Specific features of tetrahydrofuran copolymerization with lactones

European Pollmer Journal Vo[. 17, pp 1089 to 1095. 1981 Primed in Great Britain

01114-305781 101089-07502.00,0 Pergamon Press Lid

SPECIFIC FEATURES OF T E T R A H Y D R O F U R A N COPOLYMERIZATION WITH LACTONES* A. K. KHOMYAKOV, E. B. LUDVIG and N. N. SHAPETKO Karpov Institute of Physical Chemistry, U1. Obucha I0, Moscow 107120, U.S.S.R.

Abstract Mechanistic r~atures of the cationic copolymerization of lactones with cyclic ethers are studied for fl-propiolactona ~?L) with tetrahydrofuran (THF) and E-caprolactone (CLI with THF. It is shown that, in the PL THF system at [THF]0 > [THF]~, the copolymer is considerably enriched with the more basic THF whereas at [THF]o < [THF]e anomalous enrichment of the copolymer with the less basic PL is observed. The mechanism of this phenomenon, which is applicable to many other cases and causes the formation of block copolymers from some heterocyclic monomers, is considered. At concentrations below equilibrium, THF is incorporated in the copolymer as pairs of units due to the effect of the penultimate unit on the thermodynamics of its addition.

Cationic copolymerization of heterocyclic oxygencontaining m o n o m e r s is still rather poorly understood as c o m p a r e d to their homopolymerization. To a large extent this is due to the complexity of the copolymerization. In interpreting experimental results on the copolymerization, one must take into account the reversibility of chain p r o p a g a t i o n for some monomers, the possibility of transfer to the polymer, the nature of active centres, variations in dielectric constant (e) of the system during the process, etc. Published data are available for T H F copolymerization with 1,3-dioxolane IDOL) [1 3], epichlorohydrin and 3,Y-bischloromethyloxetane [4], trioxane [5], e-caprolactone (CL~ [6, 7] a n d fl-propiolactone (PL) [8]. Generally in these investigations the authors were mainly concerned with the relative reactivities of the m o n o m e r s using the M a y o Lewis equation to estimate the copolymerization constants. In some cases the reported results seem rather inconsistent probably because the polymerizing conditions differed widely thereby affecting the equilibrium concentration of T H F twhich also depended on its initial concentration, temperature, the nature of the solvent etc.) [9]. A similar dependence of the equilibrium concentration upon polymerizing conditions was found for D O L [10]. The detailed study of copolymerizations of T H F with PL and CL reported here has shown that in some cases the mechanism of chain propagation is also strongly affected by variations in operating conditions and c a n n o t be described within the framework of the M a y o - L e w i s theory. EXPERIMENTAL PL, CL and THF were purified as described previously [11 13]. Solvents were purified by standard techniques. Syntheses of MeCO * SbClg and PhaC÷SbCI6 were as given previously [14]. Oxonium salts Et30+SbCIg and

* Presented at the 21st Microsymposium on Macromolecules. Karlovy Vary, Czechoslovakia, 22-26 September, 1980.

Et30+SbFg were made and purified as already described [15] and [16]. The kinetics of monomer consumption was studied by NMR spectroscopy at 25'. Instruments JEOL PS-100 (100 MHzi, Bruker HX-90 (90 MHz) and Bruker WH-270 (270 MHzl were used to obtain NMR spectra [1H] and [~3C]. Chemical shills (& ppm) were determined with tetramethyl silane or the solvent as internal standard. i.r. Spectra were taken on KBr pellets using an i.r.-20 spectrometer. Copolymerizations were carried out in bulk, in methylene chloride, chloroform and nitromethane. All reagents were introduced under vacuum. The final product of THF copolymerizalion with PL was evacuated to remove THF and solvent, then extracted by benzene and precipitated by diethyl ether. The product of THF copolymerization with CL was first evacuated and then extracted by diethyl ether. The isolated copolymers were dried under vacuum to constant weight at ~ 25 . The compositions and structures of the copolymers were examined by [tH]- and [t3C]NMR.

RESULTS AND DISCUSSION Copolymerization of T H F with PL (or CL) in solution at 25 was initiated by one of M e C O + S b C I 6 , E t 3 0 + SbC16, E t 3 0 + SbF6 and Ph3C + S b C [ 6 . T H F PL copolymerization

The copolymerization product was extracted by benzene for removal of h o m o p o l y m e r s lpoly-PL remained in solutionl and then precipitated by diethyl ether (poly-THF remained in solution). Usually the final products were dissolved completely in benzene and readily precipitated by diethyl ether thus indicating that a true copolymer had been formed. The i.r. spectra of the copolymer and homopolyrners are given in Fig. 1. Table 1 lists chemical shifts for some monomers, polymers and several c o m p o u n d s modelling cross p r o p a g a t i o n acts. These data were used for interpretation of the spectra obtained from the polymerizing systems. As seen from Fig. 2, a [ I H ] s p e c t r u m of the forming copolymer contains signals characterized by the chemical shifts of PL h o m o p o l y m e r [4.30 and 2.60 ppm) and those of T H F h o m o p o l y m e r 13.37 a r d

1089

A.K. KHOMAYAKOVet al.

1090

Signals of the same origin are also found in [x3C]NMR spectra of the copolymer (see Fig. 3a and Table 1). The occurrence of the cross propagations confirms that a true copolymer has been formed. The kinetics of the consumption of monomers were studied by integrating the signals of [XH]NMR spectra on the basis of the following relationships between the intensities of the signals: for PL-d/(a + c): for CL i / ( i + f ) ; for THF n/(n + 1) or m / ( m + k ) for the copolymerization with PL and CL, respectively. Figure 4 shows the kinetic curves for THF and PL consumption during polymerization for various initial concentrations of the comonomers. At initial THF concentrations exceeding the equilibrium value ([THF]e), the process can be thought to proceed in two steps (see Fig. 4A). In the early stages, there is rapid polymerization of THF with PL practically uninvolved. In the later stages, when the THF concentration has dropped to the equilibrium value, the rate of THF consumption markedly decreases (see the bend on the conversion curve) and, at the same time, polymerization of PL commences. At a fixed concentration of PL, an increased initial concentration of THF results only in a somewhat faster THF polymerization in the early stages. At an initial THF concentration below ITHF]e (the second stage at [THF]o > [THF]e and the process as a whole at [THF]o < [-THF] e) both monomers are consumed simultaneously but the rate of PL consumption is higher than that of THF (Figs 4B,b and 5). Now consider both stages of the copolymerization in more detail. The predominating THF polymerization when [THF]o > [THF]¢, responsible for the

o" E

F3oO tToo

LSOO

13oo

tK)o

9o0

700

Wavelength ,cm -i

Fig. I. i.r. Spectra of: (a) homopolymer of THF: (b) homopolymer of PL: (c) THF-PL copolymer.

1.61 ppm) as well as four additional signals of equal intensity (4.07, 3.63, 2.50 and 1.67 ppm). The chemical shifts of these additional signals should be attributed to the protons involved in the cross propagation acts (m', c', d', n'): O

O

O

II

tL

ii

,,, (OCH2CH 2C)o---OCH 2CH2 CH: CH 2--(OCH2(CH2)2CH2)q--OCH 2CH2 C--(OCH2 CH2 C ~ ~,. c

d

m'

c

n'

n

m

m

n

m

c'

d'

c

d

/ /

/ I

__._/

,io

'

;o

21o ppm

Fig. 2. [IH]NMR spectrum (270 MHz) of THF-PL copolymer in CH2C12 at 25.

Specific features of tetrahydrofuran copolymerization with lactones

1091

Table 1. Chemical shifts in NMR spectra of some monomers, polymers and several other compounds in CH2CI2

[~H] Compound

Group

PL

OCH2 CH2CO OCH2 CH2CO OCH2 CHACO

poly-PL CL

Symbol a b c d e f g h i .i k I m ;I

tCH2) 3

poly-CL

OCH2 CH2CO (CH2)3 OCH2 (CHz)2 OCH2 (CH2) 2 CH 3CO OCH2 OCH2CH2 OCH2 OCH2CH2

THF poly-THF n-Amylacetat Dibutyl ether PL-THF copolymer Block poly-PL

OCH2 CH2CO COOCH2 COOCH2CH2

Block poly-THF CL-THF copolymer Block poly-CL Block poly-THF * S singlet: T

(5. ppm)

T T T T M M M T T M M M M M S T

3.37 - 1.5

T M

58.6 38.6 60.4 33.9 68.0 33.5 28.6: 27.8:22.2 64.2 34.2 28.6; 25.7; 24.8 67.9 25.8 71.1 26.9 20.5 64.3 28.6 71.7 29.9

3.63 2.50 4.07 1.67

T T M M

66.5 35.4 64.6 25.9

c' d' m' n'

OCH2 COOCH2

3.37 4.05~..08

71.1 64.2 64.3

triplet: M--multiplet.

o

÷

Multiplicity*

4.28 3.50 4.30 2.60 4.25 2.61 -- 1.7 4.08 2.30 - 1.5 3.63 1.81 3.37 1.61 2.03 4.05

formation of the homopolymer, is due to the considerable difference in basicity of the two m o n o m e r s (pK b = 5.00 for T H F and 10.06 for P L [8]) making difficult the cross p r o p a g a t i o n :

~(~

[~C]

(6, ppm)

I I : O~C

0

"-~ ,~O(CH~},4OCH2CH2 c+

.•

CDCI 3

1

T H F / ( c a r b o n y l group of PL) basicity ratio, as estimated from the equilibrium constants of the reversible reaction of either with an acyl ion [17], is a b o u t 17. The corresponding ratio for the ester oxygen of the lactone is expected to be even higher owing to conjugation. We previously investigated [18] the cross propagation using interaction of triethyloxonium salts with P L as a model reaction. The reaction

m

8O

I

coO

I

I

4O

(b)

i

P70

Z~

I

8O

L

6'0

'

L

,

'

I

20

8,1~m Fig. 3. [13C]NMR spectra (22.63 MHz) of THF PL copolymer (a) and THF-CL copolymer {b) in CDCI3 at 25 .

1092

A. K. KHOMAYAKOVet al.

(A) 6

/ 2i

12:0

~

I I

Time, rain

'°'

(B) 6

/

(b)/i

/

4

i 8

2

20O

6O0

200

600

Time,rain

Fig. 4. Kinetic curves of THF copolymerization with PL initiated by Et30 ÷ SbCI 6 in C H 2 C I 2 a t 25. (A) I-PL]0 = 2.80 (1); [THF]o = 4.76 (2); 7.80 (3), 10.10mol/l (4); C = 3.6 x 10 - 2 mol/l (curve 1--PL, curves 2 4 THF). (B) [ P L ] o = 12.10mol/l (a and b); [ T H F ] o = 2 . 9 0 (a), 1.90mol/l (b); C = 3 x 10 - 2 mol/1 (curve 1--PL, curve 2--THF).

was shown to proceed at a low rate which strongly depended on the dielectric constant of the medium and on the degree of ionic dissociation of the oxonium salt being used. Thus, the early stage of copolymerization consists in the homopolymerization of T H F . The amount of PL virtually remains unchanged as long as there is still some "active" T H F ( [ T H F ] o - [ T H F ] ~ ) available in the system* (Fig. 4A). This distinguishing feature of the process enables us to estimate [THF]e for monomer mixtures of various compositions as the T H F concentrations remaining in the system at the starting point of PL polymerization. It is well known that the equilibrium concentration of a monomer is * A similar result was previously obtained in an investigation of the copolymerization of PL with oxacyclobutane (pK b = 3.13). [1H]NMR spectra showed that, at the beginning of the process, there was rapid and complete homopolymerization of oxacyclobutane, following which PL polymerization commenced finally giving a block-copolymer of AB type.

determined by molecular interactions between the components of the system. Therefore, it depends on the composition of the system and, more specifically, on the concentration of the monomer. Dependences of this kind were earlier observed for the homopolymerization of T H F I-9] and D O L 1-10] in various solvents. It can be thought that, in the system under study ( T H F - P L - C H 2 C I 2 ) [THF]~ is also a function of its composition. Figure 6 showsplots of [THF]e vs monomer concentration for both monomers of this system determined by the above method. These plots show that [THF]~ is actually independent of T H F concentration although markedly affected by variations in the more polar monomer, PL. The second stage of the copolymerization and copolymerization at [THF]0 < [THF]e are characterized by different kinetics thus reflecting the specific features of the second cross propagation act. It could have been expected on the basis of current theories that at [ T H F ] < [ T H F ] , the more basic T H F would enter into the reaction to yield an alternating copolymer. The kinetic data suggest, however, that at this

Specific features of tetrahydrofuran copolymerization with lactones

(a)

04

were obtained by means of [ ~ H ] N M R spectroscopy. At [ T H F ] 0 < [THF]~, whatever the natures of solvent and initiator, the ratio of intensities for m a n d m' signals is constant and equal to 3 (see Fig. 2), although c/c' increases with increasing initial concentration of PL. The intensities of the signals of m', c', d', n' (expressed in terms of the difference between n and m integrated signals) are the same for b o t h cross propagation acts in the spectra of all ,copolymers. These results show that all copolymers formed under the condition [ T H F ] o < [THF]~ contain blocks composed of only two T H F units. In this case the size of a PL block is determined by the content of PL in the initial mixture. So, the copolymers obtained under these conditions have the following structure:

..~1

02

(bl IO

/,,7

"'T

"

2 I

c

(c)

3C

,3

2c

1093

( M , ) x M z M 2 (M I)y M z M 2 (M l):

/

where M1 is PL and M z is THF. The formation of a short block consisting of two T H F units appears to result from the effect of the penultimate unit on the addition of T H F to the ion

I0

?,+ /---~ ~ 0 ( CH 2 ) 2 C&l:)-~,_.__l

120

240

on the one hand, and to the ion

Time .rain

~O(CHz

Fig. 5. Kinetic curves of THF copolymerizatlon with PL initiated by Et30+SbC16 in CH2C12 at 2 5 at equimolar ratio of monomers. [PL]o = [THF]o = 0.35 (a), 0.95 (bL 2.70 mol/l (c) (curve l PL, curve 2--THF): C = 3.6 x 10 -2 mol/1.

on the other hand. Since the T H F blocks are composed of only two units, the n u m b e r of cross propagation acts is still found to be smaller than that expected from kinetic data on the c o n s u m p t i o n of comonomer. Consider the possible causes for the anomalously low rate of T H F addition to the copolymer at [ T H F ] < [THF]~. In this range the process may include the following reactions:

stage the reaction product is considerably enriched in the less basic PL. Some additional data on the copolymer structure

I ¢.. O - -

I

I

/

(

o : c--o

o

o

Kp

I C = O

)4; Q

II

, ~ O C H z CH 2 C*

o +

( I)

[

I

K~~C"--.-O : c - - o o K, ..+_ ~ g

(2)

~,,C:-.=-O = C - - O

(3)

0 = C--O

o K

O r..~_

t k+

(5)

" ~"C " - 0 f~_~

o,+

oC:]"; o +

(I)

O

3

+

-_

'

O--C

'

-

= 0

,,,

o( c,,,°

.

~.C KP

0

It" E

O(CH2) 4

]

0

u

z - O C H z C H z C+

(7)

(a) o

u

.7

[THF}o,rno~/I

[PL]o,mOl/I

Fig. 6. T H F - P L copolymerization in CH2CI 2 (A Et30 + SbClg : ~ E t 3 0 + SbFg : 0-- Ph3C +SbCI6-). (a) [THF]~ vs [THF]0 at [PL]o = 2.80 mol/l, 25::. (bl [THF]~ vs [PL]o (curve 1 -25 , curve 2 -40 ).

1094

A.K. KHOMAYAKOVet al.

It is believed that reaction (6) is the limiting step in the formation of the THF block. The acyloxonium ion involved in the reaction is asymmetrical due to the dissimilarity of the substituents. The change in hybridization in this ion leads to the bond 2 being strengthened and the bond 1 being weakened as compared to the ion + ,',,,',q C H z

14--0:~]

In consequence the reactivity of the acyloxonium ion decreases as bond 2 opens, thereby decreasing the rate of THF consumption. THF and its blocks in the copolymer behave towards the lactone as a basic additive. This suggests that the addition of PL to the copolymer should obey the general laws applicable to lactone polymerization in the presence of basic additives [17] and in particular should satisfy the equation:

d[PL]

kpC[PL]

dt

KI[PL] + K2[THF] + Ka[block T H F ] kp[PL] C g 2[THF] "

Tentative estimates of PL consumption by the above equation agree with the experimental results. In the calculations, the data on basicities of T H F and diethyl ether (a model of the T H F block) were taken into account: AVTHF = 90, AvE,~o= 78, where Av is the shift in frequency of O---D bond of deuterated methanol in an i.r. spectrum in the presence of a corresponding ether [19]. Thus, when a basic additive is introduced to the system, the growth of PL blocks proceeds through a "lactonic" mechanism and the formation of two unit THF blocks is limited by the low rate of interaction between THF and acyloxonium ions. Another peculiarity worthy of notice is the sensitivity of the process to changes in polarity of the medium which is in turn profoundly affected by changes in PL concentration*. This can be seen from the acceleration of reaction (7) as dielectric constant (E) decreases with increasing time of the reaction or with decreasing initial concentration of PL (Fig. 4, curve 1). When the initial concentration of PL is low (< 2mol/l), the rates of consumption of both monomers are reduced, the rate of T H F consumption being higher (Figs 4B, b and 5). This phenomenon can be explained by the ionic association that commences at low concentrations of PL ( a similar situation was earlier observed for PL homopolymerization [20]) and involves ion triplets as active centres in chain propagation. The rate of PL consumption by the mechanism of homopolymerization (bypassing the cross propagation act) decreases thereby causing the observed discrepancies. When [THF] < [THF] e, THF can enter into the copolymer only if PL is also available in the system. The rate of PL consumption is much higher, therefore T H F is never depleted completely in the process. The

*

For instance at 20° = 9.1.

ECH2CI2

•PL = 4 4 . 0 ,

ETH F =

(a) 1.2

~_-0.4 2 I

.e,

I

I I

(bl

~ 5.C

~3.C 1.0 60

f~

, , A ~ (c)

L

2

I

6 [THFJo,m(W/I

L

I

I0

Fig. 7. Conversion of THF in THF-PL copolymerization in CH2C12 at 25° as a function of (a) [PL]0 at [THF]0 = 1.40mol/1, (1-in MeNO2, 2---in C~H6); (b) [PL]0 at [THF]o = 5.00ml/l; (c) [THF]0 at [PL]0 = 5.00mol/1 (curve 1) and 2.80mol/l (curve 2). Dashed line--[THF]~. THF conversion at [THF]0 > 4 mol/l was calculated on the basis of [THF]~ (see also Fig. 6). limiting conversion of THF is determined by the ratio between the monomers and by temperature but is nearly independent of the nature of the solvent (Fig. 7). From the results of [1H]NMR spectroscopy, "living" polyoxonium ions

and some residual T H F are available in the system on completion of the process. The abnormalities in the polymerization observed at [THF] < [THF]e are connected with the difference in structure of the polymers creating complications in both cross propagation acts and enabling the PL-units to grow by the mechanism of lactone polymerization in the presence of a basic additive (i.e. by-passing the cross propagation acts). This leads to the inverse ratio of the reactivities of the monomers in the region [THF] < [THF]e as compared to the region [THF] > [THF]o. The discussed mechanism of copolymerization can be applied to many other cases. For instance, it may be used in interpreting the similar kinetic dependences observed for copolymerization of THF with DOE [1-3]. The developed mechanism of copolymerization may account for the differences in behaviour of the THF-PL-CH2CI 2 and THF-CL-CH2CI2 systems. T H F - C L copolymerization

Figure 8 shows the kinetic curves for consumption of THF and CL in copolymerization initiated by Et30 ÷ SbC16. This figure shows that, at 7.6 and [THF]o > [THF]~ (curve 3), THF enters the copoly• met at a rate comparable with that of CL consump-

Specific features of tetrahydrofuran copolymerization with lactones

~~" '~

1095

chemical shifts of the p r o t o n signals for polymers of T H F and CL and those for the protons involved in the cross p r o p a g a t i o n acts c a n n o t be resolved. In b o t h systems, rise in temperature of copolymerization at [THF]o < [THF]~ leads to enrichment of the forming copolymers with lactones. Introduction of some additional water produces a similar effect upon the system; in this case, the process involves an induction period.

2 3

REFERENCES

120

360

Time, rain Fig. 8. Kinetic curves of THF copolymerization with CL initiated by EI30+ SbC1, in CHzCIz at 25 . [ C L ] o = 2 . 8 0 m o l / l (1): [ T H F ] 0 = 2 . 8 0 (11, 3.94 (2), 7.67 mol/l (3): C = 8.5 × 10 z mol/l.

tion (for a particular ratio of the monomers). The basicities of T H F and of the carbonyl group of CL were estimated from reported data [17] to give the ratio KcL/KTHr ~ 30. The basicity of the ester oxygen which determines the relative reactivity of CL during copolymerization is expected to be much lower due to the strong conjugation in the ester group of the lactone (the cycle is feebly strained). Indeed, the activities of T H F and CL in this system were rather similar. The estimation of the rate of CL c o n s u m p t i o n during polymerization (from Fig. 8) gives values much higher than those expected for a "lactonic" mechanism of chain p r o p a g a t i o n [21]. Apparently, CL adds to the copolymer mainly by the cross propagation:

(( CH2 15)

~+

which proceeds rapidly due to the high basicity of CL. This agrees with the "'instant" initiation of CL polymerization with triethyloxonium salts. At [ T H F ] o < [THF]~, the rates of c o n s u m p t i o n of b o t h m o n o m e r s are rather close. Unlike PL, C L can complete successfully with T H F in reaction (6) due to the m u c h higher basicity of CL. In this system, the microstructure of the copolymer c a n n o t be determined directly from the corresponding N M R spectra as the

1. A. I. Efremova, T. E. Ponomareva, B. A. Rozenberg and N. S. Enikolopjan, Dokl. Akad. Nauk SSSR 190, 872 (1970). 2. Y. Yamashita, S. Kozawa, K. Chiba and M. Okada. Makromolek. Chem. 135, 75 (1970). 3. E. B. Ludvig, Z. N. Nysenko, A. K. Khomyakov and S. S. Medvedev, Dokl. Akad. Nauk SSSR 186, 1351 (1969). 4. A. Ishigaki, T. Shoho and Y. Hachihama, Makromolek; Chem. 79, 170 (1964). 5. M. B. Price and F. B. McAndrew, ACS Polym. Preprints 7, 207 (1966). 6. Y. Yamashita, K. Ito and K. Kozawa. Poh'm. J. 3, 389 (1971). 7. E. A. Dzhavadjan, B. A. Rosenberg and N. S. Enikolopjan, Vysokomolek. Soedin. AI5, 1982 (1973). 8. Y. Yamashita, T. Tsuda, M. Okada ~nd S. Ikatsuki, J. Polym. Sci. A-I 4, 2121 (1966). 9. S. Penczek and K. Matyiaszewski, J. Po/ym. Sci.. Polym. Syrup. 56, 255 (1976). 10. L. I. Kuzub, M. A. Markevich, A. A. Berlin and N. S. Enikolopjan, Vysokomolek. Soedin. AI0, 2007 (1968). 11. A. K. Khomyakov, G. S. Sanina and E. B. Ludvig, J. Polym. Sci. C42, 289 (1973). 12. E. B. Ludvig and B. G. Belen'kaya, J. Macromolec. Sci. A8, 819 (1974). 13. S. Kobayashi, T. Ashida and T. Saegusa, Bull. Chem. Soc. Japan 47, 1233 (1974). 14. A. K. Khomyakov, A. T. Gorelykov, N. N. Shapetko and E. B. Ludvig, Vysokomolek. Soedin. AI8, 1699 (1976). 15. H. Meerwein, E. Battenberg, H. Gold. E. Pfeil and G. Willang, J. Prakt. Chem. 154, 83 (1939). 16. B. G. Belen'kaya and E. B. Ludvig, Vwokomolek'. Soedin. A21, 1252 (19791. 17. A. K. Khomyakov and E. B. Ludvig, Vv~okomo/ek. Soedin. BI5, 698 (1973). 18. A. K. Khomyakov and E. B. Ludvig, Vvsokomolek. Soedin. A21, 2333 (19791. 19. T. Kagaiya, J. Sumida and T. Inoue. Bull. ('hem. Sou. Japan 41, 767 (1968). 20. B. G. Belen'kaya and E. B. Ludvig, Vv.~okomolek. Soedin. A20, 565 (1978). 21. B. G. Belen'kaya, A. 1. Levenko and E. B. Ludvig, l~vsokomolek. Soedin. A20, 559 (1978).