Anionic polymerization of methacrylonitrile—I. Polymerization with n-butyllithium as initiator

European PotymerJournal. Vol. 14. pp. 189 to 198. PergamonPress 1978. Primedin Great Britain

A N I O N I C P O L Y M E R I Z A T I O N OF METHACRYLONITRILE--I P O L Y M E R I Z A T I O N WITH n - B U T Y L L I T H I U M AS INITIATOR H. VANKERCKHOVENand M. VAN

BEYLEN*

University of Leuven, Laboratory of Macromolecular and Organic Chemistry, Celestijnenlaan 200 F. B-3030 Heverlee, Belgium

(Received 5 April 1977; revised 5 August 1977) Abstract--The anionic polymerization of methacrylonitrile has been studied at - 7 5 in toluene and with n-butyllithium as initiator. The kinetics of the polymerization were investigated considering the consumption of both monomer and initiator. BuLi disappears relatively slowly and about 50-60'!,, remains unreacted. A simple kinetic scheme cannot therefore be put forward. All possible side reactions have also been examined. The molecular weight study establishes the living character of this system and gives an initiator efficiency of about 0.2. The contribution of low molecular weight products, typical of the polymerization of polar monomers, is also taken into account. In order to obtain a better understanding of the mechanism of this polymerization, in which unreacted initiator is probably engaged in very stable and inactive mixed associated particles, small amounts of THF (known frequently to break down such aggregates) were added to the system. A rather unexpected slow but complete disappearance of the initiator occurs; the conversion at which the rate of monomer consumption levels off depends on the THF concentration.

INTRODUCTION In the anionic polymerizations of non-polar m o n o m e r s such as styrene and butadiene, the initiator reacts exclusively with the double bond. The anionic polymerization of polar monomers, such as ~,/~ unsaturated esters and nitriles, is, however, characterized by a variety of side reactions with the initiator and by the formation both of substantial amounts of oligomeric material and of side products which can have an important influence on the type and on the behaviour of the active species involved in the reaction. Thus only part of the initiator is converted into growing chains and the initiator efficiency depends on the initiator. Various aspects of the polymerization of m o n o m e r s of this type have been discussed by several authors. Mechanisms were proposed on the basis both of kinetic and molecular weight studies for methyl methacrylate [1 11], acrylonitrile [10-16] and methacrylonitrile [10, 11, 16-27]. In the present work, the anionic polymerization of methacrylonitrile (MAN) has been examined in toluene with various organolithium c o m p o u n d s as initiators in an attempt not only to elucidate the mechanism of the Li-initiated polymerization but also to obtain more information on the behaviour and the initiating capacity of organolithium initiators in methacrylonitrile polymerization and to correlate the various aspects typical of the polymerization of polar monomers. This paper deals with the polymerization initiated by n-butyllithium (BuLi). A second paper will be devoted to the polymerization initiated by sec-butyllithium and l,l-diphenyl-n-hexyllithium. The polymerization is carried out at - 7 5 ° in order to reduce the rate and to avoid as much as possible the formation of insol* To whom correspondence should be addressed.

uble polymer. The study is mainly restricted to the initial stages, where polymerization occurs homogeneously. EXPERIMENTAL

All experiments were carried out in high vacuum. The monomer was freed from inhibitor, dried by stirring on calcium hydride overnight i~7 vacuo. After several degassings, it was vacuum distilled onto fresh pre-evacuated calcium hydride, stirred overnight, and finally distilled under vacuum into a glass ampoule (with break-seMi and sealed off. Toluene was dried and degassed on the vacuum line on sodium-potassium alloy. Tetrahydrofuran (THF) was treated similarly after refluxing on Na K alloy for 12 hr followed by fractional distillation under nitrogen. BuLl (Fluka or Merck) was available as a 25'~,~ solution in hexane. After removal of the hexane on the vacuum line, the initiator was diluted with purified toluene or benzene to obtain the required concentrations. Concentrations of the organolithium compound and the amount of alcoholate present were determined using the double titration metqod described by Gilman [28]. After purification and dilution, the monomer and initiator solutions were divided m;der high vacuum between several calibrated ampoules using the classical dividing vessel [29] and stored in a rcfl-igerator.

Polymerization procedure The polymerizations were carried out in 100ml fournecked vessels onto which calibrated ampoules, provided with a break-seal and containing the different reagents, were sealed. The apparatus was evacuated and the desired amount of solvent distilled into it. The apparatus was l hen sealed off and the break-seal of the ampoule containing the initiator was broken. The whole unit was then immersed in the cooling bath (isopropanol and dry ice) at 75 and thermostatted for 30 min. By breaking the break-seal of the monomer ampoule at the same temperature, the 189

E p p 14 3

L

190

H. VANKERCKHOVENand M. VAN BEYLEN

monomer solution was rapidly mixed with the initiator solution. The reaction was stopped rapidly by crushing a third break-seal covered with butanol from the outside. To this cold mixture was added the reference solution for the chromatographic analysis (a dilute solution of pentane in toluene). Samples of the cold reaction mixture were immediately injected in the gas chromatograph for determination of the amounts of butane and butyl isopropenyl ketone (BIK) (reference material 2-octanone). The column was a 4-m column of 5% SE 30 on Varaport 80/100. The polymer was precipitated in petroleum ether, filtered, dried and weighed. This procedure was repeated for various times at various concentrations of butyllithium and methacrylonitrile.

Molecular weights The molecular weights of the polymers were determined with a Mechrolab vapour pressure osmometer with dimethylformamide or acetone as solvent.

at higher conversion the higher the initiator concentration. In considering this apparently limiting degree of conversion, two factors must be considered viz. the amount of polymer produced in the system and its molecular weight. Thus, at high initiator concentrations, a higher conversion is attained at the stage where the rate levels off through the formation of a greater number of polymer molecules, giving a higher total amount of polymer but of comparatively lower molecular weight than at lower initiator concentration. A sharp decrease in the rate of polymerization has also been observed with o t h e r polar monomers, e.g. acrylonitrile [30] and vinyl chloride [31] where polymerization stops completely at 20% conversion. In Fig. 2, only the first part of the conversion-time curves, where the reaction still occurs homogeneously, is taken into consideration. At low initiator concentrations, there is an induction period

RESULTS AND DISCUSSION Cow,version ~n mole. / "1

The polymerization kinetics Once the monomer is admitted to the butyllithium solution, a yellow colour develops immediately changing rapidly through light green to dark green (almost black). After termination with butanol, the colour of the solution varies from yellow to orange depending on the degree of conversion. The polymer isolated after precipitation in petroleum ether is light yellow. The initial polymerization is very fast, and after some time the reaction mixture gels (see Fig. I). In the curves, two parts may easily be distinguished. In the first, the degree of conversion increases rapidly with time. The second part is characterized by a progressive decrease of the rate which apparently levels off at quite a low conversion (50-60%) depending on the concentrations of the reagents. However, if the polymerization is continued over a long period (12-24 hr), all the monomer is consumed. This phenomenon was observed at all monomer and initiator concentrations. It will be shown, when the molecular weights will be discussed, that the polymerization is of the living type without termination and transfer reactions, and that the slowdown of the reaction is due to a gradual gelation of the system hindering the access of monomer to the immobilized active centres rather than to termination reactions. As seen in Fig. 1, the leveling occurs

0,3

c......... in mote. l -I

0,12 I[BOLI] = 2,94 x 10"2M [Man] = 0.4 M Temp = - 7 5 o C 0.10

= 1,73 x 1(~2 M

0,08

[BuLi]= 8,66x103 M

00°°6 Bu133/ 0,02

• //"/ / ./

,, / ,/ i

..

-'=:+..... T"

l

20

Time in seconds

I

I

40

60

[ " ' n ] =0.4 M Temp=-75°C

[BuLi]o = 8,66x163M 0,2

0,1

Time in s e c o n d s

I 200

q

I 400

J

] 80

Fig. 2. Conversion of methacrylonitrile as a function of time: initial reaction period.

[~Li]o = 1,73x102M

I

I

I 600

I

I 800

I

I 1000

Fig. i. Conversion of methacrylonitrile as a function of time.

Anionic polymerization of methacrylonitrile Table 1. Anionic polymerization of MAN by n-BuLl in toluene at - 7 5 °. Influence of monomer concentration on the rate of polymerization

191

no simple kinetic law is expected to be valid throughout the process.

The consumption of the initiator: n-butyllithium [MAN]0 (mole 1 ')

Rp x 103 (mole 1-1 sec -1)

0.! 0.2 0.4 0.6 0.8

0.26 1.0 3.0 4.8 7.5

To obtain more information on the initiation, the disappearance of BuLl as a function of the time has been studied by gas chromatography. If the initiation and the side reactions (in which BuLl is also consumed) are very fast, no butane should be found after a period of a few seconds. The curves in Fig. 3 suggest that for each concentration the initiator disappears fairly rapidly at the beginning of the reaction but, after some time, the rate falls off and a stage is reached, at which the amount of butane being found after termination does not decrease further with time. As illustrated in the same figure, this amount, quite important at -75% is much smaller at higher temperatures, BuLl disappearing much faster and to a much higher extent before leveling off occurs. This butane may result from unreacted BuLl which, upon termination with butanol yields butane, but could also be formed by hydrogen abstraction from the monomer or from the polymer. However, our experiments clearly demonstrate that, with increasing temperature, the consumption of the initiator increases markedly and Tsuruta [16] showed that at room temperature (in a nonpolar solvent) no unreacted BuLl remains. Therefore, a polymerization was carried out at - 7 5 ° during 15 min to ascertain that the region where the amount of BuLl remains constant was reached. Then the polymerization mixture was slowly brought to room temperature and cooled again before stopping the reaction with butanol in order to avoid any loss of gaseous butane. If the butane if formed by hydrogen abstraction from the monomer or the polymer, the same amount of butane should be found after the mixture was brought to room temperature as a t - 75% whereas none or very little should be detected after this operation if the butane results from unreacted initiator, the latter having reacted completely because of the rise.in temperature. It is found experimentally

[BuLi]o = 1.73 x 10-2mole 1-1. increasing with decreasing initiator concentration; the rate varies little, if at all, with concentration. However, at the lowest concentration, [BuLi]o = 4.33 × 10- 3 mole 1-1, determination of the rate is difficult, as there is hardly any linear part in the curve, and there might be serious error. At higher concentrations the overall rates are high, and the conversion-time curves apparently linear. For BuLl concentrations >8.66 × 10 -3 mole 1-~, the apparent reaction order with respect to initiator exceeds one. The dependence of the rate on initial monomer concentration was similarly investigated (see Table 1). No linear relationship is observed between rate of polymerization and monomer concentrations; the apparent order deduced from a log Rp - log (MAN) plot is 1.5. The observed orders might however, not be significant since the occurrence of induction periods at low initiator concentrations, characteristic of a slow or incomplete initiation, could imply that initiation and propagation occur simultaneously. The kinetics would then be very complex and exhibit the characteristics of consecutive, competing reactions. Study of such a system by gravimetry would be impracticable or impossible. Moreover, in the case of isoprene [32, 331 the rate of initiation depends not only on the reactants (monomer and butyllithium) but also on the product, polyisoprenyllithium, and the rate of propagation is affected by remaining initiator. Under such conditions

fIBuLa] mole

[Man] = 0,4 M

× i0-2

Temp=-?SoC

II

1.5

[BuLi]o" 2,40 x IO 2 M

[BuLl]o= 1,73 x 10-2M

[BuLi]o=8~6 x 10-3M

Time in s e c o n d ~

_&OO C

20O

I ~oo

'I 600

- 2oo'~

j 800

Fig. 3. Consumption of the initiator.

i ~ooo

192

H. VANKERCKHOVENand M. VAN BEYLEN Table 2. Anionic polymerization of MAN by n-BuLi in toluene. Consumption of the initiator [BuLi]0 x 102 (mole l- 1)

[MAN]o (mole 1-1)

Temp. (°C)

~o BuLi* consumed

% BuLi* unreacted

% BIK formed

1.73 2.40 1.73 1.73 0.866 1.73 1.73

0.2 0.4 0.4 0.7 0.4 0.4 0.4

-75 - 75 - 75 - 75 75 -40 - 20

41 37 46 50 50 85 94

59 63 54 50 50 15 6

11.1 11.7 14.2 12.2 -25.5 47.5

-

* Percentages are expressed with respect to the initial initiator concentration. that after raising the temperature, hardly any butane is left. This result establishes that at - 7 5 ° a considerable amount of BuLi remains unreacted and that for the given conditions hydrogen abstraction from the monomer or polymer may be ruled out as a possible side reaction. It should be emphasized that the amount of unreacted BuLi reaches a constant value fairly rapidly and that the remaining BuLi is unable to initiate new growing chains, although the monomer continues to disappear gradually. It is noteworthy also that, at the moment when the amount of initiator becomes constant (i.e. after about 100-200sec), 5~60~o of the monomer remains. If after 10 min, a new initiator solution is added to the polymerizing mixture, in which half of the monomer is still present, one observes that of the newly added initiator, the same fraction (i.e. about 60%) again remains unreacted. On the other hand, the monomer is eventually consumed completely. The initiator added in the second step behaves as if it were involved in a separate polymerization. If, however, in the second step of the experiment, some new monomer is added instead of new initiator, no change whatsoever is observed in the amounts of the unreacted BuLi whereas all the monomer disappears. Table 2 shows the amounts of reacted and unreacted initiator under various conditions, the initiator being consumed either by starting the polymerization or in side reactions. Similar behaviour of n-BuLi was observed in the initiation of isoprene in cyclohexane by Guyot and Vialle [32, 33]. They also found that, after the formation of a certain amount of isoprenyllithium, the rate of initiation decreases to a negligible value and the consumption of BuLi reaches a limiting value, which depends on the initial monomer to BuLi ratio and increases with it, and which cannot be passed by further addition of monomer due to the formation of inactive mixed associated particles. Although in our case the differences in the limiting conversions reported in Table 2 are rather small and may be due to experimental scatter, they seem on the whole to increase with increasing initial monomer to initiator ratios, reflecting a similar dependency as for isoprene. In addition it should also be considered that, if the propagation is drastically slowed down by progressive gelation, so might be the initiation by BuLi molecules associated with growing insolubilized P M A N chains; since initiation is already slow compared to propagation, the former may be even more seriously affected. This would also explain the fate of the newly added BuLi, which also becomes unreac-

tire probably because it becomes part of a mixed complex with an increasingly insolubilized P M A N growing chain. Nevertheless, where propagation will ultimately go to completion, consumption of BuLi from a certain stage does not continue at all even after an extended period or after addition of fresh monomer. All these observations strongly suggest that the remaining unreacted BuLi molecules are engaged in very stable and inactive mixed associated particles.

Study of the side reactions As pointed out previously, the BuLl which reacts disappears not only by initiation but also by side reactions with monomer and polymer. Such reactions have been studied by several authors. Tsuruta et al. investigated the fate of n-BuLi in the initiation step of acrylonitrile and methacrylonitrile, finding hydrogen abstraction and nitrile addition to be the main processes of metal alkyl transformation [16]. All possible side reactions are therefore consecutively considered. 1. Hydrogen abstraction. It has already been demonstrated in the discussion of the initiator consumption that fl hydrogen or e-methyl hydrogen abstraction from methacrylonitrile monomer or polymer does not contribute to butane formation in toluene as a solvent. 2. Reaction on the nitrile function of the monomer. This reaction can be represented thus CH 3

CH3 I Bu Li .CHz= ~ (A)

-

CH3 I CHz=~

CN

/ Bu

CH2---- C

C~HgOH D

C

~NLi ÷ (B)

I

Bu --C~NH

(C]

t HCI CH I 3 CH2----C I Bu -- C=O

(D)

In the polymerizing mixture, the product is present as an imine ion (B). After termination a neutral imine is formed (C); upon treatment with a little diluted HC1 it is hydrolyzed to n-butyl isopropenyl ketone (BIK) (D). The formation of this compound is followed by gaschromatographic analysis of batches of' the polymerizing system terminated and acidolyzed at various stages of the reaction. As illustrated in Fig. 4, the formation of the nitrile adduct is fairly fast early in the polymerization, but decreases progress-

Anionic polymerization of methacrylonitrile

193

i

[BuLi]o=

1,73 × 102M

Temp=

7B°C

~d 3

5 1(~ ~

T,me in seconds

I

I

I

80

120

!80

Fig. 4. Formation of butyl isopropenyl ketone during the polymerization. ively to become zero at the same time as the BuLi concentration was shown not to change further in Fig. 3. Table 2 shows the extent to which the addition of the nitrile function of the monomer occurs under different experimental conditions together with the total limiting conversions of BuLL The reaction of BuLi on the nitrile function of the monomer leaves an imine ion in the reaction system, containing a double bond and negatively charged. This species could in principle therefore act as a monomer and be incorporated in a polymer chain or as an initiator and start a new growing chain. However, the amount of BIK reaches a constant value long before all the monomer is consumed; upon addition of fresh monomer to a polymerizing system in which both the BuLl and the nitrile adduct concentrations have reached their limiting values, no change is observed in the amount of BIK. It is therefore improbable that the nitrile adduct is incorporated into a polymer chain via its double bond. It is, however, not possible to exclude that the nitrile adduct initiates a new chain because, if it did, a molecule of BIK would still be split off on termination and acidolysis through the scission of the C - - N bond.

where [MAN]o and [BuLi]0 represent the initial monomer and initiator concentrations. :~ the degree of conversion and M,1 the number average molecular weight of the polymer, 67 being the M.W. of ~:he monomer. By plotting the degree of polymerization (-Xn) against the degree of conversion (Fig. 5) a straight line is obtained; the efficiency of the initiator may be calculated from the slope since M. .....

67

1 [MAN]0 = X n =

.

f

.

.

.

.

.

[BuLi]o

~.

The efficiency was 0.17 and 0.19 for an initial monomer concentration of 0.4mole 1-1 and initial initiator concentrations of 1.73 x 10 -2 and 8.66 x 10- 3 mole 1-1 respectively. The difference i n f i s small and may not be meaningful considering the small range of monomer to initiator ratios. It was shown above that the initiation step in these polymerizations, whatever its efficiency, is not instantaneous and that consequently at the very beginning of the reaction the number of active centres is not

3. Reaction of the nitrile Junction of the polymer. This side reaction [35-39] leads to the formation of six membered heterocyclic rings in the polymer chain through the attack of an imine anion on an adjacent nitrile group to give a conjugated cyclic imine. These rings are probably responsible for the coloration and insolubility of the polymer. N o direct method can determine how much butyllithium reacts in this way. This cyclization may also be induced by other compounds of ionic nature, besides the initiator.

80

--

6O

z,O

,,[

Molecular weights and efficiency As pointed out in the Introduction, only part of the initiator reacts with the double bond of the monomer to form growing chains. The efficiency, f, is defined as 67. [ M A N ] o c~ M, exp. [BuLi]o

u]

~

,o

~e

OCBuLI]o = 1,75 x I0 :' m o l e

L'

20

"/o M o n o m e r i 0

I 20

conversbon

~,

_4. hO

60

Fig. 5. Degree of polymerization vs the degree of conversion: determination o f f

194

H. VANKERCKHOVENand M. VAN BEYLEN Table 3. Change in per cent initial monomer converted to precipitant insoluble polymer Polymer precip. in petroleum ether

Polymer precip. in acidified methanol

[BuLi]0 x 102 (mole 1-1)

[MAN]0 (mole 1-1)

Time (sec)

Conversion ~

0.866

0.4

660 900 1000

51.04 ---

-46.9 48.8

1.73

0.4

30

11.9

74 1500

21.5 22.9 25.3 25.3 31.1 70.1

17 16" 20.9* 60.8

120 130 130 180 1200

-32.8 32.8 40.2 58.8

5.9 -8.5* 11.6" 37.5*

50

1.73

0.1

Conversion

* Precipitated first in petroleum ether and after redissolving reprecipitated in acidifled methanol. constant. It should therefore be emphasized that the molecular weights, on which the plots in Fig. 5 are based, were obtained in the region where the amount of unreacted initiator remains constant, i.e. where the number of growing chain ends also remains constant. At that moment only, growing chain ends are responsible for monomer consumption and only then the equation definingfis applicable. On considering these values of f, it should be stressed that they refer to the total initiation efficiency, i.e. to the sum of both the efficiency of formation of oligomer and that of polymer. Indeed in contrast to the anionic polymerization of acrylonitrile [22] where the polymer was precipitated by pouring the reaction mixture in acidified methanol in which the low molecular weight material remains soluble, the PMAN produced in our experiments was precipitated in petroleum ether in which, as shown by the virtually negligible residtie left upon evaporation of the filtrate, more nearly complete precipitation of both high and low molecular weight materials may be expected. This was clearly demonstrated by carrying out a number of parallel experiments and pouring the reaction mixtures in petroleum ether and acidified methanol, or by redissolving some polymers precipitated in petroleum ether and reprecipitating them in acidified methanol. Typical results of such experiments are collected in Table 3; it is clearly seen that, for equal or similar reaction times, the amount of polymer precipitated is always noticeably smaller when precipitated in methanol than when precipitated in petroleum ether, the percent of monomer converted into methanolsoluble material amounting to approximately 3, 10 and 25~ at [MAN]o/[BuLi]o ratios of 46, 23 and 5.8 respectively, regardless of which of the two procedures was used. Further evidence may be gained from an experiment in which a polymer sample precipitated in petroleum ether was divided into three equal parts, one of which was reprecipitated in petroleum ether, the two others being precipitated respectively in pure

methanol and in HCl-acidified methanol. The fraction precipitated in petroleum ether was completely recovered, while approximately equal amounts were lost of the fractions precipitated in pure and acidified methanol, the only difference between the two latter being the colour of the recovered polymer which was white for the polymer precipitated in acidified methanol and yellow for the other. The acidified methanol (HCI) apparently only causes the scission of the conjugated cyclic imine structures. Furthermore, the methanol filtrate was slowly evaporated to dryness and the M, of the residue was determined to be 420. The closeness of this value to 455 (calculated on the basis of one of the cyclic structures proposed by Erussalimsky [40] in the case of AN) is probably fortuitous. Indeed, in view of the experimental inaccuracy of this determination, this value may well be higher. All these experiments not only explain the higher values of the initiation efficiency than those reported in the early work on AN by Erussalimsky et al. [22-24] which, as shown later by these authors, are to be considered as 'efficiencies of formation of high polymers only, but also provide evidence for the formation of oligomers during the n-BuLi initiated polymerization of methacrylonitrile, as was also clearly established in later work on AN by Erussalimsky et al. [27, 34, 40, 41] as well as by Wiles and Bywater for methylmethacrylate [5]. Similarly to the results reported by both these authors, experiments at all concentrations indicate the formation of oligomers from the early stages of the polymerization as exemplified by the following results taken from Table 3 ([BuLi] = 1.73 x 10-2mole1-1, [MAN]0 = 0.4 mole 1-1). Time (sec) Conversion ~o Polymerization Oligomerization

30

50

74

1500

11.9 10.3

16 9.3

20.9 10.3

60.8 9.3

Anionic polymerization of methacrylonitrile Table 4. Molecular weights and efficiencies of formation of methanol-insoluble polymers [BuLi]0 x 103 (mole 1- ~)

Conversion (%)

M.....

f

8.66 8.66 17.3 17.3

46.9 48.8 60.8 61.4

13,800 22,000 13,600 12,800

0.095 0.069 0.069 0.074

Temp. - 75'; [MAN]0 = 0.4 mole 1- x. Although no further attempt was made to characterize these oligomers, they may well contain such cyclized species as claimed by Erussalimsky et al. to occur in the system AN-BuLl-toluene [40] through inter- and intramolecular cyclizations. As also shown by these authors in their work on AN as well as by Wiles and Bywater for the system MMA-nBuLitoluene, it should be stressed that the percentage of soluble polymer in the polymerization of MAN also increases markedly with decreasing ratio of MAN to BuLl. When the methanol-soluble low molecular weight products are separated from the bulk product, as during precipitating the polymer mixture in methanol, the molecular weight of the remaining material is considerably higher than the overall molecular weights, while the efficiency of high polymer formation simultaneously markedly decreases as compared to the overall initiation efficiency mentioned above. This is clearly illustrated by the data (Table 4) obtained at two monomer-to-initiator ratios by precipitating the polymer in acidified methanol. Although there is scatter in the molecular weights for the highest-monomerto-initiator ratio, the efficiency for the formation of methanol-insoluble polymer is seen to be approximately 0.07~). 10, i.e. about 2 x lower than the overall initiation efficiency. At higher temperatures ( - 4 0 and -20°), an increase of the efficiency is observed, the values of f being respectively 0.36 and 0.43. Earlier in this paper it was mentioned that, after a certain conversion, the rate of polymerization drops rapidly. This reduction might be due to termination reactions. That this is not the case follows directly from the plots in Fig. 5 which show that the degree of polymerization increases linearly with conversion, characteristic of processes without termination or transfer reactions other than with polymer. The latter type of transfer, which could occur, e.g. by proton abstraction from the polymer by a growing chain and would lead to chain branching, does not affect the number average , ~ since it does not change the number of polymer molecules in the system. It can therefore in principle not be excluded on the sole basis of the linearity of X, with conversion.. However, hydrogen abstraction from monomer or polymer by BuLl was shown not to occur and it is unlikely that a growing PMAN chain would abstract a proton from a polymer chain if the more basic BuLl does not do so. Moreover, in general, many complexities or side reactions associated with initiation of polar monomers such as methacrylonitrile and methyl methacrylate [5, 24] with n-BuLl are absent when the growing chain ends are considered. The system may therefore be considered as

195

of the "living" type; as soon as initiation is completed (or stops), the number of active sites remains constant. It seems then more plausible to attribute the decrease in the rate to progressive gelling of the reaction mixture. The polymer becomes insoluble in toluene and precipitates from the reaction medium; it is then difficult for the monomer to reach the living ends, and the very slow polymerization may be due to slow monomer diffusion to the active centres. The living character of the PMAN chains was also established by Felt and Zilkha [21] in the n-BuLl initiated polymerization of MAN in petroleum ether at - 7 8 °.

Influence t?["tetrahydrofuran on the reaction As shown in this paper, the polymerization of MAN in toluene at - 7 5 ° is characterized by a relatively slow initiation, and an important amount of unreacte'd BuLl. The fact that the initiator consumption stops early in the polymerization, while most of the monomer is still present, indicates that the unreacted BuLl is somehow engaged in mixed complexes with the growing chains which after some time become insoluble. At this stage of our investigation small amounts of THF were added to the system with the aim of breaking down these aggregates. Indeed, THF, as well as other solvating agents, is known to show a strong tendency towards coordination with organolithium compounds and to break down both homo- and mixed aggregates. Although in the presence of THF the system becomes even more complex, some interesting effects were observed upon addition of 1, 2, 5 and 10G of THF to the system. The general behaviour of this system (see Fig. 6) is identical tt~ that without THF. After a certain degree of conversion, an apparent leveling-off in the conversion-time curves is seen. However, if the polymerization is continued for a sufficiently long period, complete consumption of the monomer occurs. These apparent limiting conversions of monomer decrease with increasing THF concentration, starting at a value above that in pure toluene for 1 and 2~o, to reach a lower value for 5 and 10~o THF. On the other hand, the overall polymerization rates are found to be lower than in the absence of THF, decreasing with increasing THF concentration up to 5~o. but increasing again at 10% THF. At l'~/oand less so at 2'~',~,,sigmoidal rate curves are obtained with a fairly long induction period. At 5~%, the overall rate is low and the levelling off occurs at low conversion so that the induction period is not noticeable. The kinetic curves of BuLl consumption are identical for 1, 2 and 5~o THF so that only that for 2~o is represented in Fig. 7., together with those for 10~o THF and in the absence of THF. This figure clearly demonstrates that, in the presence of 1, 2 and 5~o THF, the initiator is consumed more slowly than in the absence of THF while the reverse is true for 10~o. In contrast, however, to the polymerization in the absence of THF, the initiator is progressively consumed throughout the entire reaction until complete exhaustion. To unravel these complex processes, a separate investigation would be required taking into account the precise influences of the factors affecting the reaction and including careful molecular weight determinations in the various conditions. Nevertheless, some interesting conclusions may be drawn. The higher

196

H. VANKERCKHOVENand M. VAN BEYLEN

f

0'3 I

T Conversion

in

0

mole. k- i

1S00"

0

0

~ •

Q

1% THF

--

02~" T"/.H FTHF.., • 15~0"

•

15~0" 5 "/, THF 1500" 10% THF

0,2 -[.I -1

o,,k/U"I / I

I

200

0

I Temp

400

Time seconds=

600

800

1000

Fig. 6. Polymerization of methacrylonitrile in the presence of T.H.F.

monomer conversion attained before the rate levels off (at 1 and 2% THF) can be accounted for by assuming formation of more growing chains, in much the same way as was shown previously that at higher initial initiator concentrations higher conversions of monomer could be reached before retardation sets in. This assumption is not unreasonable, considering that the consumption of BuLi and therefore probably also the initiation, although slower than in the absence of THF, goes on during most of the polymerization, in contrast with the polymerization in pure toluene, where complete consumption of BuLi cannot be reached. Similarly the lower conversion at which the rate levels off in the presence of 10% THF might be due to a smaller number of molecules initiating polymerization. In view of the higher rate of BuLi consumption, as well as of the higher amount of BuLl consumed than in the case without THF, this seems contradictory. However, it should be noted that BuLi is also consumed in side reactions. The number of active centres then depends on the relative impor-

I.~ t ~

tances of the following processes [30, 42]: Active centres

BuLi + M ~

Inactive derivatives of initiator It is sufficient that the side reactions are relatively more important than the combined effect of side reactions and incomplete initiation observed in the absence of THF, to reduce the number of active ends below that in pure toluene. The reaction of BuLi with the nitrile function of the monomer leading to BIK, and probably also with those of the polymer becomes progressively more important with increasing THF concentration (see Table 5). At 10% THF the retardation of the overall process occurs at about the same conversion as for 5%. This can be explained by the fact that the higher rate of BuLi consumption is coupled with a higher production of nitrile adduct. That the cyclization reaction is also favoured by THF addition is demonstrated by the fact that the

[Man] = O,4 M

EBuLt] rn°te" L4x lO-z

[SuL,]=

Temp

1,73 x to 2

:

-75"c

0"I* THF •

,o

0.5

o

o I

200

l

I

I

400

Time in

I

600 seconds =

±

r

2"/,THF I 800

I000

Fig. 7. Polymerization of methacrylonitrile in the presence of T.H.F.: consumption of the initiator.

Anionic polymerization of methacrylonitrile Table 5. Formation of B.I.K. at different THF concentrations THF (vol '+~)

Time (sec)

(BIK)* (%)

2 5 5 5 5 5 5 10

1500 5 15 30 60 300 600 1500

51.6 1.7 4.9 12.4 14.6 56.5 52.5 70.5

[BuLi]0 = 1.73 x 10 -z mole 1-1; [MAN]0 = 0.4 mole 1-t:Temp.= -75. * In "; with respect to the initial BuLi concentration.

polymers are more intensively coloured (orange) and less soluble, In pure T H F the polymer is red. CONCLUSION

In contrast to the rapid initiation of AN by n-BuLi in toluene [27], the initiation of M A N by this initiator in toluene at - 7 5 `+ is not only slow, resulting in complex kinetics due to the propagation taking place simultaneously, but also incomplete. The fact that there is an absolute limit to the consumption of n-BuLl can be explained by assuming the formation of some kind of mixed growing polymer-BuLl particles in which the complexed BuLi is very inactive with respect to initiation. The low efficiency of methanol-insoluble high polymer formation, as compared to the overall initiation efficiency, is shown to be due to the formation of some low molecular weight products. Oligomer formation was claimed to be the main cause of the: very low efficiency of polymer formation in the BuLi initiated polymerization of A N [34, 40]. However, in the latter case, adding together the efficiencies of both oligomer and polymer formatiom and taking into account that in the case of AN ~H3

/ ki 20 °1o (f=0.2)

Ct H ( C H 2- C- Li ÷ CN

kH O~io b

MAN

CH2 Li ÷ Ct, HI0÷ CH2= CN

k Cn 10 ~/° " 12 °/°

!

I CH2= ~

I

/C=

!

C H9

N

. Li

%

(C4HgLi)n UNREACTED

Acknowled.qements--The authors thank Pro£ G. Smets Ior his constant interest and his advice while this work was carried out. Thanks are also due to lic. J. Vandermeulen for his cooperation. Finally the IWONL (Belgium) is acknowledged for a grant to one of us (H.V.).

REFERENCES 1. A.A. Korotkow, S. P. Mitsengendler and V. N. Krasu-

2. D. L. Glusker, E. Stiles and B. Yoncoskie, J. Polym. Sci. 49, 297 (1961). 3. D. L. Glusker, I. Lysloff and E. Stiles. J. Polym. Sei. 49, 315 (1961). 4. D. L. Glusker, R. A. Galluccio and R. S. Evans, J. Am. chem. Soe. 86, 187 (1964). 5. D. M. Wiles and S. Bywater, Polymer 3, 175 (1962). 6. D . ' M Wiles and S. Bywater. Chem. Ind. 1209 (1963). 7. D. M. Wiles and S. Bywater, d. phys. Chem. 68, 1983 (1964).

I 3

C,~HgLI + CH2 = i

°/o - 6 0

the nitrile addition reaction was shown to be negligible [16], still leaves approximately 50-60% of initiator to be accounted for; Erussalimsky et al. considered metallation of the monomer. This amount comes very close to the amount of unreacted BuLi found in this work by direct chromatographic determination of the remaining BuLi. However, unlike the case of MAN where hydrogen abstraction from monomer or polymer may be ruled out and the unreacted n-BuLi has therefore to remain unchanged, this remaining BuLi, in the case of AN, may very well react with the ~ hydrogen not only of the monomer, as claimed by Erussalimsky et al., but also of polyacrylonitrile to form butane because, as Tsuruta [11] pointed out, the propagation is rapid and polyacrylonitrile is produced before n-BuLi is completely consumed. Therefore it may prove useful to extend the investigation of the fate of n-BuLl throughout the polymerization of AN to the experimental conditions used by Erussalimsky et al. in order to establish a detailed balance of the utilization of the initiator. Considering the different reaction possibilities for n-BuLi in the system BuLi/MAN/toluene at - 7 5 ' studied in this work, approximately complete balance for the initiator is found; a comprehensive reaction scheme can be established as represented in Fig. 8. Finally it is noteworthy that at this low temperature the polymerization of M A N by BuLi in toluene, despite its complexities, displays the features typical of the living system.

8. D. M. Wiles and S. Bywater, Trans. Faraday Soc. 61,

CH

50

197

CH

[

[

I 3

k Cycl L

~ iO']o

CH:t-- C - -

CH

~

"C--

CH

"C

[ [ cygC~N/ C~N/"C'~-N I

\

Fig. 8. Reaction scheme: anionic polymerization of MAN at - 7 5 + in toluene.

150 (1964). 9. N. Kawabata and T. Tsuruta, Makromolek. Chem. 86, 231 (1965). 10. Y. Yasuda, N. Kawabata and T. Tsuruta. J. Maeromolek. Sci. AI, 662 (1967). 11. T. Tsuruta, Prowl Polym. Sci., Japan 3, 1 (1972). 12. M. L. Miller. d. Polym. Sci. 36, 203 (1962). 13. A. Zilkha and B. A. Feit, J. appl. Polym. Sei. 5, 251

(1961). 14, A. Ottolenghi and A. Zilkha, J. Polym. Sei. AI, 687

(1963). 15. B. A. Felt, D. Mirelman and A. Zilkha. J. appl. Polym. Sci. 9. 2459 (1965). 16. N. Kawabata and T. Tsuruta, Makromolek. Chem. 98, 262 (1966).

198

H. VANKERCKHOVENand M. VAN BEYLEN

17. C. G. Overberger, E. M. Pearce and N. Mayes, J. Polym. Sci. 34, 109 (1959). 18. C. G. Overberger, H. Yuki and N. Urukawa, J. Polym. Sci. 45, 127 (1960). 19. C. G. Overberger, H. Yuki and N. Urukawa, J. Polym. Sci. 49, 231 (1961). 20. A. Zilkha, B. A. Feit and M. Frankel, J. Polym. Sci. 45, 127 (1960). 21. B. A. Feit, E. Heller and A. Zilkha, J. Polym. Sci. A1, 1151 (1966). 22. I. V. Kulevskaya, B. L. Erussalimsky and V. V. Masurek, J. Polym. Sci., USSR 8, 962 (1966). 23. B. L. Erussalimsky, I. V. Kulevskaya and V. V. Masurek, J. Polym. Sci. C16, 1355 (1967). 24. B. L. Erussalimsky, Plaste und Kaur 15 (11), 788 (1968). 25. B. L. Erussalimsky et al., International Symposium on Macromolecular Chemistry, Budapest. Preprints, Vol. II, pp. 91-94 (1969). 26. B. L. Erussalimsky, J. Polym. Sci., USSR hi3, 1452 (1971). 27. B. L. Erussalimsky and A. V. Novoselova, Faserforsch. Textiltechn. 26 (4), 293 (1975). 28. H. Gilman and A. H. Houben, J. Am. chem. Soc. 66, 1515 (1944). 29. M. Szwarc, Carbanions, Living Polymers and Electron Transfer Processes, Chap. 4. Interscience, New York (1968).

30. B.L. Erussalimsky and I. Krassnoselskaya, Macromolek. Chem. 123, 80 (1969). 31. A. Guyot and P. Q. Tho, J. Polym. Sci. C4, 290 '(1963). 32. A. Guyot and J. Vialle, d. Polym. Sci. B6, 403 (1968). 33. A. Guyot and J. Vialle, J. Macromolek. Sci. A4, 79 (1970). 34. A. V. Novoselova, B. L. Erussalimsky, V, N. Krasulina and Y. V. Zashtcherinsky, J. Polym. Sci., USSR A13, 99 (1971). 35. N. Grassie and I. C. McNeill, J. Polym. Sci. 39, 211 (1959). 36. J. C. Galin, J. Herz and P. Rempp, d. Parrod, Bull. Soc. Chim. Ft. 1120 (1966). 37. N. Grassie and I. C. McNeill, J. Polym. Sci. 27, 207 (1958). 38. N. Grassie and I. C. McNeill, J. Polym. Sci. 30, 37 (1958). 39. A. Blumstein and P. Rempp, Bull. Soc. Chim. Fr. 1018 (1961). 40. B. L. Erussalimsky, I. G. Krasnoselskaya, V. N. Krasulina, A. V. Novoselova and E. V. Zashtersherinsky, Europ. Polym. J. 6, 1391 (1970). 41. I. G. Krasnoselskaya and B. L. Erussalimsky, J. Polym. Sci., USSR AI2, 2532 (1970). 42. I. V. Kulevskaya, B. L. Erussalimsky and V. M. Alekseyev, J. Polym. Sci., USSR A9, 117 (1967).