Radical Polymerization of Vinyl Monomers Initiated by a Mono-Captodatively Substituted Ethane

HITOSHI TANAKA,* HlROAKl KANETAKA, and TAKAHIRO HONCO

Department of Optical Science and Technology, Faculty of Engineering, Tokushima University, 2-1 Minamijosanjima, Tokushima 770, Japan

SYNOPSIS

Homopolymerization of styrene and methyl methacrylate was carried out a t 60-130°C in the presence of a mono-captodatively (cd) substituted ethane bearing nitrile and ethylsul- fenyl substituents on the same carbon atom. It was found that the cd-ethane accelerated both styrene and methyl methacrylate polymerizations with no induction period, but the polymerization mode of methyl methacrylate was different from that of styrene. The po- lymerization rate of styrene was proportional to the 0.46th power of the cd-ethane con- centration. However, the cd-ethane produced a reversible radical termination in the case of methyl methacrylate. The mechanism of both polymerizations is discussed in terms of the kinetic and ESR data. 0 1996 John Wiley & Sons, Inc. Keywords: radical polymerization captodative initiator block copolymerization pri- mary radical termination ESR

INTRODUCTION

It is well known that there are some disadvantages in the conventional azo and peroxy initiators, namely an evolution of nitrogen gas, induced deg- radation, and high sensitivity to metals. To over- come such disadvantages and to introduce new functionality, a number of ethane derivatives, so- called “ethane initiators,” have been investigated as a new type of initiator in free radical polymeriza- tion.’,2 1,2-Disubstituted 1,1,2,2-tetraarylethanes including tetraphenylsuccinonitrile are known to dissociate into diarylmethyl radicals to initiate the radical polymerization of vinyl monomers including styrene and methyl methacrylate (MMA).3

In addition, these ethane initiators, in common with disulfides and dithiocarbamates, have another interesting property. In contrast to conventional azo and peroxy initiators, diarylmethyl and thiyl radicals can also act as terminators and as re initiator^^^^ to give block copolymers.

* To whom all correspondence should be addressed. Journal of Polymer Science: Part A; Polymer Chemistry, Vol. 34,1945-1949 (1996) 0 1996 John Wiley & Sons, Inc. CCC OsS7-624X/9S/l01945-05

On the other hand, it has been known that the odd electron on the carbon atom substituted by both a donor and an acceptor group, so-called captodative (cd) substituted carbon,‘ is stabilized by resonance stabilization to give a persistent radical. For in- stance, the bond energy of the ethane C-C bond where both carbons are substituted by cd groups, for instance, di-cd substituted ethane is anticipated to be about one-half to one-third of that of a con- ventional C-C bond (80 kcal/mol) from kinetic studies using ESR.7-9 Therefore, such ethanes easily dissociate into persistent radicals, which can initiate radical polymerization of vinyl monomers including styrene and MMA.

In particular, cd radicals from cd ethanes are generally less persistent than diarylcyanomethyl radicals from tetraphenylsuccinonitriles. Therefore, cd radicals can even initiate polymerization of un- conjugated vinyl monomers such as vinyl acetate in contrast to diarylcyanomethyl radicals.”

It has been reported that the MMA polymer capped with a diphenylcyanomethyl fragment a t its chain end can reinitiate MMA polymerization more efficiently than the styrene polymer ~ a p p e d . ~ In this report we prepared the mono-cd-substituted com- pound 1, which is a model compound for the cd

1945

1946 TANAKA, KANETAKA, AND HONGO

fragment capped-MMA polymer, and examined the initiation and reinitiation ability of 1 comparing it with that of diarylethanes and the di-cd-substituted ethane 2."

$Xi3 d I 1 c y 3

y H 3 S C Z H 5 F H 3 H3C-C-CH2-C-C-CH2-C-CH3 I I I

H3C-7-CH2-7- F-CH, R C d R

COOCH3 cN COOCH3 c=captive and &dative groups 1 R=CH3, CN, COOCH,, CH2C(CH3),

2

EXPERIMENTAL

1 was prepared by the photoreaction of commercially available dimethyl 2,2'-azobisisobutylate (6.0 g) and a-ethylsulfenyl acrylonitrile ( 2.9 g) , which was synthesized by the same method as previously reported" in benzene (7.0 mL) under a nitrogen atmosphere at ambient temperature. After reaction for 20 h under irradiation with a 100 W high-pres- sure mercury lamp, the benzene was evaporated. Then, 1 was isolated from the residue by column chromatography using a mixed solvent of chloroform (90 vol % ) and ethyl acetate ( 10 vol 96 ) . The R, of 1 was 0.8.

ANAL. Calcd: C, 57.12%; H, 7.99%; N, 4.44%. Found: C,

cm-'. 'H-NMR (60 MHz, CHC13): 1.16 (singlet, 12H) , 1.32 (singlet, 3 H ) , 2.00-2.70 (multiplet, 2H) , 2.70-3.35 (multiplet, 2H ) , 3.63 (singlet, 6H) ppm.

56.82%; H, 7.87%; N, 4.19%. IR: 2232 ( v C N ) , 1732 ( ~ c o )

Commercially available styrene and MMA were pu- rified by fractional distillation just before use.

Polymerization was carried out in a sealed am- poule with shaking at a given temperature. The am- poule which contained the required amounts of re- agents was degassed several times by a freeze-thaw

Table I. at Various Temperatures

Polymerization of Styrene in Bulk with I

[11 Temp. Time Yield (x103 mol/L) ("C) (min) (%)

10 0

10 0

10 0

10 0

130 130 110 110 80 80 60 60

20 20 20 20

180 180 200 200

16.9 7.4

11.2 1.4 8.5 1.3 0.8 0.3

0

Reaction time (min)

Figure 1. Conversion-time curves in bulk polymeriza- tion of styrene with various concentrations of 1 a t 100°C.

1.25 X (X), 6.25 X (+), and 0 (A) mol/L. [11 = 1.00 x (01, 5.00 x 10-3 (n), 2.50 x 10-3 (o),

method and then sealed under vacuum and placed in a constant temperature bath. The resulting poly- mer was isolated by pouring the contents of the am- poule into a large amount of methanol. In the case of oligomerization of MMA using large amounts of 1, n-hexane was used as a precipitant instead of methanol. The ESR was recorded on a JEOL FE- 2XG spectrometer equipped with an X-band micro- wave unit and 100 kHz field modulation. Molecular weights ( M , and M,) of the polymers were deter- mined by SEC using a Toyosoda HLC-8020GPC based on standard polystyrene in dichloromethane at 35°C.

RESULTS A N D DISCUSSION

Polymerization of Styrene

Polymerization of styrene in bulk by 1 at various temperatures is listed in Table I. It can be seen from this table that 1 effectively accelerates styrene po- lymerization especially above 80°C.

Figure 1 shows the conversion-time curves for styrene polymerization a t 100°C. The polymeriza- tion was found to proceed linearly as a functional of reaction time without an induction period as has been reported for di-cd substituted ethane ( c = CN, d = SCzH5, R = CN in 2) .lo The molecular weight of the polymer obtained did not vary with the con- version, namely M , = 1.32 X lo5, 1.35 X lo5, and 1.38 X lo5 in 3.0, 5.5, and 8.3% conversions, re-

RADICAL POLYMERIZATION OF VINYL MONOMERS 1947

4.2’ 1 I I I I I I J

-3.2 -2.6 -2.0

log[l] (mol/L)

Figure 2. styrene on concentration of 1 a t 100°C.

Dependence of polymerization rate (R,) of

spectively, in the polymerization of [ 1 ] = 0.01 mol/ L. The relationship between the overall rate (R,) of styrene polymerization and cd-ethane concentra- tion is shown in Figure 2, in which Rp is calculated according to the equation of Rg = RZ-obs - R$ and Rp.obs and Rpo are the observed and blank polymer- ization rates, respectively. It is clear from this figure that the polymerization rate increases with increas- ing concentration of 1 and is proportional to the 0.46th power of the cd-ethane concentration, indi- cating very small or negligible primary radical ter- mination under such polymerization conditions.

To estimate an activation energy for the decom- position of 1, the overall rate of styrene polymer- ization was plotted against the absolute temperature according to the Arrhenius equation as indicated in Figure 3. From the slope of this line, an overall ac- tivation energy (E,) for styrene polymerization with 1 was estimated to be 21.0 kcal/mol within the tem- perature range 80-1 10°C. Therefore, according to the equation of E, = E d / 2 + ( E , - E , / 2 ) , where E d , Ep, and E, represent the energies of activation for initiator decomposition, propagation, and ter- mination, respectively, the Ed value was apparently calculated to be 31.2 kcal/mol assuming ( Ep - E,/ 2) = 5.4 kcal/mol.” The corresponding Ed value for the di-cd substituted ethane ( c = CN, d = SC2H5, R = CN in 2) was calculated to be 25.0 kcal/mol within the temperature range of 50-80°C on the ba- sis of the data previously reported.“ The E d value for the mono-cd substituted ethane is higher than that of the di-cd substituted ethane and also higher than that of tetraphenylsuccinonitrile or tetra-p - chlorophenylsuccinonitrile ( 22..!j4 or 19.013 kcal/mol

I I I I 2.6 2.7 2.8

10”rr (K-’)

Figure 3. styrene on temperature. [ 11 = 5.00 X

Dependence of polymerization rate (R,) of mol/L.

for the former and 15.64 kcal/mol for the latter). This is presumably due to a less weakened C-C bond in the case of the mono-cd substituted ini- tiator 1.

Polymerization of Methyl Methacrylate

MMA polymerization in bulk with 1 at various tem- peratures is listed in Table 11. It can be seen from this table that 1 also accelerates MMA polymeriza- tion.

As observed in styrene polymerization, MMA po- lymerization also proceeded linearly as a function of reaction time without an induction period. The Rp of MMA polymerization, however, did not in- crease linearly with increasing cd-ethane concen- tration in the cd-ethane concentration range 6.25 X 10-4-l.00 X lo-* mol/L. The maximum for Rp was near [ 11 = 2.5 X mol/L, as can be seen in

Table 11. in Bulk with 1 a t Various Temperatures

Polymerization of Methyl Methacrylate

111 Temp. Time Yield (x103 mol/L) (“C) (min) (7%)

10 0

10 0

10 0

10 0

130 130 110 110 80 80 60 60

20 20 20 20

180 180 200 200

13.3 1.3 6.4 0.8 4.8 0.9 1.8 0.9

1948 TANAKA, KANETAKA, AND HONGO

-3.4 I

-4.6 I I I I

-3.5 -2.5 -1.5 -0.5

log[l] (mol/L)

Figure 4. methyl methacrylate on concentration of 1 at 100°C.

Dependence of polymerization rate (R,) of

Figure 4. In addition, the molecular weight of MMA polymer obtained varies and increased with conver- sion, as seen in Figure 5, although M,/M,, is around 2.0. This suggests that 1 terminates MMA poly- merization reversibly.

To confirm the reversibility of the termination step, polymerization of styrene using MMA oligomer ( M , = 2.9 X lo3) prepared using 1 was carried out at 110°C. In the polymerization of styrene ( 2 mL) with MMA oligomer 33.5 mg, polymer was obtained in a 29.7% yield after a reaction time of 3 h, which was higher than the yield (13.5%) for the sponta- neously-obtained polymer. Such a rate acceleration, therefore, indicates homolysis of the oligomer, probably a t the cd fragment-attached chain end. In support of this hypothesis, the cd radical was de- tected during a thermolysis of the oligomer by means of ESR spectroscopy, as seen in Figure 6. The com- puter-simulated spectrum with the values of aN (CN)

G (g = 2.0052) agrees very well with the observed spectrum.

- - 3.1, as.H(C-CH2) = 11.0,anda,.H(SCH2) = 2.5

Mechanism

I t has been reported that a large excess of radicals is produced a t one time by the dissociation of ary- lethanes, and in particular in MMA polymerization, it is accompanied by an induction period and a step- wise polymerization that follows a preferential pri- mary radical termination and re in i t ia t i~n .~ However, disulfides as well as cd ethanes have been known to participate in initiation, reinitiation, and termina- tion, and do not show an induction period in vinyl

3

2

z s X I

1

0 I I I I

10 20

Conversion (%)

Figure 5. Dependence of molecular weight on conver- sion for the polymerizations of (A) styrene and (0) methyl methacrylate at 100°C. [ 11 = 1.00 X lo-' and 2.50 X mol/L for styrene and methyl methacrylate polymeriza- tions.

p~lymer iza t ion .~~ '~ The higher E, value of the cd ethanes may result in slower decomposition and, therefore, slower initiation and reinitiation, resulting in no apparent induction period.

From the results obtained, the mechanism of sty- rene and MMA polymerizations is suggested in Scheme 1. In initiation [eq. ( l ) ] , the cd radical is considered to initiate both polymerizations consis- tent with that observed with di-cd substituted ethanes ( c = CN, d = SCzHs, R = CN in 2) .lo In

1 OG -

Figure 6. ESR spectra (a) observed during the thermal reaction of methyl methacrylate oligomer in toluene at 110°C and (b) simulated.

RADICAL POLYMERIZATION OF VINYL MONOMERS 1949

- less termination

Scheme 1.

termination [eqs. ( 2 ) and ( 3 ) ] , however, the cd radical is considered to be a nucleophilic species be- cause the electronic factor ( e ) in the Alfrey-Price equation is known to be -1.9 for cu-ethylsulfenyla- crylonitrile." Hence, electronically the cd radical should combine preferentially with electron-accept- ing radicals such as poly (MMA) radical [ eq. ( 3 ) ] , rather than electron-donating radicals such as the polystyryl radical [ eq. ( 2 ) ] .

This research was supported in part by Nagase Science and Technology Foundation, to which the authors are grateful.

REFERENCES AND NOTES

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Received July 19, 1995 Accepted December 18, 1995