Dehydrochlorination of polymers—IV. Polyvinylidene chloride copolymers

European Polymer Journal, 1968, Vol. 4, pp. 83--92.

Pergamon Press.

Printed in England.

DEHYDROCHLORINATION OF POLYMERSqIV. POLYVINYLIDENE CHLORIDE COPOLYMERS G. M . B U R ~ F r a n d R. A. HALDON* Chemistry Department, The University, Aberdeen, Scotland and J. N. HAY Chemistry Department, The University, Birmingham, 15 (Received 20 June 1967) Abstraet--A study of the effect of different comonomers on the dehydrochlorination reaction of polyvinylidenechloride is reported. In general, all the copolymers were very much less stable thermally than the homopolymers and the copolymers showed anomolous adjacent group interaction. Methyl methacrylate copolymers eliminated methyl chloride as well as hydrogen chloride; y-unsaturated y-lactone groups were produced in the solid residue. Methacrylonitrile copolymers exhibited no neighbouring group interactions but there was an increase in rate of elimination due to increased chain mobility. Styrene units clearly catalysed the elimination reaction; there was evidence for adjacent group interaction.

INTRODUCTION P ~ w o u s study o f the d e g r a d a t i o n o f c o p o l y m e r s (1) suggested that, when d e g r a d a t i o n is a c c o m p a n i e d by p r o d u c t i o n o f insolubility, the reactions can profitably be studied to higher conversions in copolymers. T h e c o m p l e t e insolubility o f polyvinylidene chloride (PVdC) resulting f r o m dehydrochiorination b e y o n d 1 p e r cent m a k e s difficult the e l u c i d a t i o n o f the mechanism. (2) F o r this reason, the reactions o f V d C c o p o l y m e r s were s t u d i e d a l t h o u g h it was realized t h a t the difference in structure might alter the mechanism. C o p o l y m e r s o f V d C with m e t h y l m e t h a c r y l a t e ( M M A ) , methacrylonitrile ( M A N ) were s t u d i e d ; they were c o m p a r e d with styrene c o p o l y m e r s for which the characteristics o f d e h y d r o c h l o r i n a t i o n were considered in a p r e v i o u s c o m m u n i c a t i o n . (3)

EXPERIMENTAL The experimental procedure has been described elsewhere.( 21 Copolymerizations were carried out in bulk at 60° with benzoyl peroxide, as initiator, and taken to 5 per cent conversion. Reactivity ratios for the systems were determined from Finemann-Ross plots making use of chemical, i.r. and u.v. analyses of the copolymers. The accuracy of the measurements of rate of degradation, using 50-mg samples, has been gauged previously at I per cent; the monomer molar rate of volatilizationof all the copolymers was independent of sample size up to and beyond I00 mg. All rate studies were carried out on about 50 mg of sample. * Present address: Department of Chemistry, University of Southern California, Los Angeles, California, U.S.A. 83

84

G.M. BURNETT, R. A. HALDON and J. N. HAY COPOLYMER SYSTEMS

The characteristics o f all the copolymers are listed in Table 1 and the reactivity ratios in Table 2; the values agree with those in the literature. (4, 5) TABLEI. COPOLYMERCHARACTERISTICS Mole fraction

Comonomcr

Serial number

Initiator conch. (wt. 90

'Feed

Copolymer

A1 A3 A7 A8 A9 A10 B1 B2 B3 B4 C1 C2 C3 (24 C5 C6

0-48 0.62 0.53 0-48 0.49 0"48 0.50 0.50 0.50 0"50 0"50 0"50 0"50 0"50 0"50 0"50

0.58 0.34 0.10 0.10 0.08 0-03 0"59 0-44 0-23 0"08 0"78 0"54 0"46 0.32 0"19 0"13

0-75 0-62 0.38 0.39 0.32 0.14 0-77 0-63 0-40 0-14 0"86 0"72 0"62 0.48 0"32 0"23

Styrene*

MMA

MAN

* Taken from Ref. (3). Them 2. REACTIVrrYRAT:OS System

rt

r2

Styrene (1)-VdC (2) MMA (1)-VdC (2) MAN (1)-VdC (2)

1-75 + 0.10 2.60_ 0.10 2.15_+0.10

0.12 + 0-02 0.58 + 0-02 0-47_+0-02

The sequence o f blocks o f m o n o m e r units in the copolymer chains was calculated f r o m the general equation derivable from simple copolymerization theory, (6) i.e. the probability o f obtaining the sequence A - - ( B ) n - - A in the c o p o l y m e r o f A and B, is Pn =

~

PA~

PBA

where d A / ( d A + dB) = mole fraction o f A in the copolymer,

1 PAn= 1 +rt '

and and

f PBA __----f+Jr2

r2 P a B - - f + r2

A ('f= B = m o n o m e r feed mole ratio).

This equation has m a n y limitations. (7)

(1)

Dchydrochlorinationof Polymers--IV

85

RESULTS (1) MMA copolymers (a) General characteristics of the elimination. The copolymers eliminated volatiles much more quickly than PVdC homopolymer. Below 200 °, no M M A monomer was present in the volatiles but methyl chloride and hydrogen chloride were found. Auramine preferentially adsorbed all the hydrogen chloride leaving pure methyl chloride (g.l.c. and i.r. gas cell analysis) and this method was used to measure the volumes of each of the products continuously. Agreement was obtained between the extent of degradation measured by chemical analysis, weight loss, and the volume of volatiles. Marked changes occurred in the i.r. spectra of the copolymers--the strong M_MA carbonyl band at 1720 cm -1 progressively decreased with reaction, but at the same time a new band appeared at 1800 cm -1. The relative changes in these bands varied with extent of reaction and also with composition of the copolymers. Other bands appeared between 1550 and 1650 cm -1 consistent with the development of carbon double bonds. Bands at 1800 and 1625-1 were assigned to unsaturated 7-1actone groups in the residue. The discolouration and insolubility of the copolymer residues depended on the extent of reaction and copolymer composition, The copolymer containing least VdC was completely soluble at all stages and only slightly discoloured, while the copolymer richest in VdC became completely insoluble within the first few per cent of reaction, and rapidly darkened (see Table 3). TABLE3. MMA COPOLYS~gS.DEGRADATIONCHARACTERISTICS(150° for I hr) Residue VdC content (mole ~:o)

Reaction extent (~o)

, Colour

~ Solubility

23 37 60 86

49-5 52.5 32"8 15"7

white red brown black

100 100 90 5

(b) Kinetic study. A first-order rate expression could be fitted to the evolution of HC1, in the temperature range 120-220 °, assuming one mole of HC1 per VdC unit (see Fig. la). The rate constants showed a temperature dependence corresponding to 27-0 + 1.5 kcal/mole. Similar first-order dependences were obtained for the volumes of methyl chloride evolved up to 30 per cent reaction. However, in determining the rate constant, it was necessary to take the initial concentration of units producing methyl chloride as being proportional to the concentration of those monomer units present in the copolymer in the smaller amount. Analysis of the copolymer composition by Eqn. (1) to establish the fraction of adjacent V d C - M M A units showed that this approach was meaningful. Sample B1 exhibited first-order dependence on VdC with rate constants given by (-260(0 / kM~ct=2×109exp\ RT ] '

86

G . M . BURNETT, R. A. H A L D O N and J. N. HAY

HC1 evolution was also first order with respect to VdC with rate constants given by (-27,005 k s c t = l ' 4 × 1 0 t°exp \ R T ] " The rate of HC1 evolution increased regularly with VdC content in copolymer (see Fig. l b) while that of MeCI remained effectively independent of composition (see Table 4). TABLE 4. EUMrNA'nONE.ATES. M M A COt'OLYMERSAT 150 °

Serial number

, VdC content

B1 B2 B3 B4

23 37 60 86

Reaction rates A ~oHCI per hr ~ MeCIper hr 11"5 34"0 53"0 51"0

20"5 24'0 25-0 26.5

(c) Discussion. The value of 1.5 for r t r 2 suggests that there is little tendency to produce alternating copolymers. Sequence group distribution analysis, however, indicates that the maximum probability of finding adjacent MMA-VdC units occurs with copolymer B3; in the other copolymers, the monomer unit present in the smaller amount is isolated so explaining the observations on the rate of MeC1 evolution. Poly-a-chloroacrylate was pyrolysed to compare reaction products with those from the copolymers. Hydrogen chloride, methyl chloride, monomer and a cross-linked coloured residue, showing an i.r. spectrum which indicated five-membered ring lactones, were found. Marvel and Cowan,(8) and Crawford and Plant (9) studied in detail the degradation of poly methyl a-bromoacrylate and poly (+) sec-butyl-a-chloroacrylate respectively, and proposed ionic mechanisms. The product was optically active secbutyl chloride and so a radical process was eliminated. By analogy, the following reactions are suggested as occurring in the copolymers, CH3

Cl

I

I

CH3

-M~Cl.

--CHz--C--CH2--C--I [

Cl

1 --CH,--C--CH2--C---I

>

I

CH} - HCI

CH~ ~CH,~

1I C H "

C

\\

o//C'~o / Colouration occurs on dehydrochlorination but MMA clearly acts as a blocking unit. Chain scission also occurred prior to the onset of crosslinking. This may be associated with a radical elimination of HC1 from the isolated VdC units.

Dehydrochlorination of Polymers--IV

87

BI

x I o

20 4= r~

I 0

I

40

20

Tkne,

rain

FIG. la.

.~ 40

0

I

O.4

I

O.8

MMA mole fraction FIG. lb. FIG. 1. MMA eopolymers; a--elimination rate; b---monomer dependence.

(2) MAN copolymers (a) Kinetic study. Only HC1 was evolved in the temperature range 120-200 ° but the rate was greater than for the homopolymer. The colour and solubility of the residues were dependent on composition in a way analogous to that of the M M A copolymers-see Table 5. The i.r. spectra also showed that the intensity of the cyanide band at 2240 cm -1 did not appreciably alter during reaction. New bands appearing at 1640 and 1580 cm -1 were consistent with the development of unsaturation. First-order kinetics fitted the evolution of HC1 from aU the copolymers as shown in Fig. 2; at constant temperature, the reaction rate constant increased linearly with the concentration of M A N in the copolymer.

88

G.M. BURNETr, R. A. HALDON and J. N. HAY

I'0~ _ ~ k ~

~

166"

g, 0.5

0.0

I 0

Time,

I

120 min

F[o. 2. MAN copolymers;eliminationrate. The first-order rate constants of all the copolymers had a temperature dependence corresponding to 37.0 + 1.5 kcal/mole; the frequency factor, A, increased with MAN content from 2 x 10t3 to 10 x 1013 sec-1. The activation energy should be compared with 31.0 + 2-0 kcal/mole for PVdC. (b) Discussion. There is no evidence for neighbouring group activation by the MAN units. The copolymers are less stable than the homopolymers, and this instability increases with MAN content. The increased rate is due to increased collision number, i.e. increased chain mobility, rather than change in activation energy. The activation energy is greater with the copolymers rather than the homopolymers by about 6 kcal/ mole, suggesting that dehydrochlorination is not initiated at unsaturated double bonds. The reactivity ratio product is 1.0, showing that there is tittle tendency for alternation in the copolymers. Long sequences of VdC are present in the VdC rich copolymer thus accounting for the development ofcolour in these copolymers. In PVdC, a radical elimination reaction is initiated at unsaturated chain ends; this mechanism cannot operate in MAN rich copolymers as the zip-length would be drastically reduced, and the reaction would not be propagated at increasing rate with decreasing concentration of eliminating groups. The elimination must be from isolated VdC units probably by a unimolecular reaction involving a semi-ion pair transition state as postulated by Maccoll(1°) and Benson and Bose(H) to explain the vapour phase decomposition of alkyl hatides. (3) Styrene copolymers (a) General considerations. The sole volatile product from the styrene copolymers was HC1; there was no monomer below 200 °. The residues were either black or brown at complete dehydrochlorination even for copolymers containing very tittle VdC. The styrene rich copolymers were completely soluble at all extents of reaction, and only when the VdC content exceeded 60-70 per cent. was there any appreciable amount of insoluble material in the residues (see Table 7). There was also a random decrease in molecular weight, followed in the VdC-rich copolymers by cross-linking.

Dehydrochlorination of Polymers--IV

89

TABLE5. MAN COPOLYME~DEGRADATIONCHARACTERISTICS(RESIDUE2 hr at 166°) Residue Serial number

Original VdC content (mole ~Q

, Colour

~ Solubility

CI C2 C3 C4 C5 C6

14 28 38 52 68 77

white red brown dark brown dark brown black

100 100 100 90 80 40

TABLE6. RATESTtJDtr.SAT 166° Serial number

VdC content (mole ~

Initial rate (~, conv./hr)

Rate constant (x 10s. see-t)

C1 C2 C4 C5 C6 PVdC

14 28 52 68 77 100

22.2 23.4 23.8 23.0 18-0 5.0*

7.24 5.95 5-34 4-68 4.32

* M01. wt. dependent, and zero order characteristics.

The i.r. and u.v. spectra of the solid residues were consistent with the development of conjugated polyenes, with strong bands appearing at 1660 and 1625 cm -1 increasing in intensity. The kinetics o f H C I elimination from styrene copolymers C3) have been previously interpreted as indicating activation by styrene units but retardation by the products o f the reaction (see Fig. 3b). Accordingly, the rate o f elimination decreased more rapidly than could be accounted for in terms o f concentration changes (see Fig. 3a). The initial rates obtained by extrapolation increased linearly with styrene content (Fig. 3c). The temperature dependence of these initial rates also decreased with styrene content, see Table 7.

TABLE 7. STYRENE COPOLYMER~DEGRADATION CHARACTERISTICS (18 hr AT 155 °)

Serial number

Styrene content (mole ~,)

Solubility (wt. ~

Activation energy (kcal/mole)

A1 A3 A7 A8 A9 A10 PVdCI

25 38 62 61 68 86 100

100 100 95 95 50 70 1

24"0 26.0 29"0 28'0 31"0

90

O . M . BUR_NETT, R. A. HALDON and J. N. HAY 166" AI

A3

-

A7

h.m

o~

Time,

rain

40

F[o. 3a.

166"

I

._¢ .5

o.to

I

60 Time, rain FZO.3b.

166"

17.--

/

I

.~

59 °

~

o

I 151°

0.4 (2-8 Styrene mole fraction

FIe. 3c. Fio. 3. Styrene copolymers; a---cllm{nation rate; b--retardation by product; c---monomer rate dependcnc~.

Dehydrochlorinationof Polymers--IV

91

(b) Discussion. The proportion of VdC units adjacent to styrene units in the chains was calculated by the simple copolymerization theory, but no correlation was obtained between this proportion and the activated fraction deduced from experimental results. The kinetics of the elimination are, instead, consistent with retardation by the products of the reaction. (3' 12) It is concluded that the elimination reaction is not initiated at chain ends as there is no dependence on the degree &polymerizations; the variation of the rate and decrease in activation energy with styrene content are consistent with initiation at units adjacent to styrene. The rapid decrease in molecular weight, the competing development of cross-links, and the retardation characteristics of the kinetics are consistent with a radical process. The exact nature of the styrene activation is not clear but may involve a bridge intermediate similar to that proposed by Sykes to explain the anomalously high rates of hydrolysis of 2-phenyl ethyl chloride.

A similar bridge radical intermediate can be invoked. GENERAL DISCUSSION The copolymers were studied in order to obtain further evidence concerning the mechanism of elimination from PVdC. However, it is very apparent that conclusions drawn from these systems are conflicting. Environmental changes completely alter the characteristics of the elimination reaction. The normal rate for PVdC is reduced by the high degree of crystallinity and all the copolymers eliminate at much higher rates. The different efficiencies of the comonomers in accelerating the reaction cannot be attributed entirely to reduced crystallinity and inter-molecular attraction--as was possible with the copolymers ofacrylonitrile,m Neighbouring group interactions are clearly apparent in the cases where styrene and MMA are comonomers. PVdC eliminates HC1 by a radical process initiated at terminal unsaturated groups; in the presence of comonomers, the elimination processes are more complex since the kinetic zipp length is drastically reduced. A radical process will occur only if an initiation mechanism occurs for each isolated VdC sequence, e.g. at each styrene unit, or at each MMA unit; otherwise, a unimolecular mechanism predominates, e.g. when MAN units are present. The two mechanisms are substantiated by the considerable increase in activation energy for the MAN copolymers. In VdC rich copolymers the normal elimination may also be present. REFERENCES (I) (2) (3) (4) (5)

N. Grassie and J. N. Hay, Soc. Chem. monogr. 13, 184 (1961);./.Polym. SoL 56, 189 (1962). G. M. Bumett, R. A. Haldon and J. N. Hay, (part I) Europ. Polym. J. 3, 449 (1967). R. A. Haldon and J. N. Hay, (part 3) I. Polym. Sci.,in press. (1967). K. W. Doak, J. Am. chem. Soe. 70, 1525 (1948). F. M. Lewis, F. R. Mayo and W. F. Hulse, J. Am. chem. Soc. 67, 1701 (1945).

92

G . M . BURN"ETT, R. A. H A L D O N and J. N. HAY

(6) T. Alfrey, J. J. Bohrer and H. M. Mark, Copolymerization, High Polymer Series no. XIII, Interscience New York (1952). (7) E. Mentz, T. Alfrey and G. Goldfinger, J. chem. Phys. 12, 205; G. E. Ham, J. Polym. Sci. 45, 169 (1960). F. P. Price, J. chem. Phys. 36, 209 (1962). (8) C. S. Marvel and J. C. Cowan, J. Am. chem. Soc. 61, 3156 (1939). (9) J. W. C. Crawford and D. Plant, J. chem. Soc. 4492 (1952). (10) A. Maccoll, Chem. Soc. Spec. Publ. No. 16, p. 159 (1962). (11) S. W. Benson and A. N. Bose, J. chem. Phys. 39, 3463 (1963). (12) R . A . Haldon and J. N. Hay, (part 2) J. Polym. Sci., in press. R~sumg---On a ~tudi~ l'effet de diffgrents comonom~res sur la d~chlorhydratation du poly(chlorure de vinylid~ne). D'une mani~re g~n~mle tousles colpolym6res ~taient thermiquement beaucoup moins stables que les homopolym~res et il se produisait dam les copolym~res une interaction anormale des groupements adjacents. Dans le cas des copolym/:res avec le m&hacrylate de m~thyle il y avait ~limination de chlorure de m&hyle a eOt~ de l'acide chlorhydrique; des groupements 7-1actone y-insatur~s ont ~t~ d~celgs dans le r6sidu solide. D a m les copolym~res avec le m6thacrylonitrile il n'y avait pas d'interaction avec les groupes voisins mais le d~gagement d'acide chlorhydrique &ait acc~l~r6 en raison d'une plus grande mobilit~ des chaines. Les groupes styrene catalysaient nettement la r~action d'glimination. Selon certaines indications il existe des interactions de groupements adjacents dam ce syst~me. Sommnrio---E' riportato uno studio sull'effetto di diversi comonomeri sulla reazione di perdita di acido cloridrico del cloruro di polivinilidene. In generale tutti i copolimeri risultano molto meno stabili termieamente di quanto non lo siano gli omopolimeri e presentano delle interazioni anomali tra i gruppi adiacenti. Copolimeri con il metil metacrilato eliminano sia cloruro di metile che acido cloridrico; nel residuo solido si formano gruppi 7-1attonici 7-insaturi. Nei copolimeri con il metacrilonitrile non si hanno interazioni tra i gruppi vicini, ma si ha un aumento della velocith di eliminazione a causa di un aumento della mobilit/t della catena. Unitb. stireniche catalizzano senza dubbi la reazione di eliminazione; non si osservano interazioni tra i gruppi adiacenti. Zusammenfassung--Es wird fiber den EinfluB verschiedener Comonomerer auf die Reaktion der Chlorwasserstoffabsplatung aus Polyvinylidenchlorid berichtet. Ira allgemeinen hatten aUe Copolymeren sher viel geringere thermische Stabilitit als die Homopolymeren und die Copolymeren zeigten anomale Nachbargruppeneffeckte. Methylmethacrylat Copolymere semen sowohl Methylchlorid als auch Chlorwasserstoff frei; im festen Rtickstand wurden 7-ungesattigte 7-Lactongruppen gebildet. Methacrylnitril Copolymere zeigten keine Nachbargruppeneffekte, aber eine gr6ssere Abspaltungsgeschwindigkeit anf Grund st~kerer Kettenbeweglichkeit. Dutch Styroleinheiten wurde die Abspaltungsreaktion deutlich katalysiert; es liegen Hinweise fur einen Nachbargruppeneffekt vor.