JOURNAL OF POLYMER SCIENCE Polymer Chemistry Edition VOL. 15, 1549-1554 (1977)

Grafting onto Wool. 11. BPO-Initiated Grafting of Polystyrene

P. S. CHANDEL and B. N. MISRA, Chemistry Department, Himachal Pradesh University, Simla 171001, India

Synopsis

Grafting of polystyrene (PS) onto wool has been carried out in aqueous medium by use of benzoyl peroxide (BPO) as initiator in the presence of an acetic acid-pyridine mixture which acted as a pH modifier. Percent grafting was found to be dependent on concentration of acetic acid and pyridine, concentration of monomer, concentration of BPO, and reaction temperature. The role of pH modifier upon BPO-initiated grafting is established by the observation that no grafting occurred when one of the components of the pH modifier was absent.

INTRODUCTION

Grafting onto wool has not been extensively studied. Recently, the present authors' have reported on ceric ion-initiated grafting of poly(methy1 acrylate) onto wool and observed that graft copolymerization was catalyzed by nitric acid. Similar observations have been reported by Kenyon and Garnett,2 who found that polystyrene could be conveniently grafted onto wool swollen by methanol in the presence of mineral acid which acted as catalyst. However, they have not described a possible mechanism for such graft copolymerization reactions. Negishi et al.3 have reported grafting of vinyl monomers onto wool fiber by use of the LiBr-S20s2- initiating system. In such systems, addition of a suitable organic solvent or surface-active agent was required. In wool grafting, the physical structure of the fiber seems to have a profound effect on graft co- polymerization reactions. It is also known that in many different initiating systems, the particular conformation of the fiber which renders especially thiol groups available for reaction favors grafting of vinyl monorner~ .~-~ Relatively few studies of grafting onto wool by use of conventional free-radical initiators have been reported. Recently Arai and co-workers have used AIBN-DMS07 and BPO-methanols to effect grafting of poly(methy1 methacrylate) onto wool. In this paper, we report grafting of polystyrene on wool by use of BPO as free-radical initiator as a function of various parameters.

EXPERIMENTAL

Materials

Purification of Himachali wool by an acetone extraction method has been described.l Commercially available styrene was washed with 5% sodium hy-

1549

0 1977 by John Wiley & Sons, Inc.

1550 CHANDEL AND MISRA

droxide solution, followed by washing with deionized water and drying over anhydrous sodium sulfate. Styrene was then distilled under reduced pressure and stored in a refrigerator until use. Glacial acetic acid, pyridine, and BPO were of reagent grade.

Graft Copolymerization

Graft copolymerization was carried out essentially by the same procedure (with minor variations) as described earlier.' Pure wool samples were dispersed in cold deaerated water in a three-necked flask. Nitrogen gas which was purified by passage through alkaline pyrogallol solution and concentrated sulfuric acid was used for purging the reaction mixture for about 20 min. Weighed amounts of catalyst (BPO) was dissolved in required amount of glacial acetic acid-pyridine mixture, and the resulting solution was added to the reaction flask. A definite amount of monomer was added gradually to the reaction mixture under stirring. The copolymerization reaction was carried out in a nitrogen atmosphere for 3 hr a t four different temperatures, viz., 40,50,60, and 7OOC. Graft copolymer was separated by the usual procedure' and purified by extracting homopolymer with benzene. Percent grafting and efficiency of grafting were calculated as follows:

% Grafting = [(W, - Wo)/Wo]lOO

% Grafting Efficiency = [( W1 - W0)/Wp]100

where WI, WO, and Wz denote, respectively, the weight of grafted wool after benzene extraction, the weight of the original wool sample, and the weight of monomer used. Percent grafting and efficiency of grafting are expressed as functions of various reaction parameters.

RESULTS AND DISCUSSION

The mechanism of grafting of wool by one-electron oxidants resembles that of grafting onto cellulose and starch by redox systems. In ceric ion-initiated systems it was observed that both wool and cellulose combine with ceric ion to form a complex which then decomposes to generate active sites on the polymeric backbone (MP-) by a one-electron transfer process.'

It is presumed that grafting on wool by conventional free-radical initiator also follows the same mechanism as holds for grafting of cellulose and other synthetic polymeric backbones ( ~ P W ) in the presence of initiator. As early as 1937, Flory8 suggested that in presence of free-radical initiators all polymeric backbones undergo grafting by a chain-transfer mechanism. A general scheme for such a mechanism elaborated by Hofmanng is described in eqs. (1)-(7).

1.2 - 21. I + M - I M

I M + M - I-M-M

-P- + IMM - -P- + IMMH -P- + I -. -P- + IH

GRAFTING ONTO WOOL. IT 1551

TABLE I Effect of Concentration of BPO on Percentage and Efficiency of Graftinga

Efficiency Concn BPO, Grafting, of grafting,

No. mole/l. % %

0.001 0.003 0.005 0.008 0.010 0.015

13.50 22.89 38.00 35.00 33.00 28.00

2.97 5.04 8.37 7.71 7.27 6.17

a Composition of reaction mixture: 1.0 g wool in 300 ml water; acetic acid, 0.2910 mole/l., pyridine, 0.2160 mole/l., styrene, 0.1450 mole/l.; temperature, 60°C; reaction time, 3 hr.

P

I M + M -* IM+Mf.MH (7) The use of BPO as initiator in graft copolymerization of cellulose, starch, and

other polymeric backbones has been rather limited because in the presence of such free-radical initiators the reaction is seldom selective, and a considerable amount of homopolymer is formed by the competitive process [eq. (7)]. How- ever, in the case of wool, assuming that the above mechanism holds, the situation is little different. Since wool contains a number of functional groups, possibility exists for the formation of a large number of active sites on the polymeric back- bone (-P-) by processes (4) and (5). Homopolymer formation also occurs in wool grafting, but certainly not to the extent to prevent grafting onto wool by this procedure. Table I illustrates the effect of concentration of BPO on percent grafting; it is observed that under suitable conditions BPO-initiated grafting results in the formation of graft copolymer with an efficiency of grafting as high as 8.37%. An increase in percent grafting with increase in initiator concentration (Table I) is in agreement with the proposed mechanism. Increased initiator concentration would lead to the formation of large number of active sites on the polymeric backbone (-P-). However, a maximum grafting (38%) is obtained when the initiator concentration reaches 0.005 mole/l. Beyond this concen- tration, percent grafting decreases; this is explained by assuming that a t higher concentration initiator fragments are destroyed by a combination of several processes, viz., (a) cage recombination, (b) chain transfer with solvents, (c) chain transfer with monomer, and (d) addition of the growing radical to monomer to form homopolymer. The relative importance of these processes can be deter- mined by carrying out copolymerization reactions with a variety of monomers and chain-transfer agents. Chain transfer with solvent may be ignored, since in all the experiments water which is a poor chain-transfer solvent has been used. Work along this line is in progress, and the results will be reported later on.

Table I1 shows the effect of monomer concentration on percent grafting. An increase in the efficiency of grafting with an increase in monomer concentration will be expected if grafting onto wool occurs by chain-transfer mechanism. However, beyond a certain monomer concentration the growing radical prefer-

1552 CHANDEL AND MISRA

TABLE I1 Effect of Concentration of Monomer on Percentage and Efficiency of Graftinga

Concentra- tion of Efficiency

monomer, Grafting, of grafting, No. mole /l. % %

1 0.0870 17.80 6.54 2 0.1450 38.00 8.37 3 0.2900 35.58 3.92 4 0.4350 14.89 1.09

a Composition of reaction mixture: 1.0 g wool in 300 ml water; BPO concentration, 0.005 mole/l.; acetic acid, 0.2910 mole/l.; pyridine, 0.2160 mole/l. temperature 60°C; reaction time, 3 hr.

TABLE 111 Effect of Concentration of Acetic Acid on Percentage and Efficiency of Graftinga

Concentra- tion of Efficiency

acetic-acid, Grafting, of grafting, No. mole/l. % %

0.1746 0.2328 0.2910 0.3492 0.4074

15.40 18.26 38.00 22.60 19.54

3.39 4.02 8.37 4.98 4.30

a Composition of the reaction mixture: 1.0 g wool in 300 ml water; BPO concentra- tion, 0.005 mole/l.; pyridine, 0.2160 mole/l. temperature, 60°C; reaction time, 3 hr.

TABLE IV Effect of Concentration of Pyridine on Percentage and Efficiency of Graftinga

~~

No.

Concentra- tion of

pyridine, m ole/l .

% 7% Grafting Efficiency

0.07 20 0.1440 0.2160 0.2880 0.3600

14.80 26.00 38.00 35.00 31.10

3.26 5.73 8.37 7.71 6.85

a Composition of reaction mixture: 1.0 g wool in 300 ml water; BPO concentration, 0.005 mole/l.; acetic acid, 0.2910 mole/l.; temperature, 60°C; reaction time, hr.

entially combines with the monomer to give homopolymer and results in a de- crease in percent grafting.

From the foregoing discussion it appears that wool in presence of free-radical initiators (BPO) also, in principle, follows the chain-transfer mechanism during grafting. Wool, however, differs considerably from cellulose or starch and the difference is reflected in its reactivity. Wool is a polyamide with a crosslinked structure, and it is very likely that many of the functional groups which are embedded in the rigid structure are not available for reaction. Arai et al.1° and

GRAFTING ONTO WOOL. I1 1553

TABLE V Effect o f Temperature o n the Percentage and Efficiency o f Graftinga

No.

Tempera- Efficiency ture, Grafting, of grafting,

"C % %

40 50 60 70

3.00 15.00 38.06 35.50

0.66 3.30 8.37 7.82

a Composition o f the reaction mixture: 1 . 0 g wool in 300 ml water; BPO concentra- tion, 0.005 mole/l .;acetic acid, 0.2910 mole/l.; pyridine, 0.2160 mole/l .; reaction time, 3 hr.

Kenyon and co-workers2 observed that pretreatment of wool is required for successful grafting. During pretreatment it is likely that fiber undergoes swelling which makes functional groups available to the surface where reaction is initiated. BPO alone was found ineffective in initiating the graft copolymerization reaction. A suitable combination of BPO with acetic acid-pyridine mixture was required to produce graft copolymer. Percent grafting was found to be dependent upon concentration of both acetic acid and pyridine. In all wool grafting work'." carried out in this laboratory, it is observed that a certain amount of [H+] was needed to initiate grafting. Perhaps acid aids in the scission of disulfide bonds present in the wool, resulting in the production of more thiol groups which are known to give rise higher percentage of grafting.'O Williams and Stannett studied the grafting of ethyl acrylate to wool12 in water and concluded that new radical centers are produced upon chain scission. At lower acid concentration, the disulfide bond scission is prevented'O due to faster thiol-disulfide bond in- terchange reaction. This observation clarifies the role of pyridine which probably modifies the pH of the solution to the desired level. Tables I11 and IV show the effect of composition of acetic acid-pyridine mixture on percent grafting. In the absence of either acetic acid or pyridine, grafting did not occur. Similar behavior of acetic acid-pyridine mixture was observed by Suzuki et al.14 during grafting of acrylic acid onto poly(ethy1ene terephthalate) fibers in the presence of free-radical initiators (BPO).

Finally, the effect of temperature on efficiency of grafting (Table V) is also indicative of a chain-transfer mechanism. Chain-transfer reactions usually have higher activation energy and the rate of grafting would be expected to increase with increase in temperature. Maximum grafting was obtained at 6OoC, and at higher temperature (7OOC) percent grafting decreased, due perhaps to accel- eration of some of the termination processes which remain to be elucidated.

The authors are grateful to Professor B. S. Jogi, Vice Chancellor, Himachal Pradesh University, for his keen interest in this work, and to Professor K. C. Malhotra, Head of the Chemistry Depart- ment, Himachal Pradesh University, for providing necessary facilities.

References

1. B. N. Misra and P. S. Chandel, J . Polym. Sci. Polym. Chen. Ed., 15,1545 (1977). 2. R. S. Kenyon and J. L. Garnett, J . Polym. Sci. Polym. Letters Ed., 11,651 (1973). 3. N. Negishi, K. Arai, S. Okada, and I. Nagakura, J . Appl. Polym. Sci., 9,3465 (1965).

1554 CHANDEL AND MISRA

4. M. Lipson and J. R. Hope, Austral. J . Sci. Res., 3,324 (1950). 5. L. J. Wolfram and J. B. Speakman, J. SOC. Dyers Colourists, 77,477 (1961). 6. M. Negishi, K. Arai, and S. Okada, J. Appl. Polym. Sci., 11,115 (1967). 7. K. Arai, M. Negishi, S. Komine, and K. Takeda, in Proceedings of the Fourth International

Wool Textile Research Conference (Appl. Polym. Symp., 18), L. Rebenfeld, Ed., Interscience, New York, 1971, p. 545.

8. P. J. Flory, J . Amer. Chem. Soc., 59,241 (1937). 9. A. S. Hoffman and R. Bacskai, in Copolymerization (High Polymers, Vol. XVIII), G. E. Ham,

Ed., Interscience, New York, 1964, p. 335. 10. K. Arai, M. Shimizu, and M. Shimada, J. Polym. Sci. Polym. Chem. Ed., 11,3283 (1973). 11. B. N. Misra, and P. S. Chandel, paper presented at the Annual Convention of Chemists, Ind.

12. J. L. Williams and V. Stannett, Text. Res. J., 38,1065 (1968). 13. J. L. Williams and V. Stannett, J. Polym. Sci. B, 8,711 (1970). 14. K. Suzuki, I. Kido, and K. Nanbu, Sen-i Gakkaishi, 29,419 (1973).

Chem. Soc., 1975, Abstracts, p. 131 (1975).

Received June 18,1976