Anionic polymerization in the homogeneous phase: combination, formation of graft polymers and crosslinking by the action of carbanions on the ester group

ANIONIC POLYMERIZATION IN THE HOMOGENEOUS PHASE: COMBINATION, FORMATION OF GRAFT POLYMERS AND CROSSLINKING BY THE ACTION OF CARBANIONS ON THE ESTER GROUP* 1~. R E M P P , V. I . VOLKOV, Z H . P A R R O a n d SH. SADRO1% I n s t i t u t e of Hetero-organic Compounds, U.S.S.R. Academy of Sciences Centre for Research on Maeromolecules, Strasbourg (France)

(Received 12 May 1960) IN recent years ionic polymerization has been the subject of many investigations. However of greatest interest is anionic polymerization in the homogeneous phase, which makes possible the preparation of block copolymers with different numbers and different consecutive order of units in the chain, and also of a, o~difunetional polymers. Szwarc [1] was one of the first to synthesize and explain the mechanism of formation of the polymers which he called "living" polymers. A polymer of this type remains in solution and its two earbanionic ends are capable of initiating the polymerization of a second monomer, giving a block eopolymer [2, 3, 4]. Moreover these carbanions possess high chemical activity toward all proton donors and toward a large number of compounds containing electron-accepter groups [5, 6]. Thus a, w-diketone polymers are obtained b y the interaction of esters and "living" polymers. For example ethyl benzoate reacts with them in the following w a y

~CH~--CH: (-) + C~H0~C--~--~C2H~OJ~(--) ÷ ,,-CH~--CH--C--(-~ cp (~ 0 This is a general reaction since there are many papers in which the combination of carbanions with esters of dibasic acids is described. In addition to that already mentioned [7] m a y be added the combination of a carbanion with dimethyl terephthalate. The anionic polymerization of methyl methacrylate. I t seemed to us to be of interest to investigate the phenomena occurring during the anionic polymerization of methyl methacrylate (MMA). This vinyl monomer possesses a powerful electronegative group and can be polymerized in the presence of basic catalysts, but at the same time it is an ester and hence in this case the occurrence of auto* Vysokomol. soedin. 2: 1%o. 10, 1521-1531, 1960. t This work was carried out at the Centre for Research on Macromolecules, St.rasbourg (France). 398

Anionic polymerization in the homogeneous phase

399

deactivation is possible. This was pointed out in the work of Szwarc [2] and moreover the occurrence of the spontaneous deactivation of the earbanions of polymethylmethaerylate (P!~,LMA) has been reported [8, 9]. The fact t h a t MMA can be polymerized anionically was confirmed in references [8, l0 and 11] where 9-fluorenyllithium and organo-magnesium compounds were used as initiators. The sodium-naphthalene complex also initiates the polymerization of vinyl monomers at low temperatures. We used this catalyst and also the "living tetramer" obtained b y reacting ~-methylstyrene with excess sodium in tetrahydrofuran (THF) at room temperature [6]. The results showed t h a t the carbanionic affinity of MMA is greater t h a n t h a t of styrene: (1) whereas "living" polystyrene can be used to initiate the polymerization of MMA the reverse is not true [14]; (2) when a mixture of equimolecular amounts of styrene and MMA is polymerized in the presence of an anionic initiator only the MMA polymerizes; (3) we used a new anionic catalyst (1,1,4-tetraphenyl-butane-l,4-carbanion), obtained b y the action of excess sodium on 1,1-diphenylethylene at room temperature, and it was found t h a t this compound catalyses the polymerization of MMA at a low temperature in T H F but does not bring about polymerization of styrene. These three series of experiments indicate t h a t the MMA ion CH3 l( ) ~CH~--C: 0 (;--OCH~

(--) is a more stable earbanion t h a n t h a t of styrene ~ C H 2 - - C H : , and the latter

I is more reactive toward the ester group t h a n is the MMA ion. The reaction of styrene carbanions with the ester groups of P M M A . The mechanism of the reaction between styrene carbanions and the ester groups of various compounds has been discussed previously by one of us [7]. The infrared spectra of the products obtained clearly indicate the presence of ketonic groups in these compounds. The ester groups in PMMA are less reactive, as in the case of their saponification, but if the reaction in which we are interested is possfl31e then the coloured solution of "living" polymer should be decolorized, indicating t h a t reaction has occurred. The "living" polymer obtained by the action of sodium-naphthalene or "living tetramer" on the monomer is bifunctional and PMMA is polyfunctional. I f reaction takes place then as a result of reaction between the earbanions and the ester groups of different chains we should expect crosslinking cf these chains. On reacting the red solution of "living" polystyrene with a solution of PMMA in dry T H F (in an atmosphere of argon) we observed a progressive increase in viscosity and then gelation with the simultaneous disappearance of the colour of "living" polystyrene. 27 Polymer 3

P. REMPP et al.

400

The white polymer, after treatment with methanol to remove sodium methylate, was insoluble in all the usual solvents, in some of which it swelled a little.

The spontaneous deactivation of "living" P M M A We carried out a few experiments with different catalyst: monomer ratios and we never observed a n y crosslinkirg. However spontaneous deactivation takes place and it proceeds more or less rapidly dependirg on the temperature and the concentration of earbanions, and it was found t h a t its rate depends on the degree of conversion of monomer to polymer. Explanations of this deactivation have been given in a number of papers. In the first of these [12] it was stated t h a t here the migration of a methyl group is possible, giving a free carboxyl group according to the scheme CH3

CH3

'( ~CH2--C -):O \ .~

I

--~CH~--:2 CH3

I//o c -sL(-)

C-OCH3

This hypothesis is supported by the presence of earboxyl ions, which are associated in THF. We also observed this phenomenon when the " l i v i r g " polymer was deactivated with dry, gaseous CO s at a low temperature. The association decreased when the polymer solution was acidified: CHa

~CH2.C

t

(-):

CH3 I

+CO~-~CH~--C--COO

I/9

CH.~

(-)

H+

*~CH~-C--COOH

' C0OCH,

, COOCH3

C--OCHj or when a solution of "living" PMMA was allowed to stand for some time, when no charge in carbanion concentration with time or g~lation of the PMMA solution was observed. Other authors [9] consider t h a t cyclization is possible, according to the scheme:

CH,

O

CHa

I

II

I

CH, ( - ) :C -COOCH a CH.~C-- C--COOCHs ,~CH.~--|/COOCH3C I (- ) I/ \ CH~ -~CH30 + ~CH~--C CH~

\

/

CH,

C--C00CH3 CH3

\

/

CH~C--COOCH3 CH3

I t is impossible to confirm or reject this hypothesis because on the one hand cyclization is very difficult to prove, either chemically or spectroscopically and on the other hand it is impossible to understand why the ester group of a third molecule in a chain is more reactive than the others. I f this were the case the polymerization of the MMA would have to be stereospecific, however

Anionic polymerization in the homogeneous phase

401

under our experimental conditions this did not occur. In our view it m a y be assumed that the carbanions are deactivated b y monomer in the following way:

CH 3 I

~-,CH~--C :(-)

~fo C--OCH 3

0%

-{-

OHiO /

/CH3 0-0

CH~ O CH, I l{ J

--+~CH~--C-- C--C-- OHm-{-CH;,O (-)

%OHm.

//o C--OCH a

I t is known that the polymeric dicarbanion is in equilibrium with its monomer a n d that (dependirg on the temperature) polymerization or depolymerization can take place i.e. the chain can remain active or inactive. It is the monomer that is in equilibrium that deactivates the "living" PMMA. B u t the monomer that has deactivated a carbanion can easily split off ~gain thus formirg monomer capable of reactirg with another carbanion etc. I n order to confirm this hypothesis the f)llowipg experiments were carried out. (1) Deactivation of "livirg" PMMA already begins a t - - 6 0 ° when polymerization has still not reached 100~/o. Moreover if after 0.5 hours after the end of polymerization the reaction mixture is allowed to heat up to room temperature deactivation proceeds more slowly. (2)When the polymerization of PMMA is initiated at room temperature b y the "living tetramer" of a-methylstyrene spontaneous deactivation proceeds more rapidly i.e. each molecule of MMA has a 60% chance of reactirg with a double bond (props g~tion) and a 40~o chance of reacting with an ester group (deactivation). It was thus established that to each molecule of "livirg tetramer" there are on the avereg~ about five molecules of MMA. I t is interestipg to note here that for cyclization there must be at least three molecules of MMA at each end of the oligomer and cyclization is impossible if there are only two. However we were unable to prove the presence of a double bond b y means of infra-red spectra, and reaction with Br~ in CC14 was very slow. We should also emphasize that under th~se conditions crosslinking did not take place. Monofunctional catalysts--graft copolymers. I f as an initiator of anionic polymerization a moncfanctional catalyst is used instead of a bifunctional, which gives rise to crosslinkirg of the PMMA chains, the possibility of obtaining graft polymers arises. However certain conditions must then be satisfied: (a) the catalyst must b e soluble in T H F and must not react with it. This requirement is not met, for example, b y butyllithium; (b) the rate of initiation must be equal to or greater than the propegation rate in order to obtain "livirg", monofunctional polymers of low molecular weight. We carried out experiments with phenyllithium, which is a good catalyst f)r anionic polymerization in the homegeneous phase, b u t it proved to be unsuitable f)r our purpose. The moncfanctional initiators used were benzylsodium (C6HsCHaNa) and phenylisopropylpotassium (C6H~C(CH3),~K). These catalysts are ionized in T H F and give a red-violet solution. A charg~ in colour to red-orarg~ occurs on addition c,f the very first drop of monomer to the catalyst solution indicating the formation of the styrene anion. 27*

402

P. RE~fl)Pet al..

w e grafted the "living" monocarbanion of polystyrene (of known molecular weight) to ~he PMMA molecule b y reaction with the ester groups i.e. PMMA molecules were prepared with some grafted polystyrene side chains. The lightscattering of solutions of all the graft polymers obtained was studied. In view of the fact that the difference between the refractive indices of PMMA and benzene is zero it is possible to measure the intensity of diffusion of polystyrene (blank test) and of a graft polymer containing a very small amount of PMMA in relation to the grafter polystyrene. B y this means it is possible to say definitely whether or not grafting has taken place. The ratio of styrene to PMMA in our Case 20:1 (by weight). For a control we measured molecular weights in other ~olvents with refractive indices different from that of PMMA (for example dioxan and but~none}, and in which the total molecular weight of the eopolymer could be determined. As Table 2 shows it differs little from the molecular weight determined in benzene. EXPERIMENTAL

Purification of reagents and design of apparatus. The preparation of the catalyst, polymerization and deactivation must be carried out in an inert atmosphere free from all possible traces of oxygen and water. We worked in an atmosphere of argon in apparatus described previously [6]. The argon was purified b y passing it over copper turnings heated to 450 °, and then for complete removal of moisture it was passed through a column packed with KOH. Tetrahydrofuran (THF) was treated with K O H , distilled twice from sodium and stored over sodium. Usually the solvent contained not more than 10 mg of water per litre of solvent. Styrene and methyl methacrylate (MMA) were freed from inhibitors and then distilled twice in vacuo over sodium. Methyl methacrylate was degassed in the frozen state under high vacuum. Preparation of catalyst. Sodium-naphthalene was prepared b y a method described previously [6]. The "living tetramer" of a-methylstyrene was prepared b y adding a solution of ~-methylstyrene in T H F dropwise to an excess of sodium. A red coloration developed rapidly b u t the reaction was continued for a further f e w hours for completion. To determine the degree of polymerization of this oligomer we deactivated it with gaseous CO2 and then analysed the product and determined its acid number. The degree of polymerization of the oligomer varied between 3.8 and 4.3. The "living dimer" of 1,1-diphenylethylene was prepared b y the same method. However the eolour of the solution was originally blue and this subsequently turned to red. The purity of the product was determined b y the method described previously [13] for 2,2,5,5-tetraphenylhexanedicarboxylic acid. Benzylsodium and phenylisopropylpotassium were obtained from the firm O R G M E T in the form of suspensions in toluene or hexane. When these hydrocarbons were decanted off and T H F was added to the residue stable, dark-red solutions were obtained, typical of carbanionie initiators.

Anionic polymerization in the homogeneous phase

403

Polymerization. The method was the same as t h a t described previously byi one of us [6]. A concentrated solution of monomer (for example l0 g of monomer in 20 ml of THF) was added dropwise to a known quantity of catalyst dissolved in 100 of T H F and cooled t o - - 7 8 °. One portion of the dicarbanionic polymer, prepared in this way, was removed and deactivated with methanol, while a. deactivating agent--monomer or polymer--was added to the other Imrtion, Some deactivation reactions were curried out a t - - 7 8 ° and others a t highe~ temperatures. RESULTS

(a) Combination with dimethyl terephthalate. This reaction was carried out with both the "living tetramer" o£ ~-methylstyrene, regarded as a short chain polymer, and with the monocarbanion of po]ystyrene obtained by polymerization with phenylisopropylpotassium us initiator. The infra-red spectra of films deposited on NaC1, recorded in a Perkin-Elmer Model 21 spectrometer, indicated the presence o£ carbonyl groups, C=O, in the a-position to the benzene ring

Ar-G={

r

2/z

4

#

#

/4

i0

/5' ''

i [

Ar-C=0 (b)

2#

4

#

8

i,

lO

12

1~ ~,'! ,i.15

FIG. I. Infra-red spectrum: a--of the product obtained by combination of the monocarbanion of polystyrene with dkmethy] terephthalate (Exper!: ment No. 295); b--of the polycondensation product from "livingtetramcr" and dimethyl terephthMate.

P. REMPP ¢t aZ.

404

(1680 cm-1), and also of substitution in the para position (830 era-l). An increase in viscosity in toluene from [~]tetramer= 2"8 to [~/]reaotionproduct=7"24 and the infra-red spectra indicate t h a t reaction had taken place (Fig. la and b). The reaction between the monocarbanion of polystyrene and dimethyl terephthalate, i.e. dinlerization, also proceeded satisfactorily. From a comparison of the molecular weights of the original polystyrene (mol. wt. 12,500) and of the reaction product (tool, wt. 18,600) it is seen t h a t of the four ester groups of dimethyl terephthalate (present in the mixture) three had reacted. (b) Polymerization of a mixture of styrene and M M A . This reaction was carried out with sodium-naphthalene or "living tetramer" as catalysts. The mixture of monomers was added to a solution of the catalyst in T H F a t - - 7 8 °. The polymers obtained were precipitated, washed, dried and analysed. As is seen from Table 1. all the polymers obtained are practically pure PMM_A (calculated content, %: C 59-85, H 7.96). TABLE 1. COMPOSITIONOF PRODUCTOF POLYMERIZATION OF MIXTUREOF STYRENEAND MMA

"

Experiment No. 246 208 213 247

Catalyst

Volume of polymer (%) H C

Sodium-naphthalene Tetramer Sodium-naphthalone

248

60.32 60.59 59.02

8-11 8-26 8-10

63.10 60-71

8.23

T h e infra-red spectrum, corresponding here to sample No. 246 (see Fig. 2), also indicates the absence of polystyrene.

I

Ester ~e

4"

8

8

10

fZ

•

/4

/5

FIG. 2. The infra-red spectrum of the product from the polymerization of a mixture of styrene and methyl methacrylate in the presence of the soditun-naphthalato complex (Experiment No. 246).

Anionic polymerization in the homogeneous phase

405

(c) Experiments on the crosslinking of PMMA. These experiments were carried out under various conditions, with varying molecular weights of the dicarbanion of polystyrene and of the PMMA, which was added to a solution of the "living" polymer. Normally the styrene was polymerized at a low temperature and a control sample (for determination of molecular weight), removed just before addition of the PMMA, was deactivated with methanol. After addition of the PMMA an increase in viscosity, gel formation and progressive disappearance of the red co]our was observed. Network formation (crosslinking) was very slow at --75 ° but was more rapid at room temperature. When the PMMA used for crosslinking had a high molecular weight crosslinking commenced much earlier than with a sample of low molecular weight. Crosslinking was general because the polymer obtained was insoluble in all solvents and was only slightly swollen in C6H e and T t t F and no soluble fraction was isolated in these cases. Example. 20 g of styrene was mixed with 12 ml of a solution of "living tetramer". The molecular weight of the polystyrene obtained (control sample), determined b y the ]ight-scattering method in benzene, was 12,000. 2 g of PMMA (tool. wt. 69,000) in 10 ml of T H F was added. Cross]inking was complete in 20 minutes at --30 °. (d) Spontaneous deactivation of "living" PMMA. The first experiments on polymerization of M~IA were conducted at --75 ° or -- 100 ° and samples of"living polymer" removed were deactivated with methanol or CO 2. The remaining "living" polymer was kept for some time (2-3 hours) at room temperature. I t was found t h a t during this time spontaneous deactivation occurred but the molecular weight remained unchanged. Example. The molecular weight, determined b y the light-scattering method, was 57,000 for the control sample and 54,000 for the spontaneously deactivated polymer. A second series of experiments on the polymerization of MMA was carried out at room temperature, again using the "living" tetramer of a-methylstyrene as catalyst. Here an oligomer was not obtained because there is competition between the propagation and deactivation reactions. Elementary analysis of the products shows t h a t 4 to 5 molecules of methyl methacrylate are a t t a c h e d to each molecule of tetramer if it is assumed t h a t two of them have lost the methoxy-group by reaction.

Example: Experiment No. 249 254 Carbon content, %: found 78.66* 79-37t calculated 78.4" 79"5t The infra-red spectrum of sample No. 249 indicated the presence of C= 0 groups (Fig. 3). Under the same conditions methyl acrylate reacts more readily * For 5 molecules of I~IMAper mole of tetramer. t For 4.5 molecules of ~ _ per mole of tetramer.

406

P. REm'P et at.

I I I

2,~

I

f~

i ~ter CO~ Ketone I

4

8

8

I

10

12

14

15

FIG. 3. The mfr~-red ~pec~rLun of thv produc~ ~b~ah~c4 by Giu ac~iol~ oi methyl methacrylate on "living tetramer" at room temperature (Experiment No. 249). with its own ester group and is correspondingly less reactive toward its own double bond. Only three molecules were combined with one molecule of the tetramer of a-methylstyrene.

Example: Experiment No. 260 Carbon content, %: found 83.3 calculated 83.2

(e) Graft polymers. The same experimental method was ~used here as for polymerization. The polymerization of styrene was initiated by benzylsodium or phenylisopropylpotassium at a low temperature, with stirring and in an inert atmosphere. A portion of the red solution of "living" polymer was removed and deactivated with methanol. Then at a low temperature a concentrated solution of PMMA in dry T H F was added rapidly, after which progressive disappearance of the red colour of the styrene carbanion began, but no gelation occurred in this case. The polymers obtained (polystyrene and eopolymer) were precipitated with methanol, washed and dried. The elementary composition was then determined, the infra-red spectra plotted and, finally, the light-scattering was measured. The difference between the refractive indices of pure PMMA and benzene is zero, hence in this solvent the graft polymer can be characterized in two ways: (a) dn/dc, measured for the copolymer in relation to its concentration, can be used, and from which the molecular weight can be determined; (b) again dn/dc, measured for polystyrene in relation to its concentration in the polymer, can be used, and this enables the average molecular weight of the polystyrene in the copolymer to be calculated. The latter procedure enabled us to determine rapidly the amount of grafting because there was no difference between the

407

Anionic polymerization in the homogeneous phase

molecules o f free p o l y s t y r e n e a n d the g r a f t e d molecules where one p o l y s t y r e n e chain was g r a f t e d on to a PMMA chain. I t is interesting here to show the infra-red s p e c t r u m o f e o p o l y m e r No. 313 (Fig. 4) in which the lines characteristic o f p o l y s t y r e n e a n d PMMA are clearly seen. As is seen f r o m Table 2, in four cases we o b t a i n e d an insignificant a m o u n t

I

0

5

/g

/2

1~

/5

FI(~. 4. The infra-red spectrmn of the graft copolymer of znu~liyl methacr 5 late and styrene (Experiment No. 313). of grafting of p o l y s t y r e n e to PMMA, e v i d e n t l y because the PMMA was b a d l y freed f r o m m e t h a n o l , as a result of which p r o t o n d e a c t i v a t i o n o f the monof u n c t i o n a l p o l y s t y r e n e was possible. I t should be n o t e d t h a t the fluctuation in the composition o f the various copolymers is n o t of great significance. I n the last T A B L E 2. V A R I O U S GRAFT POLYMERS

[M w

Benzene Experiment No.

Catalyst M w

trial 272 275 277

Benzylsodium

286

Phenylisopropylpotassium

297 310 313

9~

210,000 245,000 265,000 28,500 (butanone) 6840 25,800 11,600 13,950

co- M w co* polymer i polymer I and M wuse PMMA and M w co- butanone dioxane I polymer r

230,000 I 245,000 166,000;

i

202,000 202,000 205,000

I

i

13,500 44,100 453,000 585,000

--

54,800 --

i i

50,600

69,000

i 672,000

two c o p o l y m e r samples, o b t a i n e d f r o m PMMA t h a t was purified with g r e a t care, the n u m b e r of p o l y s t y r e n e grafts on the PMMA chain (mol. wt. 69,000) was high (39 a n d 42 respectively), a n d the yield o f g r a f t p o l y m e r was 40%.

408

P. REMPP et a/. CONCLUSIONS (1) T h e r e a c t i o n o f t h e s t y r e n e e a r b a n i o n , - - C H 2 - - C H ( - ) :, w i t h ester g r o u p s

I

is general, a n d w h e n t h e y a r e t h e e s t e r g r o u p s of P M M A it is posible to o b t a i n erosslinked p o l y m e r if t h e " l i v i n g " p o l y m e r s are bifunctional, a n d g r a f t p o l y m e r s if t h e " l i v i n g " p o l y m e r (grafting o n to t h e PMMA) is m o n o functional. I n t h e l a t t e r case it is possible t o v a r y t h e n u m b e r of chains g r a f t e d t o one molecule of P M M A b y v a r y i n g t h e r a t i o o f t h e t o t a l a m o u n t s o f c a r b a n i o n s t o e s t e r g r o u p s in t h e m i x t u r e . (2) T h e m e c h a n i s m o f s p o n t a n e o u s d e a c t i v a t i o n is discussed. T h e s t a b i l i t y o f t h e MMA ion does n o t allow d e a c t i v a t i o n b y t h e e s t e r g r o u p s o f P M M A t o b e considered. T h e h y p o t h e s i s is p u t f o r w a r d t h a t t h e e s t e r g r o u p o f t h e m o n o m e r is m o r e r e a c t i v e a n d is responsible for t h e p r o g r e s s i v e d i s a p p e a r ance o f t h e c a r b a n i o n o f m e t h y l m e t h a c r y l a t e .

Translated by E. O. PHILLIPS REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

M. SZWAR[~, Makromol. Chem. 35: 132, 1960 M. SZWARC and A. REMBAUM, J. Polymer Sei. 22: 189, 1956 M. LENG and P. REMPP, Compt. rend. 250: 2720, 1960 M. LEVY, J. Amer. Chem. See. (in the press) M. SZWARC, ]Nature 178: 1168, 1956 P. REMPP and M. H. LOUCHEUX, Bull. Soc. Chem., p. 1497, 1958 W. H. STOCKMAYER and P. REMPP, J. Amer. Chem. See. (in the press) D. GLUSMER, E. STILES and B. YOUKOSKIE, American Chemical Society, Boston Meeting, April 1959 H. S{~HREIBER, Makromol. Chem. 36: 86, 1959 T. G. FOX, J. Amer. Chem. See. 80: 1768, 1958 O. GHAZNAVI, G. CHAMPETIER and P. SIGWALT, Compt. rend. 250: 3836, 1960 H. BRODY, D. H. RICHARDS and M. SZWARC, Chem. Ind., p. 1473, 1958 W. SCHLENK and E. BERGMANN, Liebigs Ann. 463: 1, 1928; G. WITTIG and F . y o n LUPIN, Ber. 61: 1632, 1928 R. K. GRAHAM and D. L. DUNKELBERGER, J. Amer. Chem. Soc. 82: 440, 1960