Effect of polar compounds on copolymerization of propylene oxide and tetrahydrofuran

PolymerScienceU.S.S.I~.Vol.22, No. 10, pp. 2571-2577,1980 Printedia Poland

0032-3950/80/102571-07507.50/0 © 1981PergamonPressLtd.

E F F E C T OF P O L A R COMPOUNDS ON COPOLYMERIZATION PROPYLENE OXIDE AND TETRAHYDROFURAN*

OF

G. 1~. KO]~IRATOV, R. A. BARZYKINA a n d G. V. KOROVINA Branch of the Institute of Chemical Physics, U.S.S.R. Academy of Sciences

(Received 27 September 1979) Propionaldehyde and sodium perchlorate have a marked effect not only on kinetics of copolymerization of propylene oxide and tetrahydrofuran (THF) initiated by BF3, and also on molecular weight characteristics of the copolymer obtained. The ability of polar compounds to solvate BFs complexes and growing centres slows down all the reactions in which they take part. For example, the rate of isomerization of propylene oxide and the formation of a cyclic re.trainer decreases, while the concentration of the latter during the process passes through the maximum. This is explained by the existence of chain transfer to a cyclic tetramer with ring opening. CATIONIC p o l y m e r i z a t i o n of cyclic esters is e x t r e m e l y sensitive t o the m e d i u m , in which t h e process takes place. T h e a d d i t i o n to the p o l y m e r i z a t i o n s y s t e m o f c o m p o u n d s t h a t can solvate the growing cation or counter-ion has a m a r k e d effect on f o r m a t i o n kinetics of the p o l y m e r p r o d u c t . E v e n slight a m o u n t s o f polar additives such as water, n i t r o m e t h a n e , sodium p e r c h l o r a t e h a v e a significant effect on t h e r a t e of p o l y m e r i z a t i o n of T H F [1]. This, a p p a r e n t l y , takes place as a result of t h e fact t h a t the solvated m a c r o c a t i o n reacts at a lower r a t e with the counter-ion a n d o t h e r reactive c o m p o u n d s in t h e p o l y m e r i z a t i o n system. P r o p i o n a l d e h y d e (PA) is a m o n g the polar substances which can solvate a c t i v e centres; it is f o r m e d as a result of isomerization o f p r o p y l e n e oxide (PO), as n o t e d w h e n s t u d y i n g c o p o l y m e r i z a t i o n of P O and T H F [2] a n d h o m o p o l y m e r i z a t i o n of T H F . I t was therefore interesting to d e t e r m i n e the effect of P A i n t r o d u c e d into the reaction m i x t u r e on c o p o l y m e r i z a t i o n of P O a n d T H F . Copolymerization of 1~0 a n d THI~ w i t h o u t a n y additive is characterized b y the existence of s e c o n d a r y reactions such as chain t r a n s f e r t o the m o n o m e r , resulting in the f o r m a t i o n of low molecular weight cyclic and linear oligomers [2, 3, 4] a n d isomerization of P O [2]. T h e addition to the p o l y m e r i z a t i o n s y s t e m of c o m p o u n d s containing h y d r o x y l (water, glycols) reduces t h e r a t e of undesirable reactions [5]. Similarly, sodium p e r c h l o r a t e c o m p l e t e l y suppresses isomerization o f P O in p o l y m e r i z a t i o n o f T H F i n i t i a t e d b y B F 3 - P O and reduces the r a t e o f d e a c t i v a t i o n o f active centres to such an e x t e n t t h a t an equilibrium degree * Vysokomol. soyed. A22: No. 10, 2342-2347, 1980. 2571

G. I~T. K O M R A T O V

2572

et a/.

of conversion [1] m a y be obtained. It was therefore interesting to examine t~ what extent the addition of NaCl04 reduces the rate of secondary reactions in copolymerization of PO and T H F , initiated b y B:Fa. Kinetic curves showing the formation of a copolymer in the presence and absence of P A and NaC10~ are shown in Fig. 1. It can be seen that the addition to the reaction mixture of P A reduces process rate, however, copolymer yield increases. The addition to the polymerization system of sodium perchlorate considerably reduces not only the rate of copolymerization, b u t also copolymer yield. This differs noticeably from the effect of NaCl04 on homopolymerization of T H F , where with a considerable reduction in process rate the yield of p o l y - T H F increases reaching equilibrium value [1].

2 ~,% 60

I

~

q J

I

I

-O

"3

J

J

60

I

Time, hp

!

I

120

FIG. 1. Kinetic curves of the formation of a PO and THF copolymer in bulk at 20°, [BFs]e=9"3×10 -3 mole/1. Here and in Figs. 2-6: 1--[PO]=l.9; [THF]o = 10.7 molefl.; 2--[PO]o--[PA]0~ 1.88; [THF]o= 9.1 mole/1.; 3--[POlo= 1.9; [THF]o = 10.7; [NaC104]e=46"5× 10-3 mole/1. Figure 2a, which shows kinetic curves of the consumption of T H F indicate~ t h a t the concentration of T H F in the system at the end of the process only approximates to the equilibrium value in the presence of PA. Without additives. and particularly with sodium perchlorate, T H F consumption is much lower. P A and NaC104 have a similar effect on PO consumption in copolymerization (Fig. 2b). It is interesting to note that the relative rate of PO loss (i.e. w~o [PO]o) is higher than the relative rate of consumption of TH:F. The analysis of these values is complicated b y the fact that PO is used not only in reactions of chain extension, b u t also in reinitiation and isomerization. The latter reaction m a y somewhat reduce the rate of consumption of PA in copolymerization with P O and T H F (Fig. 2b). However, in spite of this the rate of PA loss is higher than the rate of. consumption of PO and T H F . Figure 3 shows kinetic curves of concentration variation of active centres in the absence and presence of P A and sodium perchlorate. Without additives,

Copolymerization of propylene oxide and tetrahydrofuraaa

2573

passing through a m a x i m u m ( ~ 3 0 rain) the concentration of active eentres decreases to 0 in 4 hr. PA rduces the rate of initiation; however, the rate of loss of active centres decreases even more, as reflected by the higher value of m a x i m u m concentration of active centres (2-75 x 10 -3, compared with 2.13 x 10 -3 mole/1.) 2 I

l

q I

I

I

10

30

60

9O

120

I

I

I

I

~ 1-8 b ~ 0.8

.27.

~2 I

30

I

I

I

60

80

120

I

2

g

"]-[me ~ hr

Fxo. 2. K i n e t i c curves of t h e c o n s u m p t i o n of T H F (a) a n d P O (b); 4--PA consumption.

a n d lower a rate of reduction of this value. The presence of NaC104 in the mixture zuppresses the formation and loss of active centres even more. I f we assume t h a t the rate of loss of active centres (AC) is first order in relation to their concentration from the dependence of In [AC] on time the value of 2 I

q I

I

1

77 o

30

80 7-[me ~ hp

I I#0

:FIG. 3. K i n e t i c curves of t h e c o n c e n t r a t i o n v a r i a t i o n of a c t i v e centres [AC] d u r i n g copol y m e r i z a t i o n of P O a n d T H F .

kg m a y be evaluated to a first approximation (kg is the termination rate constant of a growing cation b y the counter-ion F - [3]). In spite of the low accuracy of this determination of the value of kg it m a y be stated t h a t without additives kg~4X 10 -4 see -1, PA reduces the rate constant of the loss of active centres b y about one order of magnitude and sodium perehlorate by more t h a n two orders of magnitude.

2574

G. N. KOM~TOV et al.

I t may be assumed that the addition of polar compounds, which are not monomers, into the polymerization system reduces not only the rate of loss of active eentres, but also the rate of other reactions in which they take part, particularly chain transfer and ring formation. Using GPC the kinetics of concentration variation of the cyclic tetramer of PO (CTPO) formed during copoIymerization of PO in THF [3] may be followed. 30

~4

80

I

-"

I

I

I

r

I

2

fSO I

T / m e , hp

o

1

t

q

FIG. 4. Kinetics of the formation of CTPO. Figure 4 shows kinetic curves of the formation of CTPO during copolymerization of PO and THF. This Figure indicates that the concentration of CTPO increases to a maximum value ( ~ 4 % of the monomers used). PO slows down the formation of CTPO and its maximum concentration decreases. 1~aC]04 reduces even more the rate of formation of CTPO.

2 I

4

I

1

i

i

4

i

~

2

J

I

I

60

I

~20

I .

Z"°

I

I

20

I

eC~%

T/me ~hp

FIG. 5

I

I

60

FIG. 6

FIO. 5. Variation of the polydispersion of the product of eopolymerizationof PO and THF during the process. FIG. 6. Dependence of ~/, of the copolymer on yield.

Copolymerization of propylene oxide a~d tetrahydrofuran

2575

I t is interesting to note that the concentration of CTPO in a system containing P A slightly decreases after 3 hr. A clearer maximum is seen on the kinetic curve of concentration variation of CTPO during copolymerization of PO and T H F in the presence of sodium perchlorate. Curves 2 and 3 in Fig. 4 may, apparently, be explained b y chain transfer to CTPO with ring opening. When no additives are used the rate of loss of active centres is high and chain transfer to CTPO with rupture cannot involve noticeable changes in kinetics of formation of CTPO. When the system contains P A or NaC104, the life time of active centres is higher and chain transfer to CTPO with rupture becomes noticeable. It should be noted that the polydispersion of products of copolymerization of PO and T H F is fairly high (Fig. 5) apparently as a result of a high content of CTPO. Since chain transfer to CTPO with ring opening results, on the one hand in a reduction of CTPO concentration and, on the other, in an increase in the molecular weight of the copolymer itself, the polydispersion of the product decreases. In fact, as shown b y Fig. 5, in the presence of sodium perchlorate the .~lw/Mnratio of the copolymer decreases during the process. The addition of P A to the reaction mixture considerably increases the rate of chain transfer to the monomer, which is reflected b y Mn of the product obtained, as shown b y Fig. 6, indicating the dependence of ~l)n of the copolymer on yield. The marked effect of chain transfer to the monomer during the formation of the copolymer reduces the polydispersion of the product. Figure 6 shows that the number average molecular weight of the product of copolymerization obtained in the presence of NaCl04 during the process increases at a higher rate than yield. This effect m a y be observed either during chain transfer to CTPO with ring opening

F~ '

0,.~.,.,.~0

~""O'~'~"~"~'-~O-BF3

(crpo) where ~ - ~ -- is the end of the growing macromo]ecule containing PO, T H F (or PA), or during bimolecular deactivation of active centres [6]

~ o _ ~F3 ,H~ ~"~O-BF3 ~-'~+

+ BF3.THF ,

(If)

or in the presence of both reactions. The presence of sodium perchlorate in the reaction mixture does not eliminate the formation of P A although it considerably reduces the rate of isomerization of PO. This is a considerable difference from homopolymerization of T H F , where it was shown that the addition of NaCl04 completely suppresses t h e formation of PA.

G. N. Kom~Tov

:2576

et al.

It m a y therefore be noted that polar compounds (particularly PA and sodium perchlorate) have a marked effect not only on kinetics of cationic copelymerization of PO and THF, but also on characteristics of the product obtained. The ability to produce specific solvation of dipoles and ionic pairs is a general feature in the b e h a v i o u r of PA and NaCIO4 in this system. The formation of quadrupoles of BF3 complexes with PA or NaCl04 is one •of the possible methods of slowing down the stage of initiation.

~O=CHCH~CH3 B F 34

o~

OP

B'F3 " ~

,+i

o CH--~_O

.Na+ d o ,

BF~ •

t ~~

OP

,

' + '' r ~ Na . . . . CIu~

(lit)

'

H~ Ells

Furthermore, the following complexes are, apparently, formed:

BF 3.

+

+ _ NaCI04

.

K'~

BF 3 . ClO 4 -~+

(V)

Na

Both the formation of quadrupoles and complex formation reduces the concentration of the BF a" PO complex which is responsible for initiation. Similarly these compounds m a y solvate active eentres forming quadrupoles or, in the case of NaC104, esters A-

+

Na CIO4 (PA)

(iH~ CH~

4~f}_B

F +:~

Na CIO4 + -

.~, B-b-30.,..~..-...OCIO a

(VII)

+

Ha

Unlike NaC104, propionaldehyde m a y take part in chain growth and since its m a x i m u m temperature of polymerization i s - - 3 1 ° [7], it is unable to undergo homo-polymerization under the conditions studied. However, as a result of the relatively high concentration of P A near the growing cation as a result of solvation, it is an active chain transfer agent. The fact t h a t PA takes part in copolymerization forming hydrolytically .unstable acetal bonds and reducing ff/n of the product as a result of chain transfer,

Copolymerization of propylene oxide and totrahydrofuran

2577

m a k e s it a n u n d e s i r a b l e c o m p o n e n t o f t h e p o l y m e r i z a t i o n s y s t e m in spite o f t h e f a c t t h a t p o l y m e r yield increases in its presence. S o l v a t i o n o f a c t i v e centres t o f o r m q u a d r u p o l e s or (and) t h e f o r m a t i o n o f esters (in t h e case o f s o d i u m l~erchlorate) c o n t r i b u t e to a slowing d o w n o f t h e i n t e r a c t i o n o f t h e growing cation w i t h t h e counterion. T h e f o r m a t i o n of p e r chloric acid esters is m o r e likely for a c t i v e centres w i t h P O a t t h e e n d t h a n for t e t r a h y d r o f u r a n e n d s since in t h e f o r m e r case ~+ on t h e ~ - c a r b o n a t o m is higher t h e n in t h e l a t t e r case [8]. T h e a d d i t i o n of NaC10~ t o t h e p o l y m e r i z a t i o n s y s t e m m a r k e d l y slows d o w n all r e a c t i o n s o f t h e process, w h i c h is a v e r y u n d e s i r a b l e factor. BFs, PO and T H F were purified, according to a former description [9] and PA and NaC104, by another method [1]. The vessels were filled, polymerization carried out and volatile compounds separated from the polymer according to a previous description [9]. The volatile compound mixture was analysed using a "Tsvet-4" GL chromatograph [2]. Conditions of gas-chromatographic analysis of copolymer samples were described previously [2]; the volumes contained butadiene-styrene gel with pore size of 200, 500 and 1000

A.

The concentration of active contres was determined~by methods previously described [10]. Translated by E. SE~ER~ REFERENCES I. G. V. KOROVINA, D. D. NOVIKOV, D. Ya. ROSSINA, T. V. GRINEVICH, N. G. TAGA-

2. 3. 4. 5.

6.

7. 8. 9. 10.

NOV, R. A. BARZY]KINA, G. N. KOMRATOV and S. G. ENTELIS, Vysokomol. soyed. A18: 1253, 1976 (Translated in Polymer Sci. U.S.S.R. 18: 6, 1438, 1976) G. N. KOMRATOY, R. A. BARZYKINA and G. V. KOROVINA, Vysokomol. soyed. B21: 326, 1979 (Not translated in Polymer Sci. U.S.S.R.) D. D. NOVIKOV, Kandidatskaya dissertatsiya (Post-Graduate Thesis) I K h F AN SSSR, 1970 L. P. BLANCHARD and M. D. BALIAL, J. Polymer Sei. 5, A-l: 2045, 1967 A. L KUZAYEV, G. N. KOMRATOV, G. V. KOROVINA, C. A. MIRONTSEVA and S. G. ENTELIS, Vysokomol. soyed. A l l : 443, 1969 (Translated in Polymer Sci. U.S.S.R. 11: 2, 502, 1969) R. A. BARZYKINA, G. V. KOROVINA, O. M. OL'KHOVA, Ya. I. ESTRIN, and S. G. ENTELIS, Vysokomol. soyed. A10: 315, 1968 (Translated in Polymer Sei. U.S.S.R. 10: 2, 369, 1968) G. OUDIAN, Osnovy khimii polimerov (Principles of Polymer Chemistry). Izd. "Mir", 1974 Yu. R. EISNER and R. L. YERUSALIMSKH, Elektronnyi aspekt reaktsii polimerizatsii (Electron Aspect of Polymerization). Izd. "Nauka", 1976 G. N. KOMRATOV, R. A. RARZYKINA, C. V. KOROYINA and S. C. ENTELIS, Vysokernel, soyed. A17: 2059, 1975 (Translated in Polymer Sci. U.S.S.R. 17: 9, 2376, 1975) R. A. BARZYKINA, G. N. KOMRATOV, G. V. KOROVINA and S. G. ENTELIS, Vysokomol, soyed. A16: 906, 1974 (Translated in Polymer Sci. U.S.S.R. 16: 4, 1048, 1974)