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

STUDY OF CHAIN TRANSFER W I T H SCISSION ON THE MODEL REACTION OF ALKYL EXCHANGE B E T W E E N DIMETHOXYAND DIETHOXYMETHANE * I. A. AB~AMYAN, V. V. IVANOV, G. V. RAKOVA, A. N. GORYACttEV and N. S. YENIKOLOPYA~ I n s t i t u t e o f C h e m i c a l P h y s i c s , U . S . S . R . A c a d e m y o f Sciences

(Received

14

July

1966)

IT wAS shown in references [1-4] that in ionic polymerization, in addition to the usual reactions of initiation, propagation and termination with transfer to monomer or other substances, in the general case it is necessary to take account of special types of reaction, which consist essentially in exchange reactions between the active centres and the main chain of the polymer molecule. Reactions of this type have been given the name of chain transfer reactions with scission (CTS). CTS reactions have a substantial effect on the entire kinetics of the polymerization process and on the properties of the polymers, hence study of these, and especially determination of the basic kinetic parameters, is of considerable interest. Direct s t u d y of CTS reactions in polymeric systems involves considerable difficulties however. In the first place accurate determination of the numberaverage molecular weight of the polymers is necessary and the heterogeneity of the system often complicates this. I t is much easier to study a CTS reaction in low molecular weight model systems. We have already studied the disproportionation of the dimethyl ether of dioxymethyleneglycol as a model of CTS reactions in polymers with an acetal structure (polyoxymethylene, polydioxolane, etc.) [2,5]. The present work is a continuation of the study of model reactions between low molecular weight acetals on the example of alkyl exchange between dimethoxy- and diethoxyethane (DMM and DEM) in the presence of boron trifluoride etherate as catalyst. EXPERIMENTAL T h e sealed t u b e m e t h o d w a s u s e d for s t u d y of t h e r e a c t i o n k i n e t i c s . T h e t u b e s were first s w e p t o u t w i t h d r y a r g o n , t h e n filled w i t h t h e s t a r t i n g m a t e r i a l s i n a d r y c a b i n e t , sealed a n d p l a c e d i n a t h e r m o s t a t . A f t e r a c h o s e n t i m e t h e t u b e s were o p e n e d a n d t h e r e a c t i o n w a s s t o p p e d b y a d d i t i o n of a q u e o u s a m m o n i a . T h e c o n c e n t r a t i o n s o f s t a r t i n g m a t e r i a l s a n d r e a c t i o n p r o d u c t ( e t h o x y - m e t h o x y m e t h a n e (EMM) w e r e d e t e r m i n e d in a P e r k i n E l m e r 452 c h r o m a t o g r a p h i n w h i c h h e l i u m w a s u s e d as t h e c a r r i e r gas a n d t h e s t a t i o n a r y p h a s e w a s P E G 1500. T h e c o l u m n t e m p e r a t u r e w a s 90 ° . T h e c o n c e n t r a t i o n s of t h e c o m p o n e n t s were c a l c u l a t e d b y m e a n s of t h e f o r m u l a r~=(h~/Y, h~) × 100 m o l e % , w h e r e h, is t h e height of the appropriate peak. i * V y s o k o m o l . soyed. A9: No. 10, 2105-2108, 1967. 2377

I. A. ABRA~YAI~"et al.

2378

The DiM~ 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 the 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 the mechanism of alkyl exchange between ethylal and m e t h y l a l can be r e p r e s e n t e d as follows: ® R~OCH2ORt÷RzOCH~ ~

@ R~OCH2OR1,

I

(A)

CH~ oI

I Rz

® ® R~OCH~ORj -~ R,OCH~ORi+RjOCH,,

(B)

i CI-I, l o

l

R~ where Ri, Rj and R~ can be methyl or ethyl radicals. In this scheme the ratedetermining step is the decomposition of the oxonium ions (B). In general the rate constants k I and k 2 should be dependent on Ri, Rj and R v In 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 t h e 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 the rates o f reactions (A) and (B), are n o t v e r y different. T h e system of kinetic e q u a t i o n s corresponding to the above scheme takes t h e following form:

dr---a=k2(R~_2~+2R~_22+2R2_n+R~_2~)--2k1(R ~ + R + )r a dt

(1)

d R ? = ]~2(21~1_11_~_R1_21_~_2P~2_11~_ R2_21) __ 2]~1R1+(re ~_r0)

(2)

dR-o+ = k2(2R~_22+R2_21+2Rl_2~+Rl_21)--2klR ~ (r°l+r °)

(3)

dt dt

dRi-n dt

-- 2klR ~ r l - - 2k2R1_ll

dR1-21 -~ ]Q(I~+ r 3+ 2R1+ -~-r l ) - - 2k2R 1-21

dt

(4) (5)

Study of chain transfer

dR1-22 dt

dR2- ~ dt

2379

~ k i R ~ r a-lc~R~-22

(6)

kl R+ r 3 - 2k2R2_ll

(7)

=

dR2- ~1 --~k~R+ ra-F 2k~R + r~-- 2k2R2_~ dt

(8)

dR~-22 -~ 2 k l R + r2-- 2k2R~_~2, dt

(9)

where R + is the concentration of the carbonium ions R i - - O - - C H 2, R~_j~ the concentration of oxonium ions with the structure

Rj I

RI--O--CH2--Oe f

;

CH2 I

O--R~ and rl, r 2 and r a 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_n~-Rl_21-~ .Rl_22-FR2_n~-R2_21~-Rg_2~=C (where C is the total concentration of active centres) we obtain R + -{- R + = ]c2C/k~(r° + r°).

(10)

B y substituting in equations (2) and (3) the expression for Ri_jl found from formulae (4)-(9) we obtain: R:/R2

0 o2. -~rl/r

(11)

From equations (10) and (11) it is easy to obtain expressions for R~ and R +. By substituting these expressions in formulae (4)-(9) we obtain in a clear form expressions for Ri_jv From formula (1) it is then not difficult to find for the rate of formation of EMM drs _ k,~C [ " ' I " " r3 • -:0 -~ • (lt L(,~+r°) ~ ~l-kr,~

(1~)

Integration of expression (12) gives

,~00 -- ~rlr2

[l

o-(k~C/r~°fl-r:°)t'~

(13)

rI + r2

Figure 1 shows kinetic data for formation of EMM b y reaction of equimolar quantities of DMM and DEM (concentration of B F a etherate 6× 10 -3 mole/l, 50°). The curve corresponds to the solution of expression (13) obtained for k 2= 2.5 × 10-1 sec-1 (calculated for the full catalyst concentration).

2380

I.A.

ABRAMYAN e~ al.

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 c o n c e n t r a t i o n interval (5 × 10-3-2.5 × 10 -2 mole/1.) the reaction is of the first order. I t follows from equation (13) t h a t the reaction between DMM and D E M reaches equilibrium when the concentration of EMM is 00 0 0 r 3 : 2rlr2/(r 1+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 the 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 ° the equilibrium composition of the m i x t u r e DMM : D E M : EMM remains constant at 1 : 1 : 2. We also measured the initial rates of reaction at three 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 the same. T h e 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. T h u s 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 the r e a c t i v i t y of the acetals.

A, mole %

o

20 10 I___

I

I

I

_-h~

8 z¢ h me. hi, FIG. 1. The kinetics of formation of EMM (A). Molar ratio of initial concentrations of DMM and DEM 1 : 1, 50°, concentration of BFa etherate 6 × 10-3 mole/1. T h e 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 constant (k) was d e t e r m i n e d over the t e m p e r a t u r e interval from 15 to 60 ° , with the following results: Temperature, °C

15

k2x 1031./mole'sec

0.91

35

40

50

60

0"35 4.4

8"9

12

The e n e r g y of activation, which is 1.5 kcal/mole, was f o u n d from the dependence of log b~ on 1/T (Fig. 4). As would be e x p e c t e d from the similarity of the reaction mechanisms, this is close to the e n e r g y of a c t i v a t i o n for polymerization o f 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 a n d also in reference [5], indicates t h a t the t y p e o f alkyl radical and the n u m b e r of acetal bonds in oligomers have only a

Study of chain transfer

2381

weak 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 makes it possible to consider similar systems as models with respect to CTS reactions in acetal polymers, n o t only from the p o i n t of view of the reaction 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 parameters.

log~ A,mole%

% 15

3

/

10

/0 ° / i i ×2 lm, 20 c, 18"v,mole/l. FIG. 2

o

I

1

I

2 T~ne, hr

/ I

t

3

3

FIG. 3

I

3.5 I/T~ I0J

FIe. 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 DEM and DMM: 1 -- 1 : 1; 2-- 2 : 1; 3-- 1 : 2, 50°, concentration of BFa etherate 2.1 × 10-* mole/1. FIG. 4. Dependence of log k2 on

lIT.

CONCLUSIONS

The kinetics of chain transfer with scission h a v e been studied b y m e a n s of the model reaction of alkyl e x c h a n g e between 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 etherate w a s used as catalyst. The rate c o n s t a n t of the r a t e - d e t e r m i n i n g step of the reaction, n a m e l y d e c o m p o s i t i o n of the o x o n i u m ions, c a l c u l a t e d on t h e basis of the 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 the energy 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, Vysokoreel. soyed. 7: 1830, 1965 (Translated in Poly. Sei. U.S.S.R. 7: 10, 2015, 1965) 3. O. A. PLECHOVA, 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. B9: 61, 1967 (Not translated by PergamorL 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, Nauchnye doklady vysshei shkoly, Lekoinzhenernoye delo, 1958