Study of chain transfer with scission on the model reaction of alkyl exchange between dimethoxy-and diethoxymethane

STUDY OF CHAIN TRANSFER WITH SCISSION ON THE MODEL REACTION OF ALKYL EXCHANGE BETWEEN DIMETHOXYAND DIETHOXYMETHANE* I. A. ABR_~IYAN, V. V. IVANOV, G. V. RAKOVA, A. N. GORYACHEV a n d 1N. S. YENIKOLOPYAN Institute of Chemical Physics, U.S.S.R. Academy of Sciences

(Received 14 July 1966) IT WAS shown in references [1-4] t h a t in ionic polymerization, in addition to t h e usual reactions of initiation, p r o p a g a t i o n and t e r m i n a t i o n w i t h transfer t o m o n o m e r or o t h e r substances, in the general case it is necessary to t a k e account of special t y p e s of reaction, which consist essentially in exchange reactions bet w e e n the active centres a n d the main chain o f the p o l y m e r molecule. Reactions o f this t y p e h a v e been given the n a m e of chain t r a n s f e r reactions with scission (CTS). CTS reactions h a v e a substantial effect on t h e entire kinetics of the polym e r i z a t i o n process a n d on the properties of t h e polymers, hence s t u d y of these, a n d especially d e t e r m i n a t i o n of the basic kinetic p a r a m e t e r s , is of considerable interest. D i r e c t s t u d y of CTS reactions in p o l y m e r i c systems involves considerable difficulties however. I n the first place accurate d e t e r m i n a t i o n of the n u m b e r average molecular weight of the p o l y m e r s is necessary a n d the h e t e r o g e n e i t y of the s y s t e m often complicates this. I t is m u c h easier to s t u d y a CTS reaction in low molecular weight model systems. W e h a v e a l r e a d y studied the disproportionation o f t h e d i m e t h y l e t h e r of d i o x y m e t h y l e n e g l y c o l as a model of CTS reactions in p o l y m e r s with a n acetal s t r u c t u r e ( p o l y o x y m e t h y l e n e , polydioxolane, etc.) [2,5]. T h e present work is a c o n t i n u a t i o n of the s t u d y of model reactions between low molecular weight acetals on t h e e x a m p l e of alkyl e x c h a n g e b e t w e e n d i m e t h oxy- a n d d i e t h o x y e t h a n e (DMM and DEM) in the presence of b o r o n trifluoride e t h e r a t e as catalyst. EXPERIMENTAL

The sealed tube method was used for study of the reaction kinetics. The tubes were first swept out with dry argon, then filled with the starting materia.Is in a dry cabinet, sealed and placed in a thermostat. After a chosen time the tubes were opened and the reaction was stopped by addition of aqueous ammonia. The concentrations of starting materials and reaction product (ethoxy-methoxymethane (EMM) were determined in a PerkinElmer 452 chromatograph in which helium was used as the carrier gas and the st,ationary phase was PEG 1500. The column temperature was 90 ~. The concentrations of the components were calculated by means of the formula r~=(hJE h~) • 100 mole%, where h~ is the height of the appropriate peak. i * Vysokomol. soyed. A9: No. 10, 2105-2108, 1967. 2377

I . A . AB~AMYA~ et al.

2378

The DI~M used in the experiments was commercial, "pure" grade methylal, which was dried over calcium chloride, redistilled and stored over metallic sodium (b.p. 42~ Diethoxymethane was prepared from ethyl ether and formalin [6] and it was also dried over calcium chloride, redistilled and stored over metallic sodium (b.p. 86~ Boron trifluoride etherate was prepared by saturating diethyl ether with boron trifluoride, purified by vacuum distillation and dispensed in thin-walled ampoules. It was used in the form of a solution in benzene. DISCUSSION

According to c u r r e n t views acetals undergo scission u n d e r t h e influence of Lewis acid catalysts, forming c a r b o n i u m ions which t h e n a d d to the acetal molecules to form more stable o x o n i u m ions. T h u s t h e m e c h a n i s m of alkyl exchange b e t w e e n ethylal a n d m e t h y l a l can be r e p r e s e n t e d as follows:

| | R~OCH2ORj+R~OCH~ - ~ R~OCH~OR~,

(A)

I CH~ I o

J

Rz @ | R~OCH~ORj - ~ RtOCH~OR[~-R~OCH~,

(B)

l CH~ I o

I

R, where Ri, Rj and 1%Z can be m e t h y l or e t h y l radicals. I n this scheme t h e r a t e d e t e r m i n i n g step is the decomposition of the o x o n i u m ions (B). I n general t h e r a t e c o n s t a n t s / c 1 a n d k 2 should be d e p e n d e n t on Ri, Rj a n d R I. I n our case however one can predict t h a t this d e p e n d e n c e will n o t be strong because the electrond o n a t i n g properties of m e t h y l a n d e t h y l groups, which d e t e r m i n e t h e rates of reactions (A) a n d (B), are n o t v e r y different. The system of kinetic e q u a t i o n s corresponding to t h e above scheme takes t h e following form:

dr~ -- ]c2(1~1_21-~-21~1_22-~-21~2_11-~R2_21)-- 2]c1(P~+ -~-l~ ~ )r 3 dt dR +

dt dR2

dt

-- ]~2(2]:~1_11-~-1~1_21-~ 2R2--11 ~-R2-21)- 2]QR + (r~ ~

+

- - k 2 ( 2 1 % z _ ~ z - } - R 2 _ 2 1 + ~ R 1 _ ~ z + l % 1 _ 2 1 ) - - 2 ] Q l %I

dR1-11 : 2k11~ r 1 - 2/c2R1_11

dt

dt

-- kl(~1~73-~21~: -~-rl)- 2}21~i_21

o

(l)

~

(2)

o

(3)

(rl+r2)

(4)

(5)

S t u d y of chain ~ransfer

dR1-22

dt

dRy_ 11

2379

(6)

= klR~ ra-- k~R1-2~ ----

dt

klR + ra-- 2k2R2_ 11

(7)

(8)

dR2-21 -= klR+ r3 + 2klR + r2-- 2k2R2_21

dt

dR2-22 ----2klR+r2 - 2k2R2_29.,

(9)

dt

@

where R + is the concentration of the carbonium ions R~--O--CH2, Ri_j~ the concentration of oxonium ions with the structure R1

I

R~--O--CH~--O|

;

I

CH~

f

O--Rt and rl, r 2 and r 3 the concentrations of DMM, DEM and EMM respectively. I f we assume steady state conditions for all the intermediate ions then from equations (2) and (3) and the condition that Rl_11+Rl_21+R1_~+R2_n+R2_21+R2_22=O (where O is the total concentration of active eentres) we obtain

Ri

=k2Clkl(r~176

(10)

B y substituting in equations {2) and (3) the expression for Ri_~ found from formulae (4)-(9) we .obtain:

R?/R2 : r l /or 02.

(11)

From equations (10) and (11) it is easy to obtain expressions for 1 ~ and R +. By substituting these expressions in formulae (4)-(9) we obtain in a clear form expressions for R~_jg. From formula (1) it is then not difficult to find for the rate of formation of EMM

drs--k2C[

2r~176

L(r~_ _ r

rs 1 7.o.......5 " rl+r ~

(12)

Integration of expression (12) gives

r - 2r~176rl

e-(k~c/r?+':~

(13)

1.1 -?-T2

Figure 1 shows kinetic data for formation of EMM b y reaction of equimolar quantities of DMM and DEM (concentration of B F 3 etherate 6 x 10 -8 mole/l., 50~ The curve corresponds to the solution of expression (13) obtained for k2= 2.5 • 10-1 sec-1 (calculated for the full catalyst concentration).

2380

I . A . ABRAMYA~ et at.

The order of reaction with respect to catalyst c o n c e n t r a t i o n was examined. Figure 2 shows t h a t over the studied concentration interval (5 • 10-a-2.5 • l0 -~ mole/1.) the reaction is of the first order. I t follows from e q u a t i o n (13) t h a t the reaction between DMM a n d D E M reaches equilibrium when the c o n c e n t r a t i o n of EMM is 0 r a = 2r O0 lro/ (r 01 ~ r2).

(14)

I n c o n f o r m i t y with formula (14) we f o u n d t h a t for t h e equimolar m i x t u r e in the range of t e m p e r a t u r e from 20 to 80 ~ t h e equilibrium composition of the m i x t u r e DMM : D E M : EMM remains c o n s t a n t at 1 : 1 : 2. We also m e a s u r e d the initial rates of reaction a t t h r e e different starting ratios of DMM and DEM, n a m e l y 1 : 1, 1 : 2 and 2 : 1. I t is seen from Fig. 3 t h a t these rates are practically t h e same. The theoretical ratio of the rates for m i x t u r e s of these compositions calculated b y means of e q u a t i o n (12) are 1.1 : 1 : 1. Thus the e x p e r i m e n t a l results s u p p o r t the initial assumption t h a t the t y p e of alkyl radical has only a weak effect on t h e r e a c t i v i t y of the acetals.

A, mo/e %

~~

20~/~~-

IOvE

~-.

0

a

2

I _L__.~___ It

~

FIG. 1. The kinetics of formation of EMM (A). Molar ratio of initial concentrations os DMM and 'DEM 1 : 1, 50 ~ concentration os BF3 etherate 6 • 10.3 mole/1. The t e m p e r a t u r e d e p e n d e n c e of the r a t e c o n s t a n t (k) was d e t e r m i n e d over t h e t e m p e r a t u r e interval f r o m 15 to 60 ~, with the following results: Temperatm'e, ~

15

ks• 1031./mole.see

0-91

35

40

50

60

0"35 4-4

8"9

12

T h e e n e r g y of activation, which is 1.5 keal/mole, was f o u n d f r o m t h e dependence of log k~ on 1 / T (Fig. 4). As would be e x p e c t e d from the similarity of t h e rea c t i o n mechanisms, this is close to the e n e r g y of a c t i v a t i o n for p o l y m e r i z a t i o n of cyclic acetals. T h u s the s t u d y of the exchange reaction between low molecular weight acetals, carried out in the present work and also in reference [5], indicates t h a t the t y p e of alkyl radical a n d the n u m b e r of acetal bonds in oligomers h a v e o n l y a

Study of chain transfer

2381

w e a k effect on the r e a c t i v i t y of acetals (calculated for one o x y g e n atom). This m a k e s it possible to consider similar s y s t e m s as models w i t h r e s p e c t to CTS reactions i n . a c e t a l p o l y m e r s , n o t only f r o m t h e p o i n t of v i e w of the r e a c t i o n m e c h a n i s m , b u t also of t h e q u a n t i t a t i v e values of t h e kinetic p a r a m e t e r s . A, mole%

3

wo /5

/ I

zO

20 c~;O3, r:zole/L.

FIG. 2

O

1

2

Time, hr

3

FIG. 3

3

I

3.5 I IF~ I03

FIG. 4

FIG. 2. Dependence of the initial reaction rate (in relative units) on catalyst concentration at 50 ~. FIG. 3. Kinetics of formation of EMM (A) at various molar ratios of DEN and DMM: 1--1 : 1; 2--2 : 1; 3--1 : 2, 50 ~ concentration of BF3 etherate 2.1 • 10 -3 mole/1. FIG. 4. Dependence of log k2 on l I T . CONCLUSIONS

T h e kinetics of chain t r a n s f e r w i t h scission h a v e b e e n studied b y m e a n s o f t h e m o d e l reaction of alkyl e x c h a n g e b e t w e e n d i m e t h o x y - a n d d i e t h o x y m e t h ane. B o r o n trifluoride e t h e r a t e was used as catalyst. T h e r a t e c o n s t a n t of t h e r a t e - d e t e r m i n i n g step of t h e reaction, n a m e l y d e c o m p o s i t i o n of t h e o x o n i u m ions, c a l c u l a t e d on t h e basis of t h e full c a t a l y s t c o n c e n t r a t i o n is 2.5 • 10 -1 sec -1 (at 50 ~ a n d t h e e n e r g y of a c t i v a t i o n is 11.5 kcal/mole. Translated by E. O. PHILLIPS

REFERENCES

1. N. S. YENIKOLOPYAN, J. Polymer Sci. 58: 1301, 1962 2. V. V. IVANOV, A. A. SHAGINYAN, V. P. VOLKOV and N. S. YENIKOLOPYAN, Vysokotool. soyed. 7: 1830, 1965 (Translated in Poly. Sci. U.S.S.R. 7: 10, 2015, 1965)

3. O. A. PLECI-IOVA, V. V. IVANOV and N. S. YENIKOLOPYAN, Dokl. Akad. Nauk SSSR 166: 905, 1966 4. A. A. BERLIN, V. V. IVANOV and N. S. YENIKOLOPYAN, Vysokomol. soyed. Bg: 61, 1967 (Not translated by Pergamon Press) 5. V. V. IVANOV, V. P. VOLKOV, V. N. IVANOV and N. S. YENIKOLOPYAN, Kinetika i kataliz 7: 763, 1966 6. D. M. VINOKUROV, l~auchnye doklady vysshei shkoly, Lekoinzhenernoye delo, 1958