Lu-Ab was found to be 1586 g/mol by GPC. The number of repeating units was calculated as 3.8 confirming the chain extension reactions during the synthesis of Lu-Ab.

TGA traces of Lu-Ab (Fig. 3) exhibited decomposition maxima at around 127°C (azo groups) and 175°C (peroxygen). The calculated content of azo groups in this TGA curve of Lu-Ab was found to be 7.4% (theoretical value is 6.9% of N-N groups).

The peroxygen content found by iodometric

analysis was 8.2% which is less than theoretical value:

15.2%; the difference between the found and

calculated value can be explained as follows: During the iodometric analysis of peroxide groups in Lu-Ab,

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910 c. Volga rr al.

12.3 , n

60

Fig. 2. FT-IR spectrum of Lu-Ab. azo groups are simultaneously cleaved. causing side

reactions which tend to affect the peroxide content. Low conversion polymerization qf’ styrene and methylmethacrylate with Lu-Ah

Table 1 shows the results of free-radical polymerizations of styrene with Lu-Ab. Standard iodometric analyses gave the peroxide contents of the active polymers (Table 1). Styrene polymerization to low conversion was carried out with Lu-Ab as azoperoxidic initiator and active polystyrene samples contain- ing undecomposed peroxygen groups were obtained. Table 2 shows the results of the free-radical polymerization of methyl methacrylate with Lu-Ab, and the peroxide content of the active polymers. Higher peroxygen content in active polymers gives higher grafting efficiency on polymer backbone [ 13, 141. Peroxygen content in active polymers can be increased by initial azoperoxidic initiator. TGA curve of active polystyrene in Fig. 4 also showed peroxygen decomposition at 170°C with polystyrene hydro- carbon decomposition.

Molecular weights of active polymers were between 159,000 and 33,000 g/mol with polydispersity I.255 2.05 depending on the initial concentration of azoperoxidic initiator.

Kinetics study

Lu-Ab was used as a free-radical initiator in the polymerizations of styrene and methyl methacrylate at 60°C. Conversions were kept below 16%, for proper analyses of the kinetic results. The results are given in Tables 1 and 2. Polymerization rate, R,, was calculated by using the following equation:

R, = conversion %

t x-fp 100 (1)

Where t is polymerization time in set, p is density of

1

I I I I I

100 200 300 400 500

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Kinetic study of vinyl polymerization 911 Table 1. Polymerization of styrene with LwAb at 60°C

Run no.

Peroxide Polym. Polym. in active

VI time, yield. pokm. Rpx IO’ _[Ml [M].[I)“Z MN Mw U/P”)

mol L-’ min wt % wt % mol L-’ s-l mol L-’ (mol/L)3fz X IO-‘ X IO-’ x IV

I 0.0013 120 2.88 0.34 8.74 0.300 12.7 15.9 8.16 2 0.0025 110 4.18 0.54 8.70 0.435 9.25 13.9 11.3 3 0.0044 100 4.80 - 0.68 8.68 0.570 6.98 10.4 14.9 4 0.0088 90 7.14 0.53 1.15 8.61 0.810 3.64 7.02 28.6 5 0.0183 90 13.83 3.20 2.24 8.44 1.140 3.10 5.94 33.0 6 0.1562 60 15.96 6.24 3.87 8.25 2.020 1.80 3.30 55.5

Table 2. Polymerization of methyl methacrylate with Lu-Ab at 60°C Peroxide

Polym. Polym. in active

Run VI time, yield, _Pob. R, x IO’ _Ml [M].[I]‘,’ MN Mw U/P”) “0. mol L-’ min wt % wt % mol L-’ s-’ mol L-l (mol/L)‘” X IO-’ X 10-q X IV

7 0.0019 100 7.20 1.12 9.36 0.403 10.5 12.6 0.95 8 0.0063 50 5.25 1.12 1.64 9.20 0.730 29.6 37.5 3.38 9 0.0123 30 8.61 3.22 4.47 9.14 1.005 15.2 18.3 6.60 IO 0.0183 50 15.13 2.00 4.10 9.04 1.220 18.6 29.0 5.38 II 0.0188 30 15.58 4.16 8.10 9.03 1.233 5.80 11.7 17.0 12 0.0313 40 15.33 I .28 5.97 9.02 ,1.630 4.50 9.98 22.0 13 0.0595 20 14.06 5.76 10.9 8.84 2.160 3.47 6.30 28.8

monomer at 6o”C, M is the molecular weight of monomer. Rp values were calculated by subtracting the purely thermal rates of polymerization.

Ri and R, for a free-radical polymerization reaction

: 420°C

100 200 300 400 500 600 Temperature, “C

Fig. 4. TGA curves of active polystyrene (run no. 4 in Table 1).

are given by the following equations [4, 19,211:

and

R, = 2.j-.kd.[Z] (2)

R, = k,+V]~(fk~[Z]/k,)“~ ₍₃₎

R, = k.[M].[Z]“2 (4)

Where [I] and [M] are the initiator and monomer concentrations, kd, k, and k,, are the thermal decomposition, propagation and termination rate constants, respectively, andfis the initiator efficiency. The term k, (fkJk,)“* is often denoted k and is a measure of the initiator reactivity [20]. The value of k can be obtained from the slope of the plot of R, vs

[M].[Z]“*. For our systems the values of k are

estimated from Fig. 5 as 1.86 x lo-“ and

3.73 x 10m4 L mol-’ s-’ for styrene and methyl methacrylate, respectively. 12

r

IO

I-

8 “0 ‘; 6 4 0 0 0.5 1 1.5 2 2.5 MIl”*

Fig. 5. Square root dependence of the polymerization rate, at 60°C: n styrene with Lu-Ab, 0 styrene with AIBN, + methyl methacrylate with AIBN, 0 methyl methacrylate

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912 C. Volga ef al

Table 3. Experimental rate constant at 60 Monomer Initiator k. L” mol I2 s’) A = k,/k; (L ’ mol) St Lu-Ab 1.86 x IO ’ 1.22 x IO’

AIBN 3.88 x IO ’ 0.90 x IO’ MMA Lu-Ab 3.73 x IO ’ 0.21 x IO’ AIBN 8.40 x IO * 0.15 x IO’

Table 3 indicates the overall rate constants for polymerizations with Lu-Ab or AIBN. One can say that the oligo-azoperoxidic initiator is half as effective as AIBN in vinyl polymerization at 60°C. For free-radical polymerization, without using solvent average degree of polymerization (P.) is related to R, thus.

+=G+R ₍₅₎

” &++&t]’

Here, CM and C, represent the chain transfer constants to monomer and initiator, respectively. If C, = 0, a plot of l/P” vs R, should yield a straight line satisfying to the equation.

1

z = CM + R, k> _* (6)

Figure 6 shows the linear plots of I/P, vs R, for vinyl polymerization with Lu-Ab or AIBN. The monorad- ical lines were also obtained for the low conversion vinyl polymerization with AIBN in our polymeriz-

60 -

.

0 20 40 60 80 100 I20 140 160

(Rp/[M]‘) x IO’

Fig. 6. Plot of l/P, vs (R,/[M]?) for styrene and methylmethacrylate polymerization at 60°C initiated by Lu-Ab: n styrene with Lu-Ab, 0 methyl methacrylate with Lu-Ab, 4 styrene with AIBN, 0 methyl methacrylate with

AIBN.

ation system. The slopes and intercepts of these lines are calculated as described in the literature [19,22]. The curvatures observed at high R&4]* for both monomers result from chain transfer to initiator. The intercepts of these curves with the l/P, axis gives CM. The values of chain transfer constants for styrene and methyl methacrylate monomers are determined to be 1.86 x 10m4 and 3.73 x 10m4 L mol-’ s-l, respectively. 9. 10. I I. 12. Ii. 14. 15. 16. 17. IX. 19. 20. 21. 22. 23. 24. REFERENCES

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