The mechanism of initiation in alkylaluminium-acyl peroxide-polar monomer systems

THE MECHANISM OF INITIATION IN ALKYLALUMINIUM-ACYL PEROXIDE-POLAR MONOMER SYSTEMS* L. V. ZAMOISKAYA,S. I. VII~OGRADOVAand YE. B. /VLmOVS~AYA Institute of Macromolecular Compounds, U.S.S.R. Academy of Sciences (Received 7 January 1970)

RESULTS obtained in study of the kinetics of polymerization of vinyl acetate (VA) and aerylonitrile (AN) initiated by systems consisting of A1Rs (R--ethyl or isobutyl) and an aeyl peroxide (benzoyl peroxide (BPO) and dicyclohexylperoxydiearbonate (PC)) led to the conclusion that all the reactants, including the polar monomer (fuctioning as an electron-donating compound) take part in initiation. On the basis of evidence obtained under model conditions in conjunction with the kinetic studies we suggested that the formation of free radicals is described by equations (1) and (2) [1] . . . . All4~ ~-lVlrapid ;A1R3"M

(1)

A1Rs"M-~BPO ~(A1Rs"M'BPO) k--~ ' A1R2OCOCeHs-~R" ~-C6H5C00" ~-[M]

(2)

k2>>]~ 1

This is equivalent to the assertion that the intermediate stage in formation of free radicals is a reaction of the S~2 type, and consists in abstraction of one electron donor (the polar monomer) from its complex with A1R8 by another, more powerful complex-forming compound--the acyl peroxide, then the uncomplexed form of the organoaluminium compound reacts with the peroxide, just as occurs under model conditions. This idea was not however confirmed by further studies, in which the complex-forming function of the polar monomer (VA) was modelled in this capacity by the polar compound ethyl acetate (EA) [2, 3]. These results showed that the formation of free radicals in the presence of EA occurs by a reaction of the S~I type, i.e. it is the result of reaction of the uncomplexed organoaluminium compound with the peroxide. In the presence of VA however the complexed form of the aluminium alkyl takes part in formation of free radicals.t It was evident from this that the role of the polar monomer in the initiation step is not limited to formation of a complex involving the polar * Vysokomol. soyed. A18: No. 7, 1484--1493, 1971. ? Shown by study of the polymerization of vinyl chloride by the system AI(C,Hs)s-BPO in the presence of catalytic quantities of EA or VA.

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Mechanism of initiation in alkylaluminium-acyl peroxide-polar monomer systems

1671

g r o u p a n d t h a t t h e f u n c t i o n o f t h e m o n o m e r in t h i s process is m o r e c o m p l e x t h a n r e a c t i o n s (1) a n d (2). W e felt t h e r e f o r e t h a t it w o u l d be a d v a n t a g e o u s for clarification of t h e fine details o f t h e initiation s t a g e in p o l y m e r i z a t i o n of t h e fine details o f t h e initiation s t a g e in p o l y m e r i z a t i o n of p o l a r m o n o m e r s to e x t e n d t h e r a n g e o f m o n o m e r s studied, w i t h t h e o b j e c t i v e s o f s t u d y i n g t h e kinetics o f p o l y m e r i z a t i o n , finding t h e effect of t h e n a t u r e o f t h e m o n o m e r a n d o f t h e a l k y l g r o u p in t h e o r g a n o a l u m i n i u m c o m p o u n d on t h e energetics of t h e process, a n d o f d e t e r m i n i n g t h e n a t u r e o f t h e e n d g r o u p s in t h e p o l y m e r . F o r kinetic studies t h e m o n o m e r u s e d w a s m e t h y l m e t h a c r y l a t e (MMA), initia t e d b y t h e s y s t e m s Al(iso-CeHg)s-BP0, AI(C=Hs)8-BPO a n d AI(C~Hs)8-PC.* EXPERIMENTAL

The teatment of the reactants and the general experimental procedure w e r e described in reference [5]. The monomer was washed free from stabilizer with 2~o alkali solution, dried over anhydrous MgSO, and distilled in v a c u o , the fraction boiling at 21.5-22.5°]20-22 m m being selected. I t was then further dried over calcium hydride and redistilled i n v a c u o . A

0"# 8 3

2 /

\

O'Z

~2 q

20

i

I

~0 80 7-(rne , rain I~G. 1

I

80

260

280

A, m/~

J00

Fro. 2

FIG. 1. Typical kinetics curves; 0°, solvent--octane. Concentrations, mole/L: M:MA--8.5; B P O - - 2 × 10-'; Al(iso-C4tt,)s × 10=--2 (1), 3 (2), 5 (3}, 6 (4). FxG. 2. Spectra of M:MA polymers prepared with BPO (1) and the BPO-~AI(iso-C,H0), system (2). Immediately before an experiment partial polymerization was carried out, using the Al(isoC,Hs)-BPO system. The monomer was stored under argon in a Schlenk flask. Al(iso-C,H0)= was purified by vacuum distillation and the fraction boiling at 43°/1 ram was used. AI(C=HB), was the commercial product, which was not further purified. The aluminium alkyls were used in solution in octane, the concentration was determined by the method of reference [6]. The reaction was stopped by cooling the dflatometers to --60- --70 °. The polymer was precipitated in methanol acidified with hydrochloric acid, washed with methanol on a * The possibility of polymerization of MMA by means of systems of this type was shown in reference [4].

1672

L. V. Z*~.,oxs*uLYA et al.

Schott filter and dried in vacuo. In the kinetic experiments the rate was calculated for the initial part of the time-conversion curve, at degrees of conversion not higher than 10-15 ~ . Typical rate curves are shown in Fig. 1. For determination of the molecular weight (M) the polymer was reprecipitated in methanol from solution in chloroform, and the viscosity was measured in chloroform at 20°. The molecular weight was calculated from the formula [~] =0-48 × 10-5 M a'', proposed for the polymer prepared at low temperatures [7]. The endgroups in the polymer were found by means of an SFD-2 spectrophotometer. The spectra were recorded with solutions in chloroform. For calculation of the concentration of benzoate groups we used the formula A =soy, where A is the optical density, 8 the extinction coefficient, c the concentration in moles/l, and V the volume, which in our case was unity. For 8 the extinction of ethyl benzoate was used, for which log 8 =2.97 at Imaz =273 m/~ [8]. In the polymers Imaz is shifted a little and appears at 275-276 m~. The spectra of ~ & polymers obtained with different initiators are shown in Fig. 2. Before t h e kinetic e x p e r i m e n t s were set u p it was established t h a t t h e interaction b e t w e e n MMA a n d Al(iso-C4H~) 8 is limited to f o r m a t i o n o f a complex t h r o u g h the e a r b o n y l g r o u p o f the monomer. U n d e r t h e c o n c e n t r a t i o n a n d t e m p e r a t u r e conditions chosen practically no irreversible reaction occurs a n d t h e c o n c e n t r a t i o n o f t h e o r g a n o a l u m i n i u m comp o n e n t remains constant. This was verified b y t h e fact t h a t w i t h a c o n c e n t r a t i o n o f Al(iso-C,Hg)3 of 0.645 mole/l, a n d a m o l a r r a t i o o f MMA/A/(iso-C,Hg)8 of 100 : 1, a t 0 °, a f t e r i h r t h e s y s t e m c o n t a i n e d 99~/o a n d after 4 hr 940/o o f t h e original a m o u n t o f t h e o r g a n o a l u m i n i u m c o m p o n e n t . RESULTS AND DISCUSSION

Kinetics of polymerization of M M A . Table 1 gives d a t a on p o l y m e r i z a t i o n o f MMA with a s y s t e m of the a l k y l a l u m i n i u m - a c y l p e r o x i d e t y p e . I t is seen t h a t all t h e systems studied are effective for polymerization. P o l y m e r i z a t i o n t a k e s place a t low t e m p e r a t u r e s a n d even at - - 4 0 ° t h e p o l y m e r i z a t i o n r a t e is high. One's a t t e n t i o n is d r a w n to the c o m p a r a t i v e l y low molecular weight o f all t h e polymers. Possible causes of this will be discussed below. T h e kinetics o f p o l y m e r i z a t i o n o f MMA were studied in detail w i t h t h e Al(isoCaIIg)a-BPO s y s t e m at 0 °. I t follows f r o m Fig. 3 t h a t t h e relationship b e t w e e n t h e p o l y m e r i z a t i o n r a t e a n d the c o n c e n t r a t i o n of the c o m p o n e n t s of t h e initiating s y s t e m a n d of t h e monom.er is described b y the expression

(3)

v--k[M] [A1R3]°'~[BPO] °'~.

These results show t h a t the order o f the reaction with respect to the compon e n t s o f t h e initiating s y s t e m corresponds to the relationship characteristic o f a radical process (with bimolecular chain t e r m i n a t i o n ) . Mayo's e q u a t i o n (equation (4)) was used to d e t e r m i n e the ratio o f t h e propagation a n d t e r m i n a t i o n r a t e constants, a n d the r a t e c o n s t a n t of t r a n s f e r to solvent 1

1 ]¢t v

~---~ ]--~[M]---~ -~-C m -~-O s

[solvent]

[~

[I]

-~C| --[M']

(4)

Mechanism of initiation in alkylaluminium-acylperoxide-polar monomer systems

1673

Here k t and /% are the termination and propagation rate constants, and Cm, C a and Ci the rate constants of transfer to monomer, solvent and initiator. The ratio kp/k~t found from Fig. 4 is 0-95 × 10-~, which differs considerably from the value 3.69 × 10 -~ found in references [9] and [10]. We consider below one of the posssible causes of this difference. The intercept on the ordinate corresponds v. tO*.,',o/e# . ~ec

2

oy o

o

1

l

2 3 [M]. [At ~s.BPO] °'5.10

FIG. 3. Dependence of polymerization rate on the concentrations of the components of the initiating systems and of the monomer. to the sum of Cm+C~ ([solvent]/[M]) and is equal to 0.1 × 10 -4. The constant of transfer to monomer (1 × 10 -5 at 60 ° [11]) can be neglected without great error and the constant of transfer to solvent (octane) is 0.9 × 10-4. Since no values of this constant are given in the literature its accuracy cannot be assessed, but its order of magnitude seems quite reasonable. I t can be compared with the known values of G8 for cyclic and aromatic hydrocarbons. ~or example for toluene and methylcyclohexane Cs is 0.5×10 -4 and 0"2×10 -4 respectively at 80 °* [12]. Thus in the process under discussion the production of polymers of comparatively low molecular weight can be due only to participation of the components of the initiating system in chain transfer, as was shown for VA in reference [13]. Since transfer to B P 0 does not occur in polymerization of MMA [14] the observed effect must be the result of reaction of the growing radical with the organoaluminium component, or more precisely with the alkylaluminium-monomer (A1Ra'M) complex. The rate constant of transfer to AIR s. M was found from equation (4) transformed to 1 ____C 1 P

~

[I]

k

Ct

--V ~[M]

(5)

Here C~ is the sum of Cs ([solvent]/[M]) -~Cm. The experimental results are shown * The energy of abstraction of a hydrogen atom from these hydrocarbons is fairly close ~o the corresponding value for octane.

1674

L . V . ZAMOISKAYAet

aL

TABLE 1. POLYMERIZATION OF ~ WITH fl-JRs--ACYL PERO:~DE SYSTEMS (Solvent-- octane) Initiating system

Concentration,peroxidemOle/1. 10~= oT~ Time,rainYield, monomer AIR3 X ~ Yo × 10=

AI(C=H6)=--PC

9"5 9"3 7"5 8"0 9"2 9"6 9"4 8.4 8"8 9.2 9"5

AI(C=H,).-BPO

AI(iso-C,H,)~-BPO

A](iso-O4H.h

1"00

1"00 2"35 5"00 2"50 2"50 2"50 3"90 2"55 4"95

2"50 4.75 6.80 9.80 2.50 2.50 2-50 3.90 2.55 4.95 6.00

0 --15 --30 --40 0 --10 --20 0 --10 --20 0

80 I 7.4 90 13-2 140 13.2 130 I 18.8 155 7-0 145 5.5 240 3.4 75 8.5 175 5.7 210 4-9 360 1-0

M X 10 -3

27.1 45.4 36.7 43.6

39.4 24.2 35.5 79.0

in Fig. 5, f r o m which t h e v a l u e o f 0.6 is f o u n d for C i . This is close t o t h e values for highly efficient c h a i n t r a n s f e r a g e n t s such as C4HgSIt, for which t h e c o n s t a n t is 0.67 a t 60 ° [11]. This is in a c c o r d w i t h i n f o r m a t i o n in t h e l i t e r a t u r e , where t h e high a c t i v i t y o f a l u m i n i u m alkyls as chain t r a n s f e r a g e n t s in radical p o l y m e r i z a t i o n h a s b e e n s h o w n [15, 16]. T h e s e results p r o v i d e a n e x p l a n a t i o n o f t h e r e a s o n for formation of ~ p o l y m e r s o f low m o l e c u l a r w e i g h t a n d show t h a t it is n o t

20 o o

o o

{I]

,,o,

28 o

o

o

12

2O

4

10 2

Zl fMF v_.E_,107

FIG. 4

I 5

I 10 10-4/v

FIG. 5

FIG. 4. Dependence of the reciprocal of the degree of polymerization on the polymerization rate. FIG. 5. Calculation of chain transfer to initiator.

Mechanism of initiation in alkylaluminium-aeyl peroxide-polar monomer systems

1675

possible to use systems containing aluminium alkyls if it is required to obtain a polymer of high molecular weight at a rate of polymerization t h a t is normal radical initiation. As was mentioned above the value found for/~p]]¢$ is very different from the published value. The reason for this difference is t h a t because of the considerable transfer to initiator it is not the true value of the ratio of the constants t h a t is measured, but the complex quantity lct/k~-bCi (Ia/c~//~pk~). I f adjustment is made for the found value of the rate constant of transfer to initiator the corrected value of kp//~ is 1.7 × 10 -2, i.e. the suggested reason for the discrepancy is quite plausible.

a

-logk

b

C

o

|

i

3.7

I

3.3

!

3"6

o

~'0

14.0

li.14 3"5

FIG. 6. Dependence of the polymerization rate constant on reciprocal temperature for the systems AI(C~Hs)a-BPO (a), AI(C~I-Is)s-PC (b) and AI(iso-CdHg)8-BPO (c).

The mechanism of initiation in polymerization of polar monomers. The relationship between the rate of polymerization and the concentration of the components of the initiating system found in the present work is in complete agreement with the relationship found in the polymerization of VA and AN [1]. This gives grounds for the assertion t h a t the stage of.formation of free radicals under the influence of A1Ra-peroxide-polar monomer systems is the same. It was of interest to determine the contribution of each of the reactants (monomer aluminium alkyl and peroxide) to the energy of activation for initiation E l . On the example of polymerization of MMA we found the effect of the peroxide and of the nature of the alkyl radical in A1R a on the value of El, which was found from the expression

where E~, is the total activation energy. Figure 6 shows the dependence of the reaction rate constant on reciprocal temperature. The values of Etot and of E l derived from this are given in Table 2, which for comparison includes this information for VA and AN [5, 18, 19]. I t follows from the results of the present work that E i is lower the higher the rate constants of decomposition of the peroxides B P 0 and PC [20, 21], which is fully in agreement with the facts found previously in polymerization of VA [18]. * For MM/k Ep--~Et was taken as 4"5 kcal/mole [17].

1676

L.V. ZAMOISKArAet

al.

I t is interesting to examine the cause of the effect of the alkyl group in the organoalumininm compound on the value of E I. I t can be shown t h a t this effect is related to the strength of the A1--C bond, which is different in AI(C~Hs) s and Al(iso-CaHg) 3. Unfortunately there axe no data for the second compound in the literature. The information available on other hereto-organic compounds is very spaxse and contradictory [22, 23], and because the relationship between the strength of the M--C bond and the nature of the alkyl radical is not uniform it is not possible to calculate the bond strength for the isobutyl group from t h a t for the ethyl group. It m a y be assumed however that these bond strengths do not differ greatly or t h a t for Al(iso-CaHg)s it is somewhat lower.* I f the relationship between the strength of the A]--C bond and El is linear the latter should be smaller for compounds of lower strength. In fact we find the opposite effect, E i being much higher for the isobutyl derivative. The must obviously be another reason for this effect. I t is well known t h a t aluminium alkyls are Lewis acids. The lower members of this series and those with an unbranched chain, because of the absence T A B L E ~. E N E R G I E S OF A C T I V A T I O N :FOR POLYMERIZATION OF POLAR MONOMERS

Monomer MMA

VA AN

E, kcal/mole

Initiating system

Eto t

E i

AI(C,Hs)s-BP0 AI(C,Hs),-PC AI(iso-C~Hg)s-BPO AI(C2Hs)3-BPO Al(C~I~h-PC AI(C~Hs)a-BPO

8.6 6.0 13.8 12.1 10.6 14.0

8.2 3-0 18-6 15.8 12.8 19.2

"I Reference

[5] [18] [19J

of steric hindrance have higher reactivity (as Lewis acids). This is shown especially by their ability to dimerize. Thus the observed increase in E~t and E i for systems containing the isobutyl derivative is in keeping with its higher Lewis acidity. It is seen irom Fig. 2 t h a t the nature oI the monomer has an enormous effect on the energetics of the process, E I for lVIMAbeing much lower t h a n for VA and AN. Within the framework of the above kinetic scheme (equations (1) and (2)) two alternative explanations of the effect of the nature of the monomer can be considered. One of these is that free radicals axe formed by the S~2 mechanism. This is not however in accord with a number of other experimental facts now known in addition to those mentioned above. Thus for all the polar monomers studied the dependence of the reaction rate on the concentrations of the components of the initiating system is expressed by equation (3), from which it m a y be concluded t h a t free radicals are formed b y * This suggestion is made by analogy with hydrocarbons with branched and unbranched chains.

Mechanism of initiation in alkylaluminium-acyl peroxide-polar monomer systems

1677

reaction of A1Rs with the peroxide in the ratio 1 : 1 (equation (2)). In contrast to this, under model conditions on the example of associated (Al(C~Ha)s) and unassociated (Al(iso-C4Hg)s) forms of the aluminium alkyl, it has been shown that the stoichiometry of the reaction corresponds to a ratio of A1 : BPO of 2 : 1 [24, 25]. The question of course arises, of why under polymerization conditions, i.e. in the reaction of the A1R a. M complex with BPO, reaction with only one carbonyl group of the peroxide is sufficient for breakdown of the latter. It has been shown under model conditions that reaction of an aluminium alkyl with benzoyl peroxide produces only alkyl and not benzoate radicals [25], while on polymerization of MMA the polymer also contains benzoate end groups (Fig. 2). It is known that no transfer of the peroxide occurs in polymerization of MMA [ 14], therefore the benzoate groups must come entirely from the initiating system.* TABLE 3. MOLECULAR WEIGHT OF POLYMERS FORMED BY MEAlqS OF AI(iso-C4Ho)a-BPO SYSTEM

Initiating system

AI(iso-C4H~)-BPO BPO

I

T, oc

0 70

c × 10s, mole/g of polymer 3.0 9.0 5.6

M × 10-4 by endgroup analysis * 33"3 11"0 23"8

viscometrict 5-6 2-4

29.8

* In calculation of molecular weight b y end-gToup analysis it w a s a s s u m e d t h a t termination occurs b y recombination. The viscosity-average molecular weight was calculated as in reference [7]. T I t w a s a s s u m e d that recombination and disproportlonation take p a r t to equal extents in termination. The viscosity-average molecular weight was calculated from the formula log/~p=3.261--1.256 log [r/].

W e u s e d t h e f o u n d c o n c e n t r a t i o n o f b e n z o a t e g r o u p s for c a l c u l a t i o n o f t h e

molecular weight (M) and compared this with M calculated from viscosity measurements. The results are presented in Table 3. While there is fairly good agreement between the molecular weights calculated from the benzoate-group concentration and from the viscosity measurements for the polymer prepared with BPO alone, the very considerable divergence of the values (by approximately an order of magnitude) for the polymer prepared with the complex systems containing BPO is noteworthy. It has been shown in the kinetic studies of polymerization of MMA that substantial chain transfer to the alkylaluminium-monomer complex occurs. The data in Table 3 confirm the important part played by chain transfer, this being to the organoaluminium component. The combination of facts presented above shows that the chemistry of the * The facts established in reference [13] do not permit unequivocal interpretation of in reference [1].

results presented

the

1678

L . V . ZAMOISKAYA ~$ a~.

formation of free radicals is different in polymerization from that under model conditions. We consider that an explanation of the observed facts can be found if it is assumed that in formation of the A1Rs.M complex (through an electrondonating atom of nitrogen or o x y g e n ) i n the rest of the monomer molecule the electron envelope shifts toward the electron aceeptor, i.e. AIRs, and consequently the monomer acquires a partial positive charge. As a result of this "excitation" of the monomer the complex acts as a bidentate acceptor in reaction with the peroxide, i.e. in addition to reaction between the aluminium alkyl and the peroxide, reaction occurs also between the double bond of the monomer as an aceeptor of electrons and the second part of the peroxide as a donor of electrons. This m a y be represented schematically as follows: X~CH R, A'I J o- O

~H~ + ~o_

C6H5"~

\

0--0

/

~--C.H,

The stoichiometry of this reaction corresponds to an AIRs : B P O ratio of unity and benzoate groups will be present as end groups in the polymer. According to this scheme the energetics of the initiation stage are dependent on a numTABLE 4. WIDTH O~ THE LIEN'EOF TIBA COMPLEXES(zlH) WITH ONE (M1) AI~TD TWO ( M I - ~ M , ) DONORS

(ztH of pure TIBA=4.8 gauss) M1 AN VA MTWA

AH, gauss 5-1~0.4 4.1±0.2 6-5~0.3

MI+M, AN+VA VA+MMA AN+MMA

AH, gauss

4-8~0.2 4.7~0.2 5.0~0.2

ber of factors, b u t primarily on the complex-forming ability of the monomer and the polarization of its double bond, on the ease of abstraction of the radical R from AIR s and on the efficiency of formation of the dialkylaluminium benzoate (with the same peroxide). Each of these stages is definitive for the observed E l and it is difficult to state the over-all situation. We attempted to find the effect of the complex-forming ability of the polar monomers on E l . Information in t h e literature on the donor properties of polar monomers is very scarce and contradictory. The latter is obviously explained b y the fact that the complex-forming ability was assessed in relation to different aceeptors of electrons [26, 27]. To characterize the quantity sought we used the N M R method, involving the AI nuclei [27]. As a characteristic of the complex we took the change in line

.~Iechanism of initiation in alkylaluminium-acyl peroxide-polar monomer systems

1679

width in comparison with the width for the pure organoaluminium compound. * The complex-forming ability was determined from the line width when two donors are added simultaneously [28]. The results are given in Table 4. T h e y show th e complex-forming ability of these monomers falls in the order AN ~ VA>>MMA. I t can be seen from the above series t h a t the energy of activation for initiation is directly related to the complex-forming ability of the polar monomer. I t seems however t h a t the observed marked difference in El for the extreme members of the series cannot be due only to difference in their strengths as donors. As was mentioned above it m a y be assumed t h a t other factors, especially the polarization of the double bond, contribute to El. Recent quantum,chemical calculations have shown t h a t the polarization of MMA itself and in a complex with LiGH3 is much higher t h a n in AN [29]. E v i d e n t l y the v e r y low value of El in the case of MMA is due to the simultaneous effect of two factors, n a m e l y the low activity of the monomer as a donor and a high degree of polarization of its double bond. CONCLUSIONS

(1) A s tu d y has been made of the kinetics of polymerization of m e t h y l methacrylate (MMA) under the influence of systems of the A1R3-aeyl peroxide type. (2) I t is shown t h a t t h e p o l y m e r s formed have a comparatively low molecular weight as a result of the considerable activity of the AlR 3. M complex as a chain transfer agent. For example Ci for AI(iso-C4Hp)3MMA was found to be 0.6 at 0 °. (3) On the basis of the results of the present st udy in con]unction with previously established factors, the formation of free radicals is a t t r i b u t e d to reaction of the peroxide, functioning as an electron donor, with t he A1R3.1~I complex, acting as a bidentate accepter. Translated by E. O. P~ILLIPS REFERENCES

1. Ye. B. MILOVSKAYA,T. G. ZHURAVLEVA and L. V. ZAlVIOISKAYA,J. Polymer Sci. C16: 899, 1967 2. Ye. L. KOPP and Ye. B. lVIILOVSKAYA,Vysokomol. soyed. All: 750, 1969 (Translated in Polymer Sci. U.S.S.R. 11: 4, 848, 1969) 3. Ye. L. KOPP and Ye. B. 1KILOVSKAYA, Seventeenth All-Union Conference on Macromolecular Compounds, p. 129, Moscow, 1969 4. Ye. B. M][LOVSKAYA, T. G. ZHURAVLEVA, P. L DOLGOPOL'SKAYAand L. I. VESELOVA, Vysokomol. soyed. 6: 412, 1964 (Translated in Polymc~ Sci. U.S.S.R. 6: 3, 467, 1964) * We used tri-isobutylaluminium (TIBA) as the aluminium alkyl because this avoided complication of the interpretation of the spectrum due to association, which occurs in the case of Al(C2Hs)3. We assumed that change in the nature of the alkyl group in AIR, will not alter the order of the relative donor activities of the polar monomers. The general method and procedural details are given in a paper now in the press. The authors express their gratitude to A. I. Kol'tsov and V. M. Donisov for carrying out this part of the%work.

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