Study of the reaction of styryl aluminium with benzoyl peroxide and its use for the synthesis of block copolymers

Reaction of s t y r y l ahuninium with benzoyl p3roxide

1873

3. 4. 5. 6. 7.

R. D. SUDUTH and C. E. ROGERS, P o l y m e r Letters 11: 241, 1973 P. DRECHSEL, J. L. H A A R D a n d F. A. LONG, J. Polymer Sol. 1O: 241, 1953 G. F. DALETSKH, Dokl. A N SSSR 54: 313, 1946; 74: 625, 1950 R. S. STEIN, J. P o l y m e r Sei. 24~ 383, 1957 V. N. TSVETKOV, K. A. ANDRIANOV, M. G. VITOVSKAYA a n d E. P. ASTAPENKO, Vysokomol. soyed. A14: 2603, 1972 (Translated ia Polymer Sci. U.S.S.R. 14: 12, 3032, 1972) 8. A. E. GRISHCHENKO, M. G. VITOVSKAYA, V. N. TSVETKOV, Ye. P. VOROB'YEVA N. N. SAPRYKINA a n d L. I. MEZENTSEVA, Vysokomol. soyed. A9: 1280, 1967 (Translated in Polymer Sci. U.S.S.R. 9: 6, 1430, 1967) 9. A. K. KHRIPUNOV, O. P. KOZ'IKINA, I. N. SHTENNIKOVA and G. I. OKHRIlVIENKO, Zh. prikl, khimii 18: 2581, 1970 10. A. N. CHERKASOV, M. G. VITOVSKAYA, N. N. MAKAROVA, K. A. ANDRIANOV a n d V. N. TSVETKOV, Vysokomol. soyed. A16: 2494, 1974 (Translated in P o l y m e r Sci. U.S.S.R. 16: 11, 2897, 1974)

STUDY OF THE REACTION OF STYRYL ALUMINIUM WITH BENZOYL PEROXIDE AND ITS USE FOR THE SYNTHESIS OF BLOCK COPOLY1KERS* L. V. ZA~OISKAYA, E. S. GAI~KINAand YE. B. MmOVSKAYA I n s t i t u t e of High Molecular Weight Compounds, U.S.S.R. A c a d e m y of Sciences

(Received 10 February 1976) S t y r y l aluminium (StA1) was synthesized with a degree of polymerization for each aluminium bond of 2:30. I t was shown t h a t under model conditions S t i l reacts with benzoyl peroxide (BP) v e r y rapidly a t 0 ° and the reaction is accompanied b y the formation of free s t y r y l radicals; these are formed over a period of t i m e with the introduction of a strong electron d o n o r - - p y r i d i n e (Py). The StA1 (p----30) - B P - P y system is used for low t e m p e r a t u r e radical polymerization of monomers of acrylic and methacrylic series. B y polymerization of methyl methacrylate (MMA) a s t u d y was made of process kinetics, a relation established between the rate a n d concentration of components of the initiating system a n d P y and overall a c t i v a t i o n energy evaluated. Using kinetic relations obtained views were expressed concerning the initiation mechanism a n d the effect of Lewis a c i d i t y of the organo-aluminium component on the chemistry of this stage of the process. A block copolymer d was isolated a n d p a r t i a l l y described for a St-MMA pair.

Interaction between aluminium alkyl (Ali%3, where R is ethyl or isobuty!) and benzoyl peroxide (BP) or percarbonate takes place as a reaction of homolytic substitution at the metal atom to fo~m free alkyl radicals [1-4]. The process takes place with practically zero activation energy and in monomer medium r e s u l t s i n l o w t e m p e r a t u r e r a d i c a l p o l y m e r i z a t i o n [5, 6]. I t w a s i n t e r e s t i n g t o * Vysokomol. soyed. A18: Iq'o. 7, 1635-1641, 1976.

L. V. ZAMOISKAYA et al.

1874

establish the effect of the type of radical and its M ~ T on the chemistry of the reaction between A1R3 and BP, in order to use it for the synthesis of block co: polymers, when R is a high molecular weight radical. J

a

~ I0~

If l /i I

X

20

I 2O

l

~ qO FIG. 1

I

1 60 (, m m

i

18

I

i

16

I

I

l/-I

I

12 v~ lO-2 cm -t

FIG. 2

Fro..1. Densitograin of a PS sample: 1--initial; 2--after reaction with BP, positions of standard PS samples and their MW: A--lxl01; B--5×10s; G-- 2 x 10s; S--degree of absorption; ~length of the plate. FIG. 2. IR spectra of reaction products of BP with A1Ets (a) and StA1 (b). This paper is concerned with the synthesis of an organ'oaluminium derivative, including a high molecular weight one, the study of its interaction with peroxide~ the process of polymerization and characteristics of the end product. Styryl aluminium (StA1) was used as organo-aluminium compound with a degree of polymerization /~ of styrol (St) per aluminium bond of 2 to 30. These compounds were obtained by polymerization of St by the action of heat in t h e presence of A1Et8 as a chain transfer agent. Under the conditions selected ([St]! ----6 mole/1, [A1]/[St]=(0"5-3)x10 -~, 120°, 8 days, toluene being the solvent} two ethyl groups are fully substituted and a third group is partially replaced by a styryl chain; the degree of substitution of the latter group in the range of ratios indicated is 40-70%. The average length of the styryl chain is given b y the chain transfer constant (Ct=20) [7], which enables MVf to be calculated from the ratio [St]/[A1].n, where n is the number of substituted groups. The accuracy of this calculation was shown using compounds with P = 2 and 4 by the cryoscopic measurement of M-W (values found: 233, 355; calculated: 238 and 422). For a compound with ~ = 3 0 Figure 1 (curve 1) shows lVlWD obtained by thin-layer chromatography. I t can be seen that MW of the main fraction agrees with t h e value calculated.* StA1 ( P = 2 - 3 0 ) , in contrast to aluminium alkyls even with comparatively high molecular weights (C14-1s), is a weak Lewis acid, which is first of all confirmed by the absence of colour transitions of the methyl violet indicator [8]. At the same time it is able to form fairly strong complexes with * Narrow fractions of PS produced by the firms Waters (U.S.A.) were used as standards.

Reaction of styryl aluminium with benzoyl peroxide

1875

strong bases, particularly with amines of aliphatic series; this property was used for the quantitative determination of an active organo-aluminium compound. In the same w a y as aluminium alkyls, StA1 reacts irreversibly with BP. The reaction takes place at a temperature of 0-20 °* and corresponds to the stoichiometry of A1 : BP----1 : 0.75 (0.8), i.e. approximately 4 : 3. In contrast to alumirdum alkyls, in the reaction of StA1 this process only takes place to the end with a fairly large peroxide excess. Complex formation between end products and the initial organo-aluminium compound is, apparently, significant in this case, this reaction impeding iurther interaction with peroxide (with a shortage of the latter) or under strict stoichiometric conditions (Table 1). TABLE 1. STOICHIOMETRYOF THE REACTIOI~OF STYRYL ALUM1NIUM WITH BP (15 rain, 0°; solvent : toluene*) Degree of polymerization

Molar ratio of AI: BP 1:1 1:1 1:2

30

Peroxide found, ~/o of the initial value 26 25* 125

1 : 0.75 1:1 1:1.5

20 74

1:0.8 1:1 1:2

16 117

8

5

* Average numbers of several experiments.

t After18hr. Interaction of StA1 with B P is preferably carried out at the A1-St bond and passes through the stage of free styryl radical formation. This conclusion follows from an analysis of results concerning M W D of PS obtained afte~ the reaction. Figure 1 (curve 2) shows a shift of the M W maximum in the direction of increase; the effect m a y only be observed in the presence of styryl radicals and subsequent recombination. The A1-Et bond is affected slightly (3-5% of the initial) and this process is not accompanied b y free ethyl radical formation, which follows from the identity of the composition of gaseous products formed before and after the reaction with BP. Free benzoate radicals are not formed. This follows from the absence of C02 and a practically complete balance at t h e benzoate groups (BG) (Table 2). * Due to the high viscosity of the operating solution, no study was carried out at lower temperature.

1876

L.V.

ZAMOISKAYA e$ a~.

According to the stoichiometry established (A1 : BP----4.3) and products isolated, the formation of both aluminium monobenzoate and dibenzoate could be anticipated. TABLE 2. PRODUCTS OF IN*I'ERACTION OF STYRY'L ALUM:INIUM ~VITH B P *

([BP]--0.18 mole/L; 0°)

P

4 30

[StAll, molo/l.

0.5 0.05 t

Molar ratio of A1 :BP 1 : 0.75 1 : 0-8

Found in reaction products of BG, % of theory A1 bonzonate 80 82

product to ] be uncreated saponified

BPt

Balar~ee according

to BG, % of theory

8

8

96

9

5

96

* Average results of several experiments. ? The viscosity of the reaction medium has a marked effect on the quantitative composition of products formed. Therefore, when studying the Interaction of StA1 ( P = 30) with BP more dilute solutious of an organo-alnmlnlum compound were used than for StA1 (P=4). $ With an A1 : BP ratio of 4 : 3 the process is incomplete (see Table 1).

I t can be seen from I R spectra of dihydroxy aluminium benzoate obtained from the reaction of A1Ets with BP, followed by the decomposition of the reaction mixture with water (Fig. 2, spectrum a) and hydroxyaluminium benzoate obtained from the reaction of StA1 with B P and corresponding treatment of the reaction mixture (spectrum b) that, the intensity of characteristic absorption bands of C O 0 - groups (range of 1560 and 1440 cm -1) is much higher in spectrum b, which confirms the earlier assumption (identical aluminium benzoate samples were used in both cases). Results enable a general layout of the process to be presented 2(St)sAIEt-t-BP ~- {2(StsAIEt).BP} --~ 2EtStA1OCOCeHsA-2S~ 2(St)sA1Et+ 2BP ~

2{St,A1Et. B P } - - : EtAI(OCOCsHs),A-2St" EtStA1OGOCeHs~-0sHsCOOSt

The possibility of controlling the rate of free radical formation, in othe~ words, the rate of interaction between StA1 and BP" is the main condition facilitating the use of the system studied as low temperature initiator. For systems based on alkyl aluminium (during the polymerization of a non-polar monomer--vinylchloride) this was achieved by further addition of an electron-donor (ED) [6]. The role of E D in this case is the formation of a complex with A1Eta, which reduces the concentration of the latter in solution. Since alkyl aluminium which has not formed complexes takes place in the reaction with BP, free radical formation slows down. During polymerization of polar monomers by the action of A1Rs-BP systems free radicM formation was controlled without further addition of

R e a c t i o n of s t y r y l a l u m i n i u m w i t h benzoyl p e r o x i d e

1877

ED by the simple separation of components of the system with the monomer (see a previous study [5] dealing with the chemistry of free radical formation). A Lewis acidity of StA1 much lower than for alkyl aluminium is observed on adding relatively weak ED, particularly ethylacetate (EA) and methyl methacrylate (MMA) and this has no effect on the rate of the reaction with BP; this reaction takes place fairly quickly in this case. * The reaction only slows down in the presence of a strong ED pyridine (Py). Corresponding results are shown in Table 3, which also shows results for an AIEt3-BP system for comparison. TABLE 3. REACTION OF S t A 1 ( P = 3 0 )

WITH B P x~q THE PRESENCE OF ELECTROIq-DOlqORS

([ED]/[A1] = 50; [BP]/[AI] = 2; 15 min, 0°; s o l v e n t - t o l u e n e *) •ED MMA EA Py MMA EAt

R e m a i n i n g p e r o x i d e , ~/o 2.7 9.0 95.0 63.0 92.0

Note P e r o x i d e excess t a k e n for t h e r e a c t i o n w i t h A1 : B P > 4 : 3, the c o l u m n does n o t c o n t a i n the "remaining peroxide" A1Et3-BP; [BP]/[AI]=0.5; [ED]/[A1]= 50; 0 ° Same, [ED]/[A1]=25; 25 °

* The order of reagent feed is always the same, namely A1Ra+F,D+BP. t Taken from a previous study [9].

According to results obtained in model experiments conditions are found for using the StA1-BP system as initiator of the polymerization of esters of acrylic and methacrylic series. The process only develops with a quantitative conversion with further addition of E D - P y to the system; on using a two component system, polymerization ceases long before the monomer is used up, this being the result o f using up the initiating system at a very high rate (Fig. 3). Kinetic relations o f the process in a StA1-Py-BP system were established by polymerization of MMA; StA1 with P = 3 0 was used as organo-aluminium compound. Experimental relations between the overall rate of the process and the concentration of BP, StA1 and Py suggest the following formula for the general rate of polymerization F A1 "30.5

(K being the overall constant of polymerization and M--monomer). This relation is identical for the A1Ets-BP system in polymerization of a non-polar monomer. It is a reflection of the fact of free radical formation as a result of the reaction of StALl with BP without complex formation, which takes * See p r e v i o u s p a p e r s [5, 6] dealing w i t h c o m p l e x f o r m a t i o n properties o f E D a n d p o l a r m o n o m e r s in relation lm alkyl a l u m i n i u m . t T h e order for t h e m o n o m e r was one.

1878

L.V.

ZAMOISKAYA et ~ .

place the same w a y as in the model experiment StAI-t-ED

x,,,,.,[SIA1.ED]

,.

StAI+BP ~ [StAI.Bp] ~ 2 SiAl $ k, St-{-Al-dibenzoate

?),.-,

(1) St+Al-benzoate

(2) (3)

When equilibrium is rapidly established, with complex formation Kequi displaced to the right and reactions with constants KzK~, to the left and rates of free radical formation are determined b y reactions with constants /~l and /~2, we derive a formula of the rate of initiation established experimentally.

0

40

-

/ / 3 ./ ~.~.....-~-~ /// ---''x'- 2 L~j ~....-

-

.....

0

5

10' 20 Time, hi,

0

i

1 0

25

FIG. 3

3"5

3"7

J'9 IOa/T, K -I

FIG. 4

Fie. 3. Polymerization using the StA1-BP system; P=30. Solvent: toluene, 00; [A1]--2× × 10-=; [BP]=3 × 10-= mole/l; 1 -- [MMA]=6.6 mole/1, without ED; 9--[MM&]=3.0 mole/]. [A1] : [Py]= 1:1; 3--[2-ethylhexylmethaerylate]= 3.0 mole/l.; A1 : P y = 1:1; g--[methylacry. late]=6.0 mole/L [AI] : [Py]=h2. 1~ o. 4. Relation between the overall rate constant of polymerization K of MMA and inverso temperature. Results confirm the effect of the acidity of the organo-aluminium component on the chemistry of free radical formation at the stage of initiation. Of the two possible methods of free radical formation, namely: participation in the reaction with B P of an organo-aluminium compound that had not formed complexes (S~z, reaction) or an A1Rs.IVI complex (evidently, ~Ns reaction), in the case of styryl derivatives of aluminium the first method is more favourable from an energy point of view. Figure 4 shows the dependence of the rate constant of the process on inverse temperature for the S t A 1 - B P - P Y system. The overall activatio~ energy is 12.8 kcal/mole, the activation energy of initiation E1=16.6 kcal/mole.* El in * The value of E l 0"5E0 was 4.5 kcal/mole [10].

Reaction of styryl aluminium with benzoyl peroxide

1879

this case also incorporates the heat of complex formation AHc, i.e. in a sense is a characteristic of the complex forming ability of the organo--aluminium derivative. In the case of the ethyl derivative (A1Ets-BP-Py system) El was 21.2 kcal/mole [6]. A comparison of these values proves that the value of AHe is higher in the case of the ethyl derivative, i.e. its Lewis acidity is higher, compared with the styryl derivative. i

tj

I

I

t.#

I

I J I I v

f~

it

a

b

I!

m, ii

2

1

3

#

2 I

q

FIa. 5. Thin layer chromatographic pattern of a PS-P1K•A block eopolymer and standard homopolymer samples in benzene (a) and methanol (b) s y s t e m s : / - - b l o c k eopolymer before fractionation; 2--after fractiooation; 3--standard PS sample, M%V~2× 10s; 4--standard sample of P]K1KA, M W = 2 X l0 t.

To examine the block copolymer formed, two samples (I and II) were used obtained under identical conditions: [StAll----[Py] = [BP] = 0.046 mole/1. (P styrylMuminium=30), concentration of the MMA comonomer=3 mole/L; solventtoluene; 0 °. TABLE 4.

SEPAII,A.TION AND CHAI~,CTEBISltIC8 OF A S f r - ~ A

BLOCK COPOLY'MER *

(10% solution in chloroform, 20 ° )

- ' y l~ ol operation

I

Reprecipitation into petroleum ether Precipitator = 5) solvent Extraction with diethyl ether Fractional precipitation (chloroform-petroleum ether, 7=0-75) I

Sample I residue

~ . , g Ist,%

t (6.8g)

,:

filtrate

Sample I I residue

t (24-8g) ~t~te

~.,g ist,% wt.,g ISt,% wt.,g Ist,% !

24-5

3.9

97

17

--

7.4

96"4

1.76

12.5

0.44

75

10.8§

16

5"7

88 "4

1.6§

12-3

2.3

i

* Styrene content was calculated by I R spectroscopy from the intensity of characteristic bands at 1603 cm -I (vibraflous of the benzene nucleus) and 1730 cm ~l (vibrations of the C = O group in M•A) [13], t The initial sample is given in brackets. ; ~,=volume of the precipitant/overall volume. § Intrinsic viscosities of pulymer fractions indicated of samples I and I I (in chloroform, 25 °) are 0.19 and 0.21 dl/g, raspectively.

1880

L . V . ZA~OIS~YA et aL

According to t h e initiation m e c h a n i s m p r o p o s e d (reaction (1-3)) t h e b l o c k c o p o l y m e r o n l y contains h o m o - P S as m p u r i t y . T h e m a i n stages o f s e p a r a t i o n (fractional p r e c i p i t a t i o n a n d selective e x t r a c t i o n ) [11, 12], q u a l i t a t i v e a n d q u a n t i t a t i v e characteristics of p o l y m e r fractions s e p a r a t e d are s h o w n in T a b l e 4. R e s u l t s of s e p a r a t i o n of s a m p l e I (Table 4) indicate t h a t t h e second stage (i.e. e x t r a c t i o n w i t h d i e t h y l ether) is sufficient to s e p a r a t e t h e b l o c k c o p o l y m e r from homo-PS impurity. T h e p u r i t y o b t a i n e d b y s e p a r a t i o n is s h o w n b y t h i n l a y e r c h r o m a t o g r a p h y (Fig. 5). R e s u l t s confirm t h a t t h e b l o c k c o p o l y m e r is quite free f r o m h o m o - P S a n d p r o v e t h e a s s u m p t i o n concerning t h e a b s e n c e o f h o m o - P M M A . T h e s t r u c t u r e o f A B a n d A B A block c o p o l y m e r s (here A is the PS b l o c k a n d B - - t h e P M M A block) c a n n o t be d e t e r m i n e d w i t h o u t f u r t h e r p h y s i c a l a n d c h e m ical investigations. E v i d e n t l y , a m i x t u r e o f b o t h t y p e s o f b l o c k c o p o l y m e r is f o r m e d as a result of opposite tendencies: on the one h a n d , r e c o m b i n a t i o n of a g r o w i n g P S - P M M A radical p r o d u c e s a t e r n a r y b l o c k c o p o l y m e r a n d on t h e other, d ! s p r o p o r t i o n a t i o n a n d chain t r a n s f e r to t h e a l u m i n i u m c o m p o n e n t , as e s t a b l i s h e d for low m o l e c u l a r w e i g h t a l k y l a l u m i n i u m [5], m a y p r o d u c e t y p e AB. T h e s t u d y o f using s y s t e m s b a s e d on higher m o l e c u l a r weight StA1 a n d investiga t i o n of t h e s t r u c t u r e o f b l o c k - c o p o l y m e r s f o r m e d continues [14]. Solvents were purified by conventional means, further dried over Oatt,. Styrene was washed to remove the stabilizer, dri£d with CaC12 and Call,, twice distilled in vacuum and prepolymerized before use over Na metal. Other monomers used in the study were propolymerized using an A1R3-BP system after conventional purification and distillation in vacuum. Synthesis of polystyryaluminium and subsequent reactions were carried out under conditions, which exclude moisture and air. To separate the StA1 formed from St that has not been polymerized, the reaction mixtt:re was repeatedly washed with toluene, which was removed every time in vacuum and the polymer dried in vacuum until the pressure gauge indicated 10 -9 torr. A solution in toluene was used in the stt~dy. The concentration of the "active" organo-alurninium compound was evaluated by reaction with (C2Hs)aN an the amount of unreacted amino determined (vacuum distillation followed by determining concentration with HC1 titration). After the interaction of StA1 with BP the reaction mixture was decomposed with water, the hydroxyahiminium benzoate separated filtered and analysed: the residue--for benzoate group content; the filtrate--for peroxide and saponifiable product contents [1, 2]. Spectra were obtained in an IS-DC-301 device (KBr pellets). Polymerization was carried out in single compartment ampoules with a graduated neck part, catalyst components and the monomer were fed from Schlenke vessels. Yield was determined from the dry residue. Thin layer chromatography was carried out according to methods adopted for polymers [15]. Glass plates of 6 × 9 cm were used in the study which were coated with KSK silica gel, the fraction was 20 to 80 rim. The samples examined were dissolved in chloroform and applied on a plate with capillaries. To detect homopolymer impurities in the block copolymer, ascending chromatography was used in benzene (PS with M up to 2 × 106 moved with the solvent front) and methanol (PMMA with M = 2 × 106 moved with the solvent front); the block copolymer remained in the starting condition. After chromatography the plate was dried and developed with a 3% KMnO,

Reaction of styryl aluminium with benzoyl peroxide

1881

solution in concentrated H~SO~. The plato was then heated in a thermostat at 160 ° for 10-15 min. The polymers were developed as black spots on white background. For the quantitative evaluation of distribution according to MW of PS samples the density of the plate was determined from the starting point along the direction of movement of the eluent using a Zeiss microphotonaeter g-II according to optical density scale S. St--MMA block eopolymers were analysed by I R spectroscopy using a UR-20 device.

Translated by E. SV.~ERE REFERENCES 1. Ye. B. MILOVSKAYA, Ye. I. POKROVSKII and Ire. F. FEDOROV, Izv. AN SSSR, ser. khim., 1093, 1967 2. L. V. ZAMOISKAYA, Ye. B. MILOVSKAYA and V. A. ORESTOVE, Same, 2053, 1971) 3. G. A. RAZUVAYEV and Ye. V. MITROFANOVA, Zh. obshch, khimii 38: 2488, 1968 4. G. A. RAZUVAYEV, L. I. STEPOVIK, V. A. DODONOV and G. V. NESTEROV, Same 39: 123, 1969 5. L. V. ZAMOISKAYA, S. I. VINOGRADOVA and Ye. B. MILOVSKAYA, VysokomoL soyed. A13: 1484, 1971 (Translated in Polymer Sci. U.S.S.R. 13: 7, 1640, 1971) 6. Ye. L. KOPP, O. S. MTKHAILYCHEVA and Ye. B. MILOVSKAYA, Same A14: 2653~ 1972 7. T. H A F F and E. PERRY, J. Amer. Chem. Soc. 82: 4277, 1960 8. A . I. GRAYEVSKII, Dissertation, 1962 9. Ye. B. MILOVSKAYA, Sb. Uspekhi khimii pereksinykh soyedinenii i autookisleniye (Progress in the Chemistry of Peroxide Compounds and Auto-oxidation). Izd. "Khimiya" 10. Kh. S. BAGDASAR'YAN, Teoriya radikal'noi polimerizatsii (Theory of Radical Polymerization). Izd. "Nauka", 1966 11. M. BEYLEN and G. SMETS, Makromolek. Chem. 69: 140, 1960 12. H. STAUDINGER and W. HENER, Z. phys. Chem. A171: 129, 1934 13. (Ed.) V. M. CHULANOVSI~H~ I K spektry pogloshcheniya polimerov i vspomogatcl'nykh veshchestv (IR Absorption spectra of Polymers and Auxiliary Substances). Izd "Khimiya", 1969 14. Ye. B. MILOVSKAYA and L. V. ZAMOISKAYA, Vysokomol. soyed. B18: 300, 197@ (Not translated in Polymer Sci. U.S.S.R.) 15. B. G. BELENKII and E. S. GANKINA, J. Chromatogr. 53: 3, 1970