- Email: [email protected]
Specific features of polymerization of formaldehyde in aromatic solvents
SPECIFIC FEATURES OF POLYMERIZATION OF FORMALDEHYDE IN AROMATIC SOLVENTS* L. L. IvA~ovA, N A GREBEbINIKOVA,G P
SOKOLOVA,
L. A. DUDINA and N. S. YENIKOLOPYAN I n s t i t u t e o f C h e m m a l P h y s m s , U . S S R A c a d e m y o f Semnces (Recewed 21 &Iareh 1974)
T h e k m e t m s o f p o l y m e r l z a t m n o f f o r m a l d e h y d e m s o l u h o n m b e n z e n e a t 40 °, w i t h B F s - e t h e r a t e as c a t a l y s t , h a s b e e n s t u d m d . A r e a c t m n s c h e m e m p r o p o s e d , w h m h i n c l u d e s a s t a g e of i n s t a n t a n e o u s m l t m t m n w i t h p a r t t a l d e a c t l v a t m d o f t h e c a t a l y s t , blmolecular propagation and hnear tel~ninatmn. The rate constants of propagatmn and termmatmn were found It was found that over the range of catalyst concentrat m n s of 6 × 10-4-3 × 10 -3 mole/1, t h e r e a c t m n r a t e is & f f u s m n c o n t r o l l e d or t h e t y p e o f c o n t r o l is t r a n s l h o n a l A t a c a t a l y s t c o n e o n t r a t t o n o f 4 × 10 -4 mole/1, t h e r e a c t t o n is m a i n l y k m e t m a l l y c o n t r o l l e d
GASEOUS formaldehyde (FA) has limited solubility in non-polar solvents [1]. Polyoxymethylene of high molecular weight is however prepared b y cationic polymerisation in such solvents [2]. Because of the low concentration of formaldehyde m the solution the process is carried out with passage of a constant stream of gaseous F A over the solution and vigorous stirring in order to create conditions most favourable for contact between the monomer and the liquid. I t would be natural to expect that in this method of polymerization a considerable part is played b y diffusion of monomer into the solution. The purpose of the present work was to s t u d y the effect of diffusion of the monomer on the kinetics of polymerization of FA in solution in benzene, with BFa-etherate as catalyst. T h e b e n z e n e w a s t r e a t e d w i t h c o n c e n t r a t e d H~SO,, w a s h e d w i t h s o d i u m c a r b o n a t e s o l u t i o n , t h e n wath w a t e r t o n e u t r a l r e a c t i o n , d r i e d o v e r e a l e m m c h l o r i d e a n d m e t a l h e s o d i u m and stored over metalho sodium. The water content of the benzene, determined by Flseher's method, was 0.001-0-002% by weight T h e b o r o n t r l f l u o m d e e t h e r a t e w a s r e d l s t d l e d i n vacuo a n d s t o r e d m sealed a m p o u l e s . P o l y m e m z a t l o n w a s e a r r m d o u t m a n a t m o s p h e r e o f d r y a r g o n . G a s e o u s F A w a s prep a r e d b y d e c o m p o m t i o n o f a - p o l y o x y m e t h y l e n e a t 150-160 ° a n d pumfled b y p a s s i n g It t h r o u g h a s y s t e m o f cold t r a p s a t g r a d u a l l y d e e r e a s m g t e m p e r a t u r e s f r o m 10 ° t o - - 1 6 °. T h e p u r i t y o f t h e g a s w a s d e t e r m i n e d b y g a s p h a s e c h r o m a t o g r a p h y . T h e p u r i f i e d F A was p a s s e d c o n t i n u o u s l y r o t e a m e t a l r e a c t o r f i t t e d w i t h a n a n c h o r s t i r r e r , c o n t a i n i n g 250 m l o f b e n z e n e . T h e flow r a t e o f F A w a s s u c h t h a t t h e r a t e o f s o l u t i o n o f t h e g a s e o u s m o n o m e r w a s a l w a y s * V y s o k o m o l . soyed. A I 7 : ~ o
6, 1229-1234, 1975. 1407
1408
L. L. I v ~ o v i
e~ al.
less than the flow rate. The excess gas was led off through a trap. After the benzene had become saturated with formaldehyde a solution of boron trifluonde etherate was added from a syringe. Polymerization was carried out at 40 °. At the end of the process supply of FA to the reactor was stopped and methanol was added ~o neutrahze the catalyst. The polymer was filtered off, washed with 10% ammoma solution, water and methanol, and dried ~n vacuo at 60°.
P,g/l. ZSO!
200
,s" °'"
/11 // ,~/
wo,#/l..min °
"
"Z¢
E
120
~
t..o
I, ,41-~
/ I
0
O0
80
I
E /0
[J
2O
T/mejm/n Fzo. I
II
i
30 co"I0 ~,,'.o/e/l.
F1o. 2
FiG. 1. Curves of the dependence of the yield of polymer on polymermation time at 40 ° m benzene, with c 0 ~ 4 × 10-~ (1, 1"), 0 (spontaneous polymenzatlon) (2, 2'), 6X 10-4 (3, 3"), 8 × 10 -4 (4, 4'), 2 × 10-3 (5, 5') and 3 × 10-a molefl. (6"), 6--yield of polymer m the diffusioncontrolled regzon, 1-6--thcoretlcal curves, pomts--expernnental results. FIG. 2. Dependence of the inltml polymermation rate on catalyst concentration. W i t h t h e p r e s e n t m e t h o d s o f purification gaseous F A contains traces o f w a t e r m e t h a n o l , f o r m i c acid etc., t h e t o t a l q u a n t i t y o f w h i c h was 0-2% in o u r case T h e impurities can initiate ionic p o l y m e r i z a t i o n o f F A a n d t h e r e f o r e we first s t u d i e d t h e possibility o f f o r m a t i o n o f p o l y m e r w i t h o u t a specially a d d e d c a t a l y s t . I t was f o u n d t h a t a t 40 ° a solution o f F A in benzene, w i t h a c o n s t a n t s t r e a m o f gaseous m o n o m e r passing over it, contains no t r a c e o f solid p o l y m e r for 20 rain, t h e n h i g h - p o l y m e r (Fig. 1, c u r v e 2) o f molecular w e i g h t m a n y t i m e s g r e a t e r t h a n t h e molecular w e i g h t o f t h e p o l y m e r o b t a i n e d b y t h e cationic m e t h o d is f o r m e d . M o r e o v e r 1,3-dioxolane, w h i c h copolymerizes w i t h F A o n l y b y t h e c~tionic m e t h o d , does n o t r e a c t u n d e r these conditions.
Specific features of polymerization of formaldehyde
1409
I f this polymer had been formed on a weak cationic catalyst or b y a pseudocationic mechanism its yield should be added to the yield of polymer formed when a cationic catalyst is added. This effect should be particularly noticeable when small amounts of a cationic catalyst are added. I t is seen from Fig. 1 (curves 1 and 3) however t h a t almost no increase in the yield of cationic polymer occurs after 30-40 min. Hence the spontaneous polymerization occurs b y an anionic mechanism. We did not set out to s t u d y this problem and therefore we based our conclusions on the assumption that the anionic catalyst, an impurity, is supplied at a constant rate to the solution from the gas and when it reaches a certain concentration (from the molecular weight of the "spontaneous polymer" this concentration is 1.0× 10-5 mole/1.) it initiates anionic polymerization of the FA ff no cationic catalyst has been added Since the purity of the gas undergoes no significant alteration during an experiment we calculated that the catalytic impurity is supplied to the solution at the constant rate of fl~--5× 10 -v mole/1./min and when cationic active centres are present it reacts with them almost instantaneously. Polymerization could have been carried out for 20 min, it being considered that during this time no appreciable amount of anionic material would have been added, b u t in production of polyformaldehyde the process lasts for some hours and description of the kinetic relationships during the first 20 min would have been incomplete. The dependence of the initial rate of polymerization on catalyst concentration is shown in Fig. 2. The following specific features are worthy of attention. 1. There is no induction period, which can be a sign of rapid initiation. I t is known from work on polymerization of styrene in the presence of BFaetherate with added FA, that the latter greatly accelerates the initial polymerization rate [3]. On the basis of these facts it m a y be assumed that in polymerization of F A on BFa-etherate initiation is instantaneous. 2. The rate of polymerization falls during the process i e the active centres decay in the course of time. Like the authors of references [4] and [5] we consider that in heterogeneous polymerization of FA "physical" decay of active centres is most probable, occurring as a result of their embedment in the mass of polymer (linear decay of the active eentres, with the rate constant kd). 3. There is a minimal catalyst concentration, below which no polymer is formed, because the curve of the dependence of the initial polymerization rate on catalyst concentration does not go to zero This has been reported before in polymerization of FA at --30 ° with SnCla as catalyst. As in reference [6] it is natural to assume that some of the catalyst is deactivated b y impurities, especially water, which is present in the solvent. The fraction of deactivated catalyst Cdea¢ can be found from the point of intersection with the abscissa of the curve of the dependence of the initial polymerization rate on catalyst concentration This fraction is 3.6 × 10 -4 mole/1 The moisture content of the benzene was less than 0.002%, i e. a concentration of water of 3.6× 10 -4 mole/1, is quite probable in " d r y " benzene.
1410
L. L. I v ~ o v A e~ al.
4. The initial propagation rates are practically the same over the range of catalyst concentrations of 8 × 10-4-3 × 10-a mole/1. This shows that the process is diffusion controlled or is transitional in this respect. Taking into account all of the above features of the process the following scheme m a y be presented. 1. Instantaneous initiation, the concentration of active centres at zero time being C0=C~--Cdea~ , where C~nt is the initial catalyst concentration. 2. Linear time dependence of the decay of active centres and instantaneous decay on anionic impurities. Then the variation in concentration of active centres is given b y dc*/dt~- --lQc*--fl (1) Integrating expression (1) and bearing in mind the zero lower limit we obtain c*---- (kdc~'-]-'6)e-k**
kd
fl
(2)
kd
3. The expression for the rate of polymerization takes the form ]%
dP/dt ~-- ~ [M] [(kac~-~fl) e-Z, '-fl],
(3)
whore kp is the propagation rate constant, [M] the instantaneous monomer concentration and P the yield of polymer. I f the polymerization rate is much lower than the rate of solution of F A in benzene the instantaneous monomer concentration [M]----[M]0 will not v a r y with time and is equal to the solubility of F A in benzene.* Then the expression for the yield of polymer takes the form P----
h'p[M]0 (kdC~_~_]~)( 1--e-kd$) k~
kp[]~J0pt ]¢a
(4)
With an initial catalyst concentration of 4 × 10 -4 mole/1, the monomer concontration, measured during the course of polymerization, was 0.4=t=0.05 mole/1, i.e. at this catalyst concovtration it m a y be considered that the polymerization is kinetically controlled. The apparent propagation rate constant can be determined from the rate of polymerization at zero time. It was found to lie within the limits of 3500-6000 1./mole/min. The value of/c a can be found b y comparing the actual yields of polymer with a series of theoretical curves constructed with different values of lca. The experimental yields are in good agreement with the curve constructed with kd=0'06 rain -1. At higher catalyst concentrations than 4 × 1 0 -4 mole/1, the instantaneous monomer concentration varies and it is necessary to take into account diffusion of the monomer into the solution. Then the dependence of monomer concentra* We determined the solubility of F A m benzene exlaerlmentally. It is 0.5 mole/1 at 40°.
Specific features of polymerization of formaldehyde
1411
tion with time takes the form
dt
-- D ([M]o-- [M])-- kr [M]c*,
(5)
where [M]o-- [M] is the difference between the saturated concentration * and instantaneous concentration of monomer, kp[M]c* the polymerization rate, equal to dP/dt and D the coefficient of diffusion of monomer into the solution (the solution rate constant). Because of imperfections m the method of measurement of the coefficient o f diffusion its value is not accurate. The saturated monomer concentration is reached in 3-5 min however, and this corresponds to D----0.45 rain -1 From equation (5) the yield of polymer in the transitional region is given by P=D[M]ot--D S [M]dtq-[M]0--[M] (6) The instantaneous monomer concentration can be found b y integrabing expression (5) after substituting expression (2) for c*.
B y substitution for the variables the integral in the square brackets can be e-x
brought to the form S ~-~ dx, the precise solution of which is dependent on the power n. I f the polymer formed does not interact at all with the solvent it will not affect the solubility of the monomer in benzene and [M]0 will not vary with time. Polyformaldehyde is a polymer that swells in benzene, therefore the limiting concentration of monomer in the solution varies with time. Thus if in the course of polymerization 12-13 base-moles/1, of polymer (360-390 g/1.) were formed the concentration of monomer in this system would be zero regardless of the time of contact of the monomer with the reaction mixture. The most probable law b y which the solubility of the monomer varies with the concentration of polymer is
[Mole=
(8)
We chose a value of ~ based on the assumption that [Mt]o=0 when P = 12.5 • The concentration corresponding to the solubdrty of the monomer under the given conditlorm.
L . L . Iva_wovx et at.
~4~2
base-moles/1., which gives ==0-04. F i g u r e 3 shows t h e t i m e d e p e n d e n c e o f t h e s a t u r a t i o n m o n o m e r c o n c e n t r a t i o n a t various c a t a l y s t concentrations. T h e yield o f p o l y m e r is t a k e n f r o m e x p e r i m e n t a l m e a s u r e m e n t s . I t is seen t h a t the s a t u r a t i o n c o n c e n t r a t i o n o f m o n o m e r varies quite considerably w i t h t i m e a n d theret
fore it is n e c e s s a r y to p u t D S [Mt]0dt in expression (6) in place o f D[M]ot. I t was 0 n o t possible t o o b t a i n this expression in t h e a n a l y t i c a l f o r m a n d t h e r e f o r e i t is f o u n d b y n u m e r i c a l i n t e g r a t i o n f r o m Fig. 3. E x p r e s s i o n (7) will n o w a p p e a r a s
F i g u r e 4 presents theoretical curves o f t h e v a r i a t i o n in t h e i n s t a n t a n e o u s m o n o m e r c o n c e n t r a t i o n w i t h time. T h e e x p e r i m e n t a l results are shown b y t h e points. A t c a t a l y s t c o n c e n t r a t i o n s o f 8 × 10-4-2 × 10 -3 mole/1, o n l y a t r a c e o f [~1] mole/l
05 ~ j ~ # [Hf]o, molo/l. 0.5 1
3
t
,
3
O3
0
2O
O0 6O Tt'me , rmn FIe.
8
8O
gO
6O 7-tree ~rain
I tO0
FIG. 4
I~IG. 3. Theore~lcal curves of the variation in the saturation coneentrataon of monomer wxth polymerxzatxon tune, with c0----2× 10 -8 (/), 8 × 10 -~ (2) and 6 × 10-4 mole/1. (3). PIG. 4. Theoretical curves of the vanagaon m the msfa~ntaneous monomer concentration with kp-----6000 1/mole/ram, /¢a----0.06 mm -z and co----2× 10 -s (1), 8)< 10-4 (2), 6)< 10-' (3t and 4 X 10-' mole/1. (g). The points represent experimental results. m o n o m e r was d e t e c t e d e x p e r i m e n t a l l y during t h e course o f polymerization. T h e a c c u r a c y o f d e t e r m i n a t i o n of t h e i n s t a n t a n e o u s m o n o m e r c o n c e n t r a t i o n falls as t h e c o n c e n t r a t i o n o f p o l y m e r increases, because o f t h e sharp r e d u c t i o n
Specffie features of polymerization of formaldehyde
1413
in the volume of the liquid part of the sample. TMs explains the divergence of the theoretical and experimental relationships at catalyst concentrations above 6 × 10 -4 mole/1 I t is seen from Fig. 4 t h a t with the given polymerization parameters, for a catalyst concentration of 4 × 10 -4 mole/1, the instantaneous monomer concentration at the beginning of polymerization is below the concentration corresponding to the solubility of the monomer under the given conditions. Takmg all the special features mentioned above into account we constructed theoretical curves of the dependence of the yield of polymer on time at various catalyst concentrations The theoretical curves, which we calculated for the conditions kp=6000 1./ /mole/min, kd=0.06 min-l,a-~0.04, f l = 5 × 10-7 mole/1./min and D = 0 . 4 5 min -1, are drawn as continuous lines in Fig. 1. I t is easy to see t h a t there is good agreement between the theoretical curves and the experimental results. Curve 6 in Fig. 1 was calculated from the formula p=
[M]0 ( l _ e _ ~ D t ) '
(10)
which was obtained from expression (5), assumirtg t h a t d[M]]dt and [M] are zero (all the monomer entering the system polymerizes under pseudo stonily state conditions), with substitution of expression (8). This curve characterized the yield of polymer when polymerization is entirely diffusion controlled. The yield of polymer found experimentally at a catalyst concentration of 3 × 10-3 mole/l. follows curve 6 approximately. Thus polymerization of F A in solution in benzene with BF3-otherato as catalyst in concentrations between 4 × 10 -a and 3× 10-3 molo]l, is controlled by the rate of diffusion of the monomer or is in a transitional state in this respect. The l~inetie parameters found describe the process satisfactorily.
Translated by E. O. PHILLIPS REFERENCES
1. J. F. WALKER, Formal'degad (Formaldehyde). p. 60, 1957 (Rusman translation) 2. N. S. YENIKOLOPYAN and S. A. VOL'FSON, Khilm'ya i tekhnologiya pohformal'degida (The Chemistry and Technology of Polyformaldehyde) p. 204, Izd. "Khnniya", 1968 3. W. FU~DA, N. OGATA and H. KAKIUglII, J. Polymer Sci. A-I, 9: 1537, 1971 4. V. JAA~KS and R. BOEI~LKE, Makromolek. Chem. 142: 189, 1970 5. Yu. N. SMIRNOV, V. P. VOLKOV, B. A. ROZENBER{~ and N. S. YENIKOLOPYAN, Vysokomol. soyed. A16: 283, 1972 (Translated m Polymer Sei. U.S.S.R. 16: 2, 327, 1974) 6. I. F. SANAYA, Dissertation, 1973
SOKOLOVA,
L. A. DUDINA and N. S. YENIKOLOPYAN I n s t i t u t e o f C h e m m a l P h y s m s , U . S S R A c a d e m y o f Semnces (Recewed 21 &Iareh 1974)
T h e k m e t m s o f p o l y m e r l z a t m n o f f o r m a l d e h y d e m s o l u h o n m b e n z e n e a t 40 °, w i t h B F s - e t h e r a t e as c a t a l y s t , h a s b e e n s t u d m d . A r e a c t m n s c h e m e m p r o p o s e d , w h m h i n c l u d e s a s t a g e of i n s t a n t a n e o u s m l t m t m n w i t h p a r t t a l d e a c t l v a t m d o f t h e c a t a l y s t , blmolecular propagation and hnear tel~ninatmn. The rate constants of propagatmn and termmatmn were found It was found that over the range of catalyst concentrat m n s of 6 × 10-4-3 × 10 -3 mole/1, t h e r e a c t m n r a t e is & f f u s m n c o n t r o l l e d or t h e t y p e o f c o n t r o l is t r a n s l h o n a l A t a c a t a l y s t c o n e o n t r a t t o n o f 4 × 10 -4 mole/1, t h e r e a c t t o n is m a i n l y k m e t m a l l y c o n t r o l l e d
GASEOUS formaldehyde (FA) has limited solubility in non-polar solvents [1]. Polyoxymethylene of high molecular weight is however prepared b y cationic polymerisation in such solvents [2]. Because of the low concentration of formaldehyde m the solution the process is carried out with passage of a constant stream of gaseous F A over the solution and vigorous stirring in order to create conditions most favourable for contact between the monomer and the liquid. I t would be natural to expect that in this method of polymerization a considerable part is played b y diffusion of monomer into the solution. The purpose of the present work was to s t u d y the effect of diffusion of the monomer on the kinetics of polymerization of FA in solution in benzene, with BFa-etherate as catalyst. T h e b e n z e n e w a s t r e a t e d w i t h c o n c e n t r a t e d H~SO,, w a s h e d w i t h s o d i u m c a r b o n a t e s o l u t i o n , t h e n wath w a t e r t o n e u t r a l r e a c t i o n , d r i e d o v e r e a l e m m c h l o r i d e a n d m e t a l h e s o d i u m and stored over metalho sodium. The water content of the benzene, determined by Flseher's method, was 0.001-0-002% by weight T h e b o r o n t r l f l u o m d e e t h e r a t e w a s r e d l s t d l e d i n vacuo a n d s t o r e d m sealed a m p o u l e s . P o l y m e m z a t l o n w a s e a r r m d o u t m a n a t m o s p h e r e o f d r y a r g o n . G a s e o u s F A w a s prep a r e d b y d e c o m p o m t i o n o f a - p o l y o x y m e t h y l e n e a t 150-160 ° a n d pumfled b y p a s s i n g It t h r o u g h a s y s t e m o f cold t r a p s a t g r a d u a l l y d e e r e a s m g t e m p e r a t u r e s f r o m 10 ° t o - - 1 6 °. T h e p u r i t y o f t h e g a s w a s d e t e r m i n e d b y g a s p h a s e c h r o m a t o g r a p h y . T h e p u r i f i e d F A was p a s s e d c o n t i n u o u s l y r o t e a m e t a l r e a c t o r f i t t e d w i t h a n a n c h o r s t i r r e r , c o n t a i n i n g 250 m l o f b e n z e n e . T h e flow r a t e o f F A w a s s u c h t h a t t h e r a t e o f s o l u t i o n o f t h e g a s e o u s m o n o m e r w a s a l w a y s * V y s o k o m o l . soyed. A I 7 : ~ o
6, 1229-1234, 1975. 1407
1408
L. L. I v ~ o v i
e~ al.
less than the flow rate. The excess gas was led off through a trap. After the benzene had become saturated with formaldehyde a solution of boron trifluonde etherate was added from a syringe. Polymerization was carried out at 40 °. At the end of the process supply of FA to the reactor was stopped and methanol was added ~o neutrahze the catalyst. The polymer was filtered off, washed with 10% ammoma solution, water and methanol, and dried ~n vacuo at 60°.
P,g/l. ZSO!
200
,s" °'"
/11 // ,~/
wo,#/l..min °
"
"Z¢
E
120
~
t..o
I, ,41-~
/ I
0
O0
80
I
E /0
[J
2O
T/mejm/n Fzo. I
II
i
30 co"I0 ~,,'.o/e/l.
F1o. 2
FiG. 1. Curves of the dependence of the yield of polymer on polymermation time at 40 ° m benzene, with c 0 ~ 4 × 10-~ (1, 1"), 0 (spontaneous polymenzatlon) (2, 2'), 6X 10-4 (3, 3"), 8 × 10 -4 (4, 4'), 2 × 10-3 (5, 5') and 3 × 10-a molefl. (6"), 6--yield of polymer m the diffusioncontrolled regzon, 1-6--thcoretlcal curves, pomts--expernnental results. FIG. 2. Dependence of the inltml polymermation rate on catalyst concentration. W i t h t h e p r e s e n t m e t h o d s o f purification gaseous F A contains traces o f w a t e r m e t h a n o l , f o r m i c acid etc., t h e t o t a l q u a n t i t y o f w h i c h was 0-2% in o u r case T h e impurities can initiate ionic p o l y m e r i z a t i o n o f F A a n d t h e r e f o r e we first s t u d i e d t h e possibility o f f o r m a t i o n o f p o l y m e r w i t h o u t a specially a d d e d c a t a l y s t . I t was f o u n d t h a t a t 40 ° a solution o f F A in benzene, w i t h a c o n s t a n t s t r e a m o f gaseous m o n o m e r passing over it, contains no t r a c e o f solid p o l y m e r for 20 rain, t h e n h i g h - p o l y m e r (Fig. 1, c u r v e 2) o f molecular w e i g h t m a n y t i m e s g r e a t e r t h a n t h e molecular w e i g h t o f t h e p o l y m e r o b t a i n e d b y t h e cationic m e t h o d is f o r m e d . M o r e o v e r 1,3-dioxolane, w h i c h copolymerizes w i t h F A o n l y b y t h e c~tionic m e t h o d , does n o t r e a c t u n d e r these conditions.
Specific features of polymerization of formaldehyde
1409
I f this polymer had been formed on a weak cationic catalyst or b y a pseudocationic mechanism its yield should be added to the yield of polymer formed when a cationic catalyst is added. This effect should be particularly noticeable when small amounts of a cationic catalyst are added. I t is seen from Fig. 1 (curves 1 and 3) however t h a t almost no increase in the yield of cationic polymer occurs after 30-40 min. Hence the spontaneous polymerization occurs b y an anionic mechanism. We did not set out to s t u d y this problem and therefore we based our conclusions on the assumption that the anionic catalyst, an impurity, is supplied at a constant rate to the solution from the gas and when it reaches a certain concentration (from the molecular weight of the "spontaneous polymer" this concentration is 1.0× 10-5 mole/1.) it initiates anionic polymerization of the FA ff no cationic catalyst has been added Since the purity of the gas undergoes no significant alteration during an experiment we calculated that the catalytic impurity is supplied to the solution at the constant rate of fl~--5× 10 -v mole/1./min and when cationic active centres are present it reacts with them almost instantaneously. Polymerization could have been carried out for 20 min, it being considered that during this time no appreciable amount of anionic material would have been added, b u t in production of polyformaldehyde the process lasts for some hours and description of the kinetic relationships during the first 20 min would have been incomplete. The dependence of the initial rate of polymerization on catalyst concentration is shown in Fig. 2. The following specific features are worthy of attention. 1. There is no induction period, which can be a sign of rapid initiation. I t is known from work on polymerization of styrene in the presence of BFaetherate with added FA, that the latter greatly accelerates the initial polymerization rate [3]. On the basis of these facts it m a y be assumed that in polymerization of F A on BFa-etherate initiation is instantaneous. 2. The rate of polymerization falls during the process i e the active centres decay in the course of time. Like the authors of references [4] and [5] we consider that in heterogeneous polymerization of FA "physical" decay of active centres is most probable, occurring as a result of their embedment in the mass of polymer (linear decay of the active eentres, with the rate constant kd). 3. There is a minimal catalyst concentration, below which no polymer is formed, because the curve of the dependence of the initial polymerization rate on catalyst concentration does not go to zero This has been reported before in polymerization of FA at --30 ° with SnCla as catalyst. As in reference [6] it is natural to assume that some of the catalyst is deactivated b y impurities, especially water, which is present in the solvent. The fraction of deactivated catalyst Cdea¢ can be found from the point of intersection with the abscissa of the curve of the dependence of the initial polymerization rate on catalyst concentration This fraction is 3.6 × 10 -4 mole/1 The moisture content of the benzene was less than 0.002%, i e. a concentration of water of 3.6× 10 -4 mole/1, is quite probable in " d r y " benzene.
1410
L. L. I v ~ o v A e~ al.
4. The initial propagation rates are practically the same over the range of catalyst concentrations of 8 × 10-4-3 × 10-a mole/1. This shows that the process is diffusion controlled or is transitional in this respect. Taking into account all of the above features of the process the following scheme m a y be presented. 1. Instantaneous initiation, the concentration of active centres at zero time being C0=C~--Cdea~ , where C~nt is the initial catalyst concentration. 2. Linear time dependence of the decay of active centres and instantaneous decay on anionic impurities. Then the variation in concentration of active centres is given b y dc*/dt~- --lQc*--fl (1) Integrating expression (1) and bearing in mind the zero lower limit we obtain c*---- (kdc~'-]-'6)e-k**
kd
fl
(2)
kd
3. The expression for the rate of polymerization takes the form ]%
dP/dt ~-- ~ [M] [(kac~-~fl) e-Z, '-fl],
(3)
whore kp is the propagation rate constant, [M] the instantaneous monomer concentration and P the yield of polymer. I f the polymerization rate is much lower than the rate of solution of F A in benzene the instantaneous monomer concentration [M]----[M]0 will not v a r y with time and is equal to the solubility of F A in benzene.* Then the expression for the yield of polymer takes the form P----
h'p[M]0 (kdC~_~_]~)( 1--e-kd$) k~
kp[]~J0pt ]¢a
(4)
With an initial catalyst concentration of 4 × 10 -4 mole/1, the monomer concontration, measured during the course of polymerization, was 0.4=t=0.05 mole/1, i.e. at this catalyst concovtration it m a y be considered that the polymerization is kinetically controlled. The apparent propagation rate constant can be determined from the rate of polymerization at zero time. It was found to lie within the limits of 3500-6000 1./mole/min. The value of/c a can be found b y comparing the actual yields of polymer with a series of theoretical curves constructed with different values of lca. The experimental yields are in good agreement with the curve constructed with kd=0'06 rain -1. At higher catalyst concentrations than 4 × 1 0 -4 mole/1, the instantaneous monomer concentration varies and it is necessary to take into account diffusion of the monomer into the solution. Then the dependence of monomer concentra* We determined the solubility of F A m benzene exlaerlmentally. It is 0.5 mole/1 at 40°.
Specific features of polymerization of formaldehyde
1411
tion with time takes the form
dt
-- D ([M]o-- [M])-- kr [M]c*,
(5)
where [M]o-- [M] is the difference between the saturated concentration * and instantaneous concentration of monomer, kp[M]c* the polymerization rate, equal to dP/dt and D the coefficient of diffusion of monomer into the solution (the solution rate constant). Because of imperfections m the method of measurement of the coefficient o f diffusion its value is not accurate. The saturated monomer concentration is reached in 3-5 min however, and this corresponds to D----0.45 rain -1 From equation (5) the yield of polymer in the transitional region is given by P=D[M]ot--D S [M]dtq-[M]0--[M] (6) The instantaneous monomer concentration can be found b y integrabing expression (5) after substituting expression (2) for c*.
B y substitution for the variables the integral in the square brackets can be e-x
brought to the form S ~-~ dx, the precise solution of which is dependent on the power n. I f the polymer formed does not interact at all with the solvent it will not affect the solubility of the monomer in benzene and [M]0 will not vary with time. Polyformaldehyde is a polymer that swells in benzene, therefore the limiting concentration of monomer in the solution varies with time. Thus if in the course of polymerization 12-13 base-moles/1, of polymer (360-390 g/1.) were formed the concentration of monomer in this system would be zero regardless of the time of contact of the monomer with the reaction mixture. The most probable law b y which the solubility of the monomer varies with the concentration of polymer is
[Mole=
(8)
We chose a value of ~ based on the assumption that [Mt]o=0 when P = 12.5 • The concentration corresponding to the solubdrty of the monomer under the given conditlorm.
L . L . Iva_wovx et at.
~4~2
base-moles/1., which gives ==0-04. F i g u r e 3 shows t h e t i m e d e p e n d e n c e o f t h e s a t u r a t i o n m o n o m e r c o n c e n t r a t i o n a t various c a t a l y s t concentrations. T h e yield o f p o l y m e r is t a k e n f r o m e x p e r i m e n t a l m e a s u r e m e n t s . I t is seen t h a t the s a t u r a t i o n c o n c e n t r a t i o n o f m o n o m e r varies quite considerably w i t h t i m e a n d theret
fore it is n e c e s s a r y to p u t D S [Mt]0dt in expression (6) in place o f D[M]ot. I t was 0 n o t possible t o o b t a i n this expression in t h e a n a l y t i c a l f o r m a n d t h e r e f o r e i t is f o u n d b y n u m e r i c a l i n t e g r a t i o n f r o m Fig. 3. E x p r e s s i o n (7) will n o w a p p e a r a s
F i g u r e 4 presents theoretical curves o f t h e v a r i a t i o n in t h e i n s t a n t a n e o u s m o n o m e r c o n c e n t r a t i o n w i t h time. T h e e x p e r i m e n t a l results are shown b y t h e points. A t c a t a l y s t c o n c e n t r a t i o n s o f 8 × 10-4-2 × 10 -3 mole/1, o n l y a t r a c e o f [~1] mole/l
05 ~ j ~ # [Hf]o, molo/l. 0.5 1
3
t
,
3
O3
0
2O
O0 6O Tt'me , rmn FIe.
8
8O
gO
6O 7-tree ~rain
I tO0
FIG. 4
I~IG. 3. Theore~lcal curves of the variation in the saturation coneentrataon of monomer wxth polymerxzatxon tune, with c0----2× 10 -8 (/), 8 × 10 -~ (2) and 6 × 10-4 mole/1. (3). PIG. 4. Theoretical curves of the vanagaon m the msfa~ntaneous monomer concentration with kp-----6000 1/mole/ram, /¢a----0.06 mm -z and co----2× 10 -s (1), 8)< 10-4 (2), 6)< 10-' (3t and 4 X 10-' mole/1. (g). The points represent experimental results. m o n o m e r was d e t e c t e d e x p e r i m e n t a l l y during t h e course o f polymerization. T h e a c c u r a c y o f d e t e r m i n a t i o n of t h e i n s t a n t a n e o u s m o n o m e r c o n c e n t r a t i o n falls as t h e c o n c e n t r a t i o n o f p o l y m e r increases, because o f t h e sharp r e d u c t i o n
Specffie features of polymerization of formaldehyde
1413
in the volume of the liquid part of the sample. TMs explains the divergence of the theoretical and experimental relationships at catalyst concentrations above 6 × 10 -4 mole/1 I t is seen from Fig. 4 t h a t with the given polymerization parameters, for a catalyst concentration of 4 × 10 -4 mole/1, the instantaneous monomer concentration at the beginning of polymerization is below the concentration corresponding to the solubility of the monomer under the given conditions. Takmg all the special features mentioned above into account we constructed theoretical curves of the dependence of the yield of polymer on time at various catalyst concentrations The theoretical curves, which we calculated for the conditions kp=6000 1./ /mole/min, kd=0.06 min-l,a-~0.04, f l = 5 × 10-7 mole/1./min and D = 0 . 4 5 min -1, are drawn as continuous lines in Fig. 1. I t is easy to see t h a t there is good agreement between the theoretical curves and the experimental results. Curve 6 in Fig. 1 was calculated from the formula p=
[M]0 ( l _ e _ ~ D t ) '
(10)
which was obtained from expression (5), assumirtg t h a t d[M]]dt and [M] are zero (all the monomer entering the system polymerizes under pseudo stonily state conditions), with substitution of expression (8). This curve characterized the yield of polymer when polymerization is entirely diffusion controlled. The yield of polymer found experimentally at a catalyst concentration of 3 × 10-3 mole/l. follows curve 6 approximately. Thus polymerization of F A in solution in benzene with BF3-otherato as catalyst in concentrations between 4 × 10 -a and 3× 10-3 molo]l, is controlled by the rate of diffusion of the monomer or is in a transitional state in this respect. The l~inetie parameters found describe the process satisfactorily.
Translated by E. O. PHILLIPS REFERENCES
1. J. F. WALKER, Formal'degad (Formaldehyde). p. 60, 1957 (Rusman translation) 2. N. S. YENIKOLOPYAN and S. A. VOL'FSON, Khilm'ya i tekhnologiya pohformal'degida (The Chemistry and Technology of Polyformaldehyde) p. 204, Izd. "Khnniya", 1968 3. W. FU~DA, N. OGATA and H. KAKIUglII, J. Polymer Sci. A-I, 9: 1537, 1971 4. V. JAA~KS and R. BOEI~LKE, Makromolek. Chem. 142: 189, 1970 5. Yu. N. SMIRNOV, V. P. VOLKOV, B. A. ROZENBER{~ and N. S. YENIKOLOPYAN, Vysokomol. soyed. A16: 283, 1972 (Translated m Polymer Sei. U.S.S.R. 16: 2, 327, 1974) 6. I. F. SANAYA, Dissertation, 1973