Free radical polymerization of unconjugated dienes VI—Vinyl-trans-crotonate in benzene at 60°C

Free Radical Polymerization of Unconjugated Dienes V! Vinyl-transcrotonate in Benzene at 60°C LUIGI TROSSARELLI and MARINO GUAITA With the aim of checking the hypothesis that, in the free radical cyclopolymerization o] unconjugated dienes, inter- and intra-molecular chain propagation reactions have to be slow to allow the formation o] large fractions of cyclic structural units in the resulting polymers, the ]tee radical polymerization at 60°C o] vinyl-trans-crotonate in benzene solutions has been investigated. The experimental results agree with the above hypothesis and show that the crotonic and vinyl double bonds of the monomer are chiefly involved in the intermolecular and intramolecular chain propagation steps, respectively.

IN A previous paper 1, on the basis of what has been reported in the literature about the free radical cyclopolymerization of .symmetrical unconjugated dienes ~ and our own results on the free radical polymerization of some unsymmetrical unconjugated dienes 1, 3. ~, we were led to suggest that, since the ratio between cyclic and linear structural units depends on the competition between the intramolecular and intermolecular chain propagation steps, these reactions have to be slow enough to allow suitable orientation of the reacting groups in order to have the formation of a large fraction of cyclic structural units. This might occur when rather stable radicals and double bonds are involved in the process. With the aim of checking the validity of such a hypothesis, we thought it of interest to investigate the free radical polymerization of some unsymmetrical unconjugated dienes in which one of the two double bonds gives rise to a highly reactive radical and the other is so unreactive that it does not homopolymerize but only copolymerizes when attacked by suitably reactive radicals. In this paper are presented and discussed the results obtained from a kinetic investigation on the free radical polymerization at 60°C in benzene solutions of vinyl-trans-crotonate which, as already reported by Arbuzova and Rostovskii ~, undergoes cyclopolymerization. EXPERIMENTAL

Monomer Vinyl-trans-crotonate was prepared according to Swern and Jordan 6 by transvinylation from trans-crotonic acid and vinylacetate. Its purity, as checked by gas chromatography, was found to be higher than 99"9 per cent. Polymers

Polymerizations were carried out at 60°C in benzene solutions of different concentration, using ~,oY-azobisisobutyronitrile (3 × 10 -3 mole/l.) as a free radical initiator. Ampoules sealed under high vacuum were employed. The conversions to polymer were kept as low as convenient and did not exceed five per cent. 233

L U I G I TROSSARELLI and M A R I N O G U A I T A

The polymers were purified from monomer and initiator by several precipitations in ethyl ether from benzene solutions. They were easily soluble in benzene, acetone, chloroform and bromoform and insoluble in aliphatic hydrocarbons and in ethyl ether. The softening point was found near 250°C. In Figure 1 the infra-red spectrum (film from benzene) of a typical poly(vinyl-trans-crotonate) is reported.

Analysis of the polymers The mole fraction of crotonic (fo0 and of vinyl (/v) residual urtsaturations in the polymers from vinyl-trans-crotonate were determined by infra-red spectroscopy (Beckman IR 9) in three per cent CHBr.~ solutions. The C - - H out-of-plane bending vibrations at 970 cm -1 and at 950 cm -x were taken respectively as analytical bands for crotonic and vinyl residual unsaturations. The calibration plots were obtained from ethyl-trans-crotonate and vinylacetate mixtures in CHBr3, prepared in such a way as to give, in the range 900 to 1 000 cm -1, bands of shape and absorbance similar to those of the polymers under inv6stigation. The Lambert-Beer law was observed to be obeyed in the concentration range of interest. RESULTS

In Table 1 are collected, as obtained from the analysis of the polymers from the free radical polymerization at 60°C of vinyl-trans-crotonate in Table 1.

Mole fractions of structural units in the products of the free radical polymerization at 60°C of vinyl-trans-crotonate in benzene solutions. Initiator: AIBN (3 x 10-2 mole/L)

Monomer concentration (mole/L)

fv

fer

lc

4'22 2"41 2'11 1"40 0'84 0-45 0"40 0'17

0"26 0"19 0"17 0" 13 0'08 0"05 0"04 0"02

0"27 0"22 0"21 0"20 0"17 0' 14 0"14 0-11

0.47 0"59 0"62 0"67 0"75 0"81 0'82 0"87

benzene solutions, the mole fractions of residual vinyl unsaturations (Iv) and the mole fractions of crotonic residual double bonds (fc0. The presence in the polymers of both crotonic and vinyl residual unsaturations leads to consideration of the following two linear structural units:

I

t

CH--O--CO--CH--CH--CH3

CH---CO--O--CH--CH2

I

I

CH2

f

CH--CH3

J

(I)

(II)

The fact that, for all the polymers investigated, Iv + [or < 1, indicates, in agreement with Arbuzova and Rostovskiis, the presence of lactone-type cyclic structural units. The infra-red spectra of the polymers (see Figure 1), 234

FREE RADICAL POLYMERIZATION OF UNCONJUGATED DIENES VI I

I

L

I

1

1800

1600

I

I

I

I

1400

I

I

1200

I

I

1000

cm-1

Figure /--Infra-red spectrum of a typical product of the free radical polymerization at 60°C of vinyl-trans-crotonate in benzene solutions

in the region of carbonyl stretching, shows, besides a n absorption band at 1 725 cm -1 and a shoulder at about 1 780 cm -1, due to linear structural units I and II respectively, an absorption at 1 750 cm -1 most probably to be attributed to the carbonyl stretching of y-lactone-type structural units, account being taken that the overlapping of the absorption of the c~,flunsaturated ester carbonyl stretching (1 725 cm -1) might result in an apparent shift toward lower frequencies of the transmission minimum of the band of the y-lactone carbonyl stretching. On the other hand, it must be considered that, in the free radical polymerization of vinyl-trans-crotonate, the most stable radicals which can be formed, namely radicals III and IV, ~'~

CH(CH3)--(~H--CO--O--CH~CH2 (III)

CH~(}H--O~CO--CH~CH(CH3) (iv)

should give rise, through the intramolecular chain propagation (cyclization) steps, exclusively to y-lactone structural units V and VI : CH3

I

CH3 I

H2

,,c..

--CH--CH

CH--

..c~

--H2C--CH

CH-/

o~-6

o-co

(v)

(vl)

The overall mole fractions [c = 1 - (Iv + fc~) of cyclic structural units in the polymers from vinyl-trans-crotonate are collected in Table 1. 235

LUIGI TROSSARELLI and MARINO GUAITA On the basis of what has already been shown 3' 7, the mole fractions of structural units are related to the monomer concentration [M] by fv +/c, Jo -

1 1 +!v/Jfc,. r~v. l+jv./fo,.. [M]

(1)

and V

rev.

1o'

Jr.

roe,.. It,..

(2)

where for, fv, ]v. and for, are respectively the mole fractions of structural units I, II, V and VI, and roy. and r~or. are the cyclization ratios 3, 7 for radicals IV and III respectively. From equations (1) and (2) it is easy to derive (re~fOr) [M] =r~v: + r~,.. (Iv~for) (3) °

6 5

'--'3 2

1 '

o'.2

'

o14

'

o'.6

'

0'.8

'

'

fvlfcr Figure 2--Plot of the experimental data of Table 1 according to equation (3)

In Figure 2 the experimental data of Table 1 are plotted according to equation (3) a n d , by the method of the least squares, one obtains roy.=0.00.., mole/l, and r~or.=7"57 mole/1. The value r,v.=0-00... obviously means that, within the limits of the experimental errors, in the free radical polymerization products of vinyl-trans-crotonate, cyclic structural units VI are absent, and fc may be identified with fv.. DISCUSSION

The values of the cyclization ratios show the strong tendency of radicals III to undergo cyclization according to reaction (4) and the extremely low ability of radicals IV to give cyclic structural units through reaction (5). This agrees with our previous hypothesis1 that cyclization takes place only 236

FREE RADICAL POLYMERIZATION OF UNCONJUGATED DIENES VI

-- CH(CH3)--CH. f CO

kc~v-

I 0

CH3 "

I CH

I

H2

..C..

--C--CH CH" H O~C_OI

(~)

II

CH2

when the rates of the chain propagation reactions involved in the process are low enough to allow proper orientation of the reacting groups for ring ~CHz--CH. I

CH3 I

o

I

%Cr*

co

I

CH II

.CH

--CH2--CH ICHo (5) O-CO

CH(CH a)

closure. In fact, radicals III can propagate the chain either through reaction (4) or through reactions (6) and (7). Reactions (4), (6) and (7), which involve the rather stable radicals IlI and two double bonds of low reacti"~" CH(CH3)---CH" CH~-~CH

I

CO 0

~ " CHc-CH"

I

1

O +

I

O

CO

~

CO

I

CH

CH

il

li

CH~

(6)

I

CH

il

CH(CHa)

CH(CHs)

CH(CH3)---CH. CH(CH3)==CH

~ " CH(CH3)--CH.

I

I

CO

i

O

+

I

CH

I

CO

I

O

CO

kor~,.

!

--~-

O

I

(7)

I

CH

CH

CH2

CH2

vity, should be slow. On the other hand, the highly unstable radicals IV should be so quickly consumed through reactions (8) and (9) that the " ~ CH2--CH"

CH~CH

I

O

I

CO

~

I

i

O +

1

1

CO - CH

LI

H

CH (CH3)

O

kvv

I

CH

CH (CHD 237

CH2--CH"

.>

I

CO

(8)

I

CH

il

CH (CH3)

LUIGI TROSSARELLI and MARINO GUAITA " ~ CH2--CH-

CH ( C H 3 ~ H

f

~

CH2(CH~)--CH.

l

O

I

CO

I

CO

CO

J

+

kv~

0

l

I

>

0

I

CH

CH

II

II

CH (CH~)

(9)

t

CH

II

CH2

CH~

pendant crotonic double bondscannot assume the proper orientation for cyclization. According to our previous treatment of cyclopolymerization~' 7 the mole fractions [v and fo, of linear structural units II and I respectively are related to the monomer concentration by the relationship where

iv/lo,.=a {[M] + fl} / { [ M ] + "),}

(lOa)

1 + kcro,./korv oz= 1 +kvv/kvor

(lOb)

kvo~.lkvcr

fl = 1 4- kor,,/kor,or

(|Oc)

kc~v,l ko~v T = 1 + kv.o~/kv,v

(lOd)

and kv,v, kv,c,., kcr,v and kcr,o, are respectively the rate constants of the intermolecular chain propagation reactions involving cycloradicals and monomer:

H2C=~;H

CH3 H_2 I ..(-:--. .,~CH--CH "CH"

+

o-d

O I

,-,,, c H~---(~H" k~,v

CO

CO I CH ~H(CH~)

I CH

IclH(CH3)

~H3

~z ./t,\ •v~CH--CR ~:H" ~O-O

CH(CHs)=~H +

CH~

,,v,,H~--qH 'FH"

b-do

CO I

o I CH ~H2

O l

"'" CH(CH3)-~H" kv*cr--

CO I

o I CH II CH2

H2C=CH .,v~,H~C--CH, O O CO CO I I CH CH It II CH{CH:~ CH (CH::~ 238

(11 )

(12)

03)

FREE RADICAL POLYMERIZATION OF UNCONJUGATED DIENES VI

CH 3

CHICH3)=~H

L

CO

/CH ,.,-v,,,H2C - - CH "CH.

I O

+

\o&

,/v'~CH(CH3}-CH" CO I O

&:~,c,-_

C, H II

--

(1 &)

C, H II

CH2

CH2

Since r,v.=kvcr,/(kvv+kvcr), it is easy to see that it must be /3 ~ r~v. (1 + kvv/kvc,.). According to copolymerization data of crotonic derivatives and vinylacetates' 9, kvv/kvc,, can be reasonably assumed less than unity and then the value of fl is to be taken close to zero. As a consequence, equation (10a) may be rewritten to give

1~,Iv i

l + y l a [M]

(15)

c~ i

i

[

i

i

5

I

~4 ~3 2 1

0

I

I

1

2

I

I

3 4 [M] -'l (I/mole)

t

I

5

Figure 3--Plot of the experimental data of .Table 1 according to equation (15) In Figure 3 the data of Table 1 are plotted according to equation (15). The experimental points fit well on a straight line and, by the least squares method, one obtains T=0"80 mole/l, and or= 1-00. This value of a, according to our previous treatment of cyclopolymerization3' 7, might well be compared with the value 0.76 calculated from equation (10b), taking kvv/kvcr=0"33 and kc,.cr/kcrv=O'O1 as though from the free radical copolymerization at 68°C of vinylacetate and trans-crotonic acid s, account being taken of the difference in the temperatures of polymerizatioh and of the role of the substituents in determining the reactivity of the monomers and of the radicals involved. According to what has been shown already ~,7, the ratio ([v+i~,,.)/f~ between the overall mole fractions of linear and cyclic structural units is related to the monomer concentration by Iv +/c,. ]c

a[m]2+b [M] [M] + c 239

(16a)

LUIGI TROSSARELLI and MARINO GUAITA where a=

kvvk~rv + kcrerkver + 2kvorkerv kvcr,kcrv + kcrv,kvc r

(16b)

kvv + kvc~ kcrv + kc~cr kc~v*kv*v kv,v+ kv,cr + kvcr,kc~,cr kcr,v + kc~, o b=

(16c) kvcr,kcr v + kcrv,kvcr

kv.v

k~.o~

kvcr'kc~v"(kv.v+kv.c, + kc.v+k------c,.c, ) c=

(16d) kvc~,kc~v + kc~v,kv~

The constants a, b and c in equation (16a) are related, as previously shown3, to the cyclization ratios and to the constants o~,/3 and y in equation (10a) by: Ot (/3 -- C)/ (C -- "y) ----rev./rcor"

(17a)

b / a = (or~3+ y)/(ot + 1)

(1To)

In the present case, since both/3 and r~v. are in practice close to zero, it is easy to see from equation (17a) that c must also be close to zero, and then equation (16a) can be rewritten to give (]v + for)/]o = a [M] + b

(18)

Neglecting in equation (17b) or/3 with respect to y, and introducing the numerical values of ot and y, one obtains b / a = 0 . 4 0 . From the experimental values of [M], Iv+let and fo one calculates a=0.25 L/mole and b=0.!0. Figure 4 shows that the agreement between calculated and experimental data is quite satisfactory. 1"4 1.2 1"0

,,u

,.,3

0,8

+>0.6 0"4 0"2

o

i

[,'w] (mote/t.)

Figure 4--Plot of the experimental data of T a b l e 1 according to equation (18). Solid line as calculated taking b / a = 0 . 4 0

The above discussion of the experimental results has been carried out simply by considering that r,v. =0.00 . . means kvc~, ,~ kvv + kvce and 240

FREE RADICAL POLYMERIZATION OF UNCONJUGATED DIENES VI

without any assumption about the value of the individual rate constants. Since, by definition3' 7, roy represents the monomer concentration at which the ratio between the mole fractions, that is between the rates of formation, of structural units VI and I is unity, it is easy to see that, in the range of monomer concentrations usually employed, 'the rate of reaction (5) must be negligible with respect to the rates of reactions (8) and (9). Obviously, the experimental data of Table 1 could also have been interpreted by taking kvcr,=0 and therefore f l = c = 0 , by which equations (10a) and (16a) can be reduced to equations (15) and (18) respectively. The former interpretation seems, however, more correct because it does not imply taking as zero the rate constant of reaction (5), which would result in a y-lactone ring as well as reaction (4). On the other hand, such an assumption would be inconsistent with copolymerization data 8, 9, which show that vinyl radicals easily attack the crotonic double bonds. The above analysis of the experimental results indicates that, in the free radical polymerization at 60°C in benzene solutions of vinyl-transcrotonate, the vinyl double bonds are chiefly involved in the intramolecular propagation of the chain and the crotonic double bonds are predominantly involved in the intermolecular chain propagation reactions. This makes the mechanism of polymerization of vinyl-trans-crotonate completely different from the one considered by other authors 1°, 11 for the free radical polymerization of vinyl-trans-cinnamate. According to the latter mechanism, vinyl double bonds would be responsible for the intermolecular chain propagation, in contrast with copolymerization data 1°, 11, whereas the cinnamic ones would only take part in the cyclization reactions. It is to be noted that the mechanism presented here is to be regarded as more reliable since it is derived from the application to the experimental data of general equations relating cyclopolymer composition to monomer concentration, taking into account all the reactions involved in the process, and it results in fairly good agreement with what is expected from copolymerization data.

The authors thank Prof. G. Saini [or many help[ul suggestions and valuable discussions. Istituto di Chimica Analitica dell' Universith e Centro Nazionale di Chimica delle Macromolecole del C.N.R., Sezione H, Turin, ltaly (Received May 1967) REFERENCES 1TROSSARELLI,L., GUAITA,M. and PRIOLA,A. Ric. Sci. 1966, 36, 993 2 MERCIER,J. and SMEarS,G. J. Polym. Sci. A, 1963, 1, 1491 SMETS, G., Hous, P. and DEVAL,N. J. Polym. Sci. A, 1964, 2, 4825 GISBS, W. E. J. Polym. Sci. A, 1964, 2, 4815 SIMPSON, W. and HOLT,T. Proc. Roy. Soc. A, 1956, 238, 154 3TROSSARELLI, L., GUAITA,M. and PRIOLA, A. International Symposium on Macromolecular Chemistry (Prague) 1965, preprint P.442; I. Polym. Sci. In press t TROSSARELLI,L., GUAITA,M. and PRIOLA,A. Ric. Sci. 1965, 35(ii-A), 429 TROSSARELLI,L., GUAITA,M. and PRIOLA,A. Annal. Chim. (Roma), 1966, 56, 1065 TROSSARELLI,L., GUAIT^,M. and PRIOL^, A. Makromol. Chem. 1967, 100, 147

241

LUIGI TROSSARELLI and M A R I N O G U A I T A 5 ARBUZOVA,L A. and ROSTOySKII, E. N. J. Polym. Sci. 1961, 52, 325 6 SWERN, D. and JORDAN,E, F. Org. Synth. 1950, 30, 106 7 TROSSARELLI,L., GUAITA,M. and PRIOLA, A. Ric. Sci. 1965, 35(II-A), 379 8 CHAPIN, E. C., HAM, G. 'E. and MILLS, C. L. J. Polym. Sci. 1949, 4, 597 9 USrtAKOV, S. N. and TRUKHMANOVA,L. B. Vysokomol. Soedineniya, 1959, 1, 1754 10 VAN PAESSCHEN,G., JANSSEN, R. and HART, R. Makromol. Chem. 1960, 37, 46 i1 ROOVERS, J. and SMETS, G. Makromol. Chem. 1963, 60, 89

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