POLYMER LETTERS EDITION VOL. 14, PP. 73-77 (1976)

MORE ON THE DECOMPOSITION OF AROMATIC POLYRADICAL ANIONS AND THEIR MONOMERS,

UTILIZING ELECTRON PARAMAGNETIC RESONANCE

Styryl-type polymers and monomers have for some time been known to produce polyradical anions and radical anions, albeit occasionally with dif- ficulty (1-4). All of these radical anions were observed to decompose after a time (msec to days), and all decompositions appeared to be temperature dependent. However, no effort was made to determine the ultimate reduc- tion products. Rembaum et al. (2) determined that an initial decomposition product of polyvinyl aromatics was probably a chain-ruptured anion.

We have investigated the aspect of the decomposition products of several polyradical anions and the corresponding monomers by means of EPR (elec- tron paramagnetic resonance) spectroscopy. In our work the reduction was accomplished by means of potassium in dimethoxyethane (DME), initially at ambient temperatures, and if no signal resulted, at lowered or elevated tem- perature.

The samples, about 0.5 mg in 3 ml DME, were allowed to remain in con- tact with the metal as a continuing reducing source for as long as a week in some cases (Table I). The surprising result was that, in nearly all cases, whether polymer or monomer, the ultimate reduction product was a recog- nizable EPR signal of the parent hydrocarbon ring system.

Owing to the presence of some side products (dimers, etc.), some as yet unsevered polymer, and some diamagnetic anions (generally giving the signal a characteristically wide sinusoidal hump), the major splittings varied some- what (Table I). Nevertheless, we were able to pick out the two major split- tings, rather easily in most cases. Although there was some variation in the observed splittings of the products (+ 0.2 gauss for biphenyl and f 0.5 gauss for naphthalene) from the interferences noted above, these splittings remained well within acceptable limits.

To ensure that the splittings we noted were not merely fortuitously similar to those of the initial radical anion, we made HMO and McLachlan calcula- tions for the monomer and polymer molecules (5); however, in no case did the theoretical values correspond to the experimental values.

(relative time in reaching maximum EPR signal), and the relative yield of radical (relative intensity of EPR signal) (see Table 11).

Since much recent work with radical anions has involved substitution of alkyl groups into the aromatic ring system and elimination of heteroatomic groups from it, we find elimination of alkyl groups an interesting difference.

In light of the common product (the parent hydrocarbon ring system) ob- served, we suggest that similar mechanisms must be operating in both mon-

One other observation was made-that of ease of formation of the radicals

73

0 1976 by John Wiley & Sons, Inc.

74 POLYMER LETTERS EDITION

TABLE I

EPR Characteristics of the Ultimate Reduction Products of Vinyl Aromatics

~

Splittings Species a, (GI a, (GI T(”C) Color

biphenyl 5.40 2.70 25 blue-green 4-vinylbiphenyl (VB) 5.4 2.5 25 red polyf4-~inylbiphenyl)(PVB) 5.6 2.8 -80 It green

5.6 2.8 25 red naphthalene 4.90 1.83 25 red 1-vinylnaphthalene (1-VN) 4.9 1.8 25 tan polyfl-vinylnaphthalene)@-1-VN) 5.2 1.7 25 brown 2-vinylnaphthalene (2-VN) 5.5 1.4 25 brown polyf2-vinylnaphthalene)(P-2-VN) 5.4 1.6 25 yetlow vinylmesitylene (VM) a - 25 violet

poly(vinylmesitylene)@VM) b - -8O,25,4Oc It. blue vinyldurene (VD) 1.6d - 25 yellow poly(vinyldurene)(PVD) b - -8O,25,4Oc It. blue

(after 1 wk)

aSignal/noise too low to determine hyperfine lines. bA single, sharp exchange-narrowed line was observed as with polystyrene

(4). Since no hyperfine line was seen, the initially reduced species is very probably solvated electrons stabilized by the polymer or an exchange-narrowed “living polymer,” but not one of the PVB or PVN types above or that de- scribed by Shimura et al. (10).

CLithium was also used as metal reductor. daCH, = 11 G for mesitylene’+(G. Vincow, in “Radical Ions,” E. T. Kaiser

and L. Kevan, Eds., Interscience, New York, 1968, p. 198).

TABLE I1

Relative Ease. of Formation of Radicals VN,VB > VD > VM PVN,PVB > PVDPVM

Relative Yield of Radical PVB, P-1-VN, P-2-VN > PVD,PVM

(abbreviations from Table I)

omers and polymers to cleave off the vinyl groups. Considering first the mon- omers, it is likely that the first steps involve a polymerization [eq. (l)] (6 ) , followed by cleavage.

HC=CHz HC-CH2 -CH-CH2 -CH-CH2- further re- I & I % I I - duction to

Ar At Ar Ar polyradical (1) anion in

M(= monomer) eq. (6)

POLYMER LETTERS EDITION 75

Alternatively, there are possibilities of solvent or radical anion involvement [eqs. (2-5)] leading to a saturated species not unlike the polymer. Justifica- tion of the species MH- or M H is provided by the high electron density on the @-carbon of the radical anion (3, 5) and the high resonance stabilization of either species (1).

+ SH - HC-CH3 t S- I (2) Ar

Ma- MH. SHE solvent

MH- + SH - MH2 + S-

ol

MH. + SH - MH2 + S*

(4)

(5)

At this point we may consider the reduced polymer (polyradical anion), the reduced polymerized monomer (polyradical anion from eq. (l)), or (in the alternative case) the H- and e- reduced monomer (MH2 --, from eqs. (4) or (3, and later where the squiggle - cleavage sequences.

marily the Rembaum mechanism (1) [eq. (6)].

H) as undergoing essentially the same

For rupture of the Ar moiety from the alkyl portion we may invoke pri-

H H H H I I I I

I I I I Ar' Ar-- Ar Ar

-C-CH2 -C-CH2- - -C- + -CH2 -C-CH2- -

The products of eq. (6 ) are highly reactive species which may be expected either to pick up a proton from the solvent (as in eq. (4)) and become reduced, eventually losing the Are radical [eq. (7)] :

H H H H I I I I

T H 2 -C- % CH3 -C- A CH-C- - CH3 -c- + Are I I tl Ar Ar Ar -

(7)

or to immediately expel A f , which reduces another Ar moiety [eq. (S)]

76 POLYMER LETTERS EDITION

H

An immediate elimination, eq. (9), similar to that of (ArX)*- [eq. (6)], would seem td be ruled out by the Rembaum observation of conjugated anions [eq. (6)].

H H I I

<-CH2 - --+ <--CH2 - + Arm 11

Ar--

(9)

The ultimate product, then, is formed through the well known route [eq. (1011:

- Ar. t SH - ArH (ArH)-- (10)

The inclusion of vinyldurene and vinylmesitylene in this series was prompt- ed by our curiosity as to the effect of added methyl groups. Reduction of both monomers was accomplished with somewhat more difficulty than with styrene, probably owing to the electron repulsion of the methyl groups and consequent lowering of the total electron affinity of the molecule (8). More- over, this difficulty of reduction (Table 11) follows observed reductive dif- ficulties in poly(methy1 aniso1e)s (9).

The effect of the methyl groups in lowering electron affinity in the car- responding poly(viny1durene) and poly(vinylmesity1ene) was so great as to afford them a reductive resistance too great to be overcome, even at lowered or elevated temperatures with either potassium or lithium (Tables I and 11).

References

(1) A. Rembaum and J. Moacanin, J. Polym. Sci. B, 1, 41 (1963). (2) A. Rembaum, J. Moacanin, and E. Cuddlhy, J. Polym. Sci. C, 4, 529

(1964). (3) A. R. Buick, T. J. Kemp, and T. J. Stone, J. Phys. Chem., 7_4, 3439

(1970). (4) D. H. Eargle, Jr., J . Amer. Chem. SOC., @, 2567 (1964). (5) Programs MACHK, by R. S . Ernst, and SPECTRO-RUNID (BZ0652),

University of Wisconsin, Madison, Wis.; J . D. Roberts, “Calculos con Orbitales Moleculares,” Editorial Revert$, Barcelona, 1969, chap. 8.

POLYMER LETTERS EDITION I7

(6) A. R. Forrester, J. M. Hay, and R. H. Thomson, “Organic Chemistry

(7) J. F. Garst, in “Free Radicals,” J. Kochi, Ed., Vol. I, John Wiley &

(8) A. Streitweiser, “Molecular Orbital Theory,” John Wiley & Sons, New

(9) K. W. Bowers, in “Radical Ions,” E. T. Kaiser and L. Kevan, Eds.,

(10) T. Shimimura, K. J. Tolle, J . Smid, and M. Swarc, J. Amer. Chem.

of Free Radicals,” Academic Press, New York, 1968, p. 101.

Sons, New York, 1973, chap. 9.

York, 1961, p, 182.

Interscience, New York, 1968, p. 198.

SOC., 89, 796 (1967).

Jairo Marquez James Giulianelli

Dept. de Quimica Universidad de Los Andes MCrida, Venezuela

Dolan H. Eargle, Jr.*

Dept. of Pharmaceutical Chemistry University of California San Francisco, California 94143

Received July 15, 1975 Revised September 5, 1975

*To whom correspondence should be addressed.