Copolymerization of unsaturated compounds induced by high electric fields

European Polymer Journal. V o l 11, pp, 113 to 118 Pergamon Press 1975. Printed in Great Brilain

COPOLYMERIZATION OF UNSATURATED COMPOUNDS I N D U C E D BY H I G H ELECTRIC FIELDS W. WM~LAT, W. F. SCHMIDT a n d W. SCHNABEL Hahn-Meitner-lnstitut fiir Kernforschung Berlin GmbH. Bereich Strahlenchemie. 1 Berlin 39. Germany

(Received 29 July 19747 Abstract--The copolymerization of :~-methylstyrene (:t MS) with vinylcarbazole (VC), styrene (ST) and methylmethacrylate (MMA) was initiated by field emission (FE) and field ionization (FI) at - 8 : . From the dependency of the composition of copolymer (CP) on monomer composition, it is concluded that the polymerization under FI and FE conditions proceeds via free cations. Thus earlier assumptions on thc polymerization mechanism based on scavenger experiments were corroborated. In detail the following results were obtained: (at :t MS-VC. The reactivity ratios are rl (~ MS) = 0-06 ± 0.02. r_, (VC) = 51 _ 2. CP-composition and specific yield dependencies on the composition of the monomer mixture yielded k~, > k~ > k2_, > k~ t. Number average molecular weights varied from 160,000 (92 mole "~, VC in copolymer) to 15.000 (pure P u.MS). (b) ~ M ~ S t . The reactivity ratios are rl (St) = 0-1 4- 0"1 and r2 (~ MS) = 10-4 4- 2"0 and agree with values found during ?~radiation initiated copolymerization. Specific polymer )ield measurements gave: k~ 2 > k a~ > k2_, > k2t. The number of average molecular weights varied from 36.000 (pure PStl to 15.000 (pure P :~ MS). At 22 ° radical polymerization occurs in addition to cationic pol) merization. (c) ~ MS-MMA. With FI, pure MMA does not yield polymer. Monomer mixtures yield homo PaMS and CP ~ MS MMA generated via a radical mechanism (r~ (MMA) = 0"4 -'- 0'1 and r~ (~ MS) = 0'27 ± 005). I. INTRODUCTION Several recent studies have concerned free ion polymerization in the liquid phase of vinyl m o n o m e r s and related c o m p o u n d s [ I - 5 ] . Polymerization was initiated either by high energy radiation [ 1 2] or by a high inhomogeneous electric field I-3-5]. If one applies a sufficiently high voltage (several kV) to a n electrode assembly comprised of a plane electrode a n d a tip or a sharp edge, then near the surface of the tip or sharp edge field strengths of 106-108 V cm - t can be obtained. Field emission (FE) of electrons occurs when the tip (edge) is negative while molecules are ionized when the tip (edge) is positive (field ionization, FI). For our experiments, we used diode cells which consisted of a pyrex glass tube with a razor blade electrode opposite to a plane steel electrode. Voltages up to 15 kV were applied and currents of the order of microamperes flowed through the cell. In a preceding paper [6] concerned with elucidation of the mechanism of polymerization, we reported on the influence of scavengers on the polymerization of 7methylstyrene initiated by FI or FE. The results indicated that under FE conditions the overwhelming portion of the polymer is formed via a cationic mechanism. No evidence was obtained for the occurrence of anionic polymerization. However, it was observed that electrons were scavenged by SF 6 and other c o m p o u n d s and that simultaneously the specific polymer yield was reduced a n d completely suppressed at high scavenger concentrations. The proposed mechanism is shown in C h a r t 1.

Chart 1. Mechanism of polymerization of :~-mefll,xlstyrene induced b~ field emission (FE) tip negatixe

highel tield e[,.

(1)

e(,,,+ M - "M * + nM "M,7 + M - -

k,,

"M- + 2e "M,[_ l 'M, + M

12) (3) I41

M,7 + M

G

M.~. ~ M,, + M,~

(5j (6j

M,~ + M

in the presence of an electron scavenger F : e[,,, + F

~' , F/

, R" + X

171

in the presence of a cation scavenger:

M,7 + F~

, M,, + F~

I8)

In the case of field ionization, the situation appeared to be comparatively simple insofar as positive ions created at the positive electrode obviously initiated the cationic polymerization of the monomer. We now report on copolymerizations carried out to substantiate the proposed mechanism. 2. EXPERIMENTAL (a)

Purification1 oJmonomers

In order to remove the stabilizer, the monomers were washed with N a O H solution a n d distilled water 113

114

W. WABLAT. W. F. SCHMII)T a n d W. SCHNABEL

followed by drying over CaC12 and Call2. ~-Methylstyrene (~ MS) and styrene (St) were passed through columns filled with activated silica gel and AI2O 3 (W200, basic W61m) before distillation. Degassing of the monomers and the monomer mixtures was achieved by repeated pumping on the frozen systems. Methylmethacrylate (MMA) was obtained from Merck-Schuchardt, Miinchen, :~-MS, St and vinylcarbazole (VC) from Ferak, Berlin. The latter monomer was recrystallized from methanol solution three times and dried under high vacuum (m.p. 64"5°). Mixtures of VC and c~MS were prepared as follows: VC was introduced into a flask which was connected to a second flask (containing c~ MS) via a tube with a side arm leading to a vacuum line. After evacuating the VC containing flask, ~ MS was added. Mixtures of the liquid monomers were prepared similarly. All operations, including the filling of the reaction cells, were carried out under vacuum. Care was taken to exclude moisture.

(b) Characterization of the polymers The polymer generated during the experiment was separated by precipitation with methanol. Number average molecular weights were determined by membrane osmometry in toluene solution at 37 ° using a Mechrolab Highspeed osmometer. Information about molecular weight distributions was obtained with the aid of G P C using a Waters instrument with Styragel columns (solvent: tetrahydrofurane). The composition of the copolymers was determined by spectrophotometry, NMR analysis and elemental analysis. Details are given in Table 1. 3. RESULTS AND DISCUSSION

(a) c~-Methylstyr ene-N-vin ylcarbazole (al) Copolymer composition. VC is heavily favoured in the copolymerization as is shown in Fig. 1, where the copolymer composition is plotted vs the composition of the feed. The VC content increases considerably

1.0 (.3 >

e8

• : positive polarity e : negative polarity

0"-i

i

i

i

I

,

,

,

i

0-5 1.0 Mole fraction of VC in monomer mixture

Fig. 1. Copolymer composition vs composition of monomer mixtures. System: vinylcarbazole-~-methylstyrene.

with the VC fraction in the monomer mixture. Within the error limit, values obtained under FE and FI conditions form the same curve. By the direct extrapolation method 1,7] the reactivity ratios r2 (VC) = 51 + 2 and rl (ct MS) = 0'06 + 0-02 were found. The difference oft, and r 2 by several orders of magnitude is typical for ionic polymerizations. Very similar reactivity ratios for the cationic copolymerization of VC and pmethoxystyrene I-8] and of VC and styrene 1,9] were reported by other authors. When FI conditions are applied, cationic propagation appears to be the only feasible process for the formation of macromolecules. Since the same dependency of polymer composition on monomer composition was found with FI and with FE, it is concluded that field emission also initiates a cationic polymerization. This corroborates the results obtained during the scavenger work 1-6]. (a2) Specific yields. The specific yield of copolymer was significantly decreased by the addition of only small amounts of VC to st MS. By increasing the VC concentration further, the specific yield remained virtually constant over the concentration range studied. The limiting values were 45 kg/Faraday (FI) and 20 kg/ Faraday (FE).

I

Table 1. Determination of the composition of polymers Copolymer CP ~t MS-VC

Monomer determined VC

VC CP-x MS-St CP-:t MS-MMA

x-MS via C H 3 groups x-MS and MMA

Method

Solvent used during analysis

Instrument used for analysis

u.v.-Spectroscopy 2 = 300 nm E,,. = (5.7 _+ 0-4) t 0 3 M-~ cm-~ Elemental analysis* LH-NMR

Benzene

Zeiss PM QII

Carbontetrachloride

GPC ~H-NMR

Tetrahydrofurane D-chloroform

Varian A-60A (60 MHz) GPC Waters Varian A-60 A

* Carried out by Mikroanalytisches Laboratorium Bernhardt.

Copolymerization of unsaturated compounds induced by high electric fields 3.0

According to the usual concept, the formation of macromolecules should proceed via the following propagation reactions: y.MS ~' + :zMS

~",

:~MS---~.MS

~MS + + VC

J":,

: z M ~ - V C '~

VC'~'+ VC

~::,

V('--VC":

VCS + ~MS

a:',

VC--z~MS e'.

115

k~2=01 o

~-~

20

1"0

Provided that the kinetic chains are terminated exclusively by the reaction 0

M ®+ H,O

<"<,

M + H 3 0 ®,

(9)

(which is very probable in our case) and assuming that steady-state kinetics may be applied, Eqn. (10) was derived 2[M2] r2[M2] 2

[M,]

L~_Z (M L + M , ) ]

+

+

r,

-

0-1 0.2 0"3 04 05 Mole fraction of VC in monomer mixture

Fig. 2. Relative specific yield ofcopolymer vs the mole fraction of VC in the VC-:~ MS mixture. Curves were calculated according to Eqn. 110) for (k 1ilk,_2)equal to 0.1, 1.0 and 10.O M~ = e MS. M2 = VC. Experimental points obtained with positively polarized blades (FI).

-

rl[Mi]

obtained under FE a n d FI conditions. The average chain length increases with the fraction of VC in the copolymer. The length of polymer chains formed during the homopolymerization of ~ M S is determined by chain transfer to monomer. These results, therefore, indicate that the rate constant of chain transfer is significantly lower in the case of homopolymerization of VC than for 7. MS.

L [ - ( M ~ + M2)] denotes the n u m b e r of molecules of the m o n o m e r s M~ a n d M_, polymerized per charge unit passing the system. L ( - M l ) o is the respective n u m b e r of molecules converted to polymer during the polymerization of the pure c o m p o u n d M , . Figure 2 shows curves calculated according to Eqn. (10) for k I 1/k22 equal to 01. 1"0 and 10.0 using the r l a n d r 2 values obtained from Fig. 1. One recognizes that the experimental points follow rather closely the trend of the curve for (k 1 l/k,_,_) = 10. A proper curve fitting procedure requires a highly accurate controlling of the c o n t e n t of impurities, especially water. This was not possible with the necessary accuracy. However, provided that other effects like the solvation of poly:~ M S ions by VC molecules need not be considered, it is concluded that k ~ > k22 and therefore: k12 > kit > k22 > k21. (a3) Molecular weight. Table 2 contains data of n u m b e r average molecular weights of copolymers

(b) ~-Methylstj~'ene-Styrene (b l) Copolymer composition. The CP-composition as a function of monomer composition is plotted in Fig. 3. As in the case of VC ~ MS, this curve indicates an ionic mechanism. :~ MS is strongly favoured during this copolymerization. The reactivity values evaluated by the direct extrapolation method [7] are r 1 (St) = 0"1 + 0"1 and r 2 (:{ MS) = 10"4 + 2'0. These values agree fairly well with r I = 0"25 and r, = 8"5 found during the 6°Co- )' initiated copolymerization at 0 which is assumed to proceed via a free cation

Table 2. Number average molecular weight of CP-VC ~, MS generated by FI and Ff5 ~w

K:F

Molefraction VC in CP

Molecular weight of average base unit

Number average molecular weight

t)'O0 0'48 0"70 0"89 0"92 0"00 0"61 0-73 0"88

118 154 171 185 187 118 164 173 185

1"5 x 7'8 x I'1 x 1"4 x 1-6 x 1"5 x 9-4 × 8"7 x 1"3 x

I04 I()4 IOs If)~ 10s 10'* 10"* I0 "t IOs

Degree of polymerization

Polarity of blade

127 506 643 757 856 127 573 503 703

+ + + + +

-

116

W , WABLAT, W . F. SCHMIDT a n d W . SCHNABEL

co

600

1.0

I

soo 0

,.-._~ .~ o8_ o-s

400 300 -6 N 200 .~,

:I 0 2~

F

0

i

I

i

i

I

l

,

I

d 100

t

0.5 1-0 Mole froction of ~oNS in monomer mixture

i

Fig. 3. Copolymer composition vs composition of monomer mixture. System: styrene-ct-methylstyrene. mechanism [10]. Identical results were obtained with FI a n d FE indicating that polymer formation proceeds by the same m e c h a n i s m in b o t h cases. (b2) Specific yields. The addition of small portions of M S to St caused a m a r k e d decrease of the specific yield (see Fig. 4). Based o n Eqn. (10) a n d the values of r I a n d r E given in (bl), it was estimated that k 1~ > k22. This estimate involves the assumption that the impurity (water) content of all m o n o m e r mixtures studied is essentially the same. k ~ a n d k2z denote p r o p a g a t i o n constants as explained in the following scheme:

,

,

,

I

l

i

i

(}5 1.0 Mole froction styrene in monomer mixture Fig. 4. The specific yield ofcopolymer vs the mole fraction of St in St-~ MS mixtures.

St ~ + S t

~'~ . . S t - - S t e

St * + ctMS

k,:, , S t ~ M S e

molecular weight decreases indicating that chain transfer to m o n o m e r is more effective during the polymerization of ~ MS than of St. (b4) Temperature effects. Normally experiments were carried out at - 7 to - 9 ~ and polymer samples exhibited m o n o m o d a l molecular weight distributions. However, samples polymerized at + 2 2 : possessed bimodal distributions. Typical examples are shown in Fig. 5. The occurrence of the high molecular weight peak (at low elution volume) is due to macromolecules formed by a radical mechanism, which does not contribute to the polymerization at - 7 °.

k~,

(c) ~-Methylstyrene methylmethacrylate

:tMS ~ + ~MS ,~MS ~ + St

7M~_~MS~

k:,, , ~MS__St~"

It is concluded that k12 > k~ a > k22 > k2 ~. (b3) Molecular weight. Typical examples of n u m b e r average molecular weights of copolymer samples polymerized by FE a n d F I are given in Table 3. There is no drastic change of the average chain length by varying the composition of the reaction mixture. However, it appears that, with decreasing content of styrene, the

M M A was selected as a c o m o n o m e r for the following reasons: (a) it does not polymerize by a cationic mechanism; (b) it polymerizes readily by a radical mechanism, even between 0 ° a n d - 1 8 : [ 1 1 ] ; (c) it is reported [12] not to polymerize under FI conditions. The latter finding was c o r r o b o r a t e d during this work. Therefore, it was assumed that in this system field ionization should lead to the simultaneous formation of h o m o - P v. M S generated via a cationic mechanism and

Table 3. Number average molecular weights of CP-St-~ MS generated by FI and FE Molefraction ~t MS in CP

Molecular weight of average base unit

0.00 0.61 0"77 0.87 0.95 1.00 0.00 0-73 0-83 0"93 1.00

104 ll3 115 116 117 118 104 114 116 117 118

Number average molecular weight 3"6 × 2.7 × 2"0 × 2"1 x 1.4 x 1"5 × 3"0 × 1.8 × I-6 × 1"4 × I-4 X

104 l0 4

104 10 4

t04 104 ]04 104 104 104 10 4

Degree of polymerization

Polarit\ of blade

346 239 174 181 120 127 288 158 138 120 119

+ + + + + + -

Copolymerization of unsaturated compounds induced b~ high electric fields 1-0

u3

(a)

117

oE c~ o 00-5

100

jSO (b) L ~

_.C 121 1

I

i

i

I

*

i

I

*

0-5 10 Mole froction of c~-NS in monomer mixture

100 150 Elution volume

Fig. 5. Gel chromatograms of samples obtained during the FI induced polymerization of ~ MS and St at various temperatures. Composition of the monomer mixture: 73 mole % z~MS-27 mole !!,~St.

Fig. 7. Composition of copolymer formed during FI initiated polymerization of MMA and :~ MS vs the composition of the monomer mixture.

M S - M M A generated via a radical mechanism. After the initial formation of ~MS-cation radicals, long chains consisting exclusively of ~ MS are formed by the propagation reaction (3) in Chart 1. Transfer to z~ MS according to reaction (4) leads to ~ MS cation molecules whose propagation including several additional transfer steps is responsible for the formation of the homo-P~MS. The formation of the copolymer, on the other hand, is due to macroradicals formed by reaction (4). Since radical chain transfer processes to monomer are not very probable, the copolymer is expected to be of much higher molecular weight than the homo P~MS. The results confirm the expected lines. Gel chromatograms indicated bimodal molecular weight distributions in all cases. Figure 6(a) shows a typical example. The peak at the lower molecular weight(at high elution volume) refers to homo P ~ MS and the peak at higher molecular weight to the copolymer. From G P C and ~H-NMR, the composition of the copolymer was obtained. The CP-composition dependency on monomer mixture composition is shown in Fig. 7. The curve exhibits the behaviour typical for a monomer system with r~ < 1 and r 2 < 1 which has been found for radical copolymerization of many monomer pairs[13]. The reactivity ratios de-

rived from the curve in Fig. 7 are r~ (MMA) = 0-4 + 0-1 and r2 (~ M S ) = 0-27_+ 0"05. Similar reactivity ratios (r 1 = 0'5 and 1"2 0-14) have been reported for the radical copolymerization of these monomers at 6 0 by other authors [14]. Field emission experiments also yielded products with bimodal molecular weight distributions. An example is shown in Fig. 6(b). The low molecular weight peak represents homo Pz~MS and the other peak copolymer. The composition of the copolymer depends on monomer composition in the same way as it does for the case of FI induced polymerization. Since pure M M A polymerizes under FE conditions via a radical mechanism, growing radical chains will be initiated both by M M A - and by x MS-radicals. The fact that homo Pz~ MS is also formed simultaneously in this case again confirms that FE induces the cationic polymerization of ~ MS as shown in Chart 1.

a CP<

Q

Ib)

100 ®

150 (a)

100 150 E{ution volume, m[ Fig. 6. Gel chromatograms of samples obtained by El (a) and FE (b) induced polymerization of M M A and ~ MS. Polymerization temperature: - 8 °.

Composition of the monomer mixtures: (a) 56.9 mole-°~o MMA and 43.1 mole ~o ~ MS: (b) 17.8 mole ° o MMA and 82.2 mole % ~ MS.

:

Acknowledgement--The authors are grateful to Dr. K. Roth. Institut ftir Organische Chemie der Freien Universit~it Berlin, for carrying out the NMR measurements, and to Gillette Roth-Btichner GmbH for providing uncoated razor blades. Partial support by Fonds der Chemischen Industrie is appreciated.

REFERENCES I. K. Hayashi. RadiatioJ1 Induced PolvmerizatiolTs hv Ft'ee Ions, (Edited by M. Hayssinsky) Actioas_ chimiques et hiologiques des radiatiolls. Quinzihme Serie. Masson & Cie, Paris ( 1971 ). 2. T. F. Williams, Radiation Induced Polymerization. In Fundamental Processes in Radiation Chemistr.r (Edited by P. Ausloos). Interscience, New York (19681. 3. M. Lambla, G. Scheibling and A. Banderet. C. r. hehd. Si'anc. Acad. Sci., Paris 271,924 (1970); M. Lambla. R. Koenig and M. Banderet, Europ. Polym. J. 8, 1 (1972L 4. M. Brendl6 and A. M. Ilvoas-Fremond. J. Chim. ph.rs. 69, 1748(1972): M. Brendl6. C. r. hehd. S~;a;w. Acad. Sci.. Paris 272, 743 (1971). 5. W. F. Schmidt and W. Schnabel, Be;'. Btm.sen~tes Phys. Chem. 75, 654 (1971): W. Schnabel and W. F. Schmidt. J. Polym. Sci. Syrup. 42, 273 (1973).

118

W. WABLAT, W. F. SCHMIDTand W. SCHNABEL

6. W. Wablat. W. F. Schmidt and W. Schnabel, Makromol. Chem. 175, 2687 (1974). 7. M. Fineman and S. D. Ross, J. Polym. Sci. 5, 259 (1950). 8. J. M. Barrales-Rienda, G. R. Brown and P. D. Pepper, Polymer 10, 327 (1969). 9. S. Tazuke, K. Nakagawa and S. Okamura, Polym. Lett. 3, 923 (1965). 10. K. Ueno, K. Hayashi and S. Okamura, Polymer 7, 431 (1966).

11. D.S. Ballantine, P. Colombo, A. Glines and B. Manowitz, Chem. Enqng Proqr. 50, 267 (1954). 12. M. Lambla, doctorate thesis, Universit6 Louis Pasteur de Strasbourg, France (1971). 13. Compare e.g.B. Vollmert, Grundril; der Makromolekularen Chemie, p. 78. Springer Verlag, Berlin-GtittingenHeidelberg (1962). 14. C. E. Walling, E. R. Briggs and K. B. Wolfstirn. J. Am. chem, Soc. 70, 1543 (1948).