Anionic Polymerization of Cyclosiloxanes with Cryptates as Counterions

2 Anionic Polymerization of Cyclosiloxanes with Cryptates as Counterions

Downloaded by MICHIGAN STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: August 16, 1985 | doi: 10.1021/bk-1985-0286.ch002

SYLVIE

BOILEAU

Laboratoire de Chimie Macromoléculaire associé au CNRS, Collège de France, 11 place Marcelin Berthelot, 75231 Paris Cédex 05, France

The anionic polymerization of cyclosiloxanes was examined in benzene and toluene with lithium cryptates as counterions. Only one type of active species is observed in the case of Li + [211]; thus, the kinetics of the propagation and of the by-product cyclosiloxanes formation can be studied in detail for the first time. The reactivity of cryptated silanolate ion pairs toward the ring opening of D is greatly enhanced compared to that of other systems. The amount of cyclic by-products is very low when the polymer yield reaches the maximum value. PDMS of narrow molecular weight distributions were obtained using a cryptand larger than the [211]. +

3

The anionic polymerization of cyclosiloxanes has been studied for a long time. However the knowledge of the mechanism of the process i s s t i l l unfortunately rather limited (1^2) · This i s p a r t i a l l y due to the fact that the polymerization i s of the equilibrium type and besides linear polymers, c y c l i c structures exist i n s o l u t i o n . I t has been shown that under certain conditions, hexamethylcyclotrisiloxane (D3) polymerizes to give linear polymers of n e g l i g i b l e cyclosiloxane content and of narrow molecular weight d i s t r i b u t i o n at a much more rapid rate than octamethylcyclotetra siloxane (D4) ( 3 ) · This can be explained by the ring s t r a i n of D3 which enhances the r e a c t i v i t y of the Si-0 bonds. A suitable i n i t i a t o r such as butyllithium can be mixed with D3 i n hydrocarbon solvents to form Bu(-Si (CH3)2 0)3Li (4). No polymerization occurs even i n the presence of excess D3 u n t i l a donor solvent l i k e THF ( 3 , 5 ) , diglyme ( 6 ) , triglyme ( 5 ) , DME (7^), HMPA (7) or DMSO (3) i s added; then a reasonably rapid polymerization starts to give near monodisperse polymers. Only few k i n e t i c investigations have been performed on these systems though the side reactions which involve i n t r a and i n t e r molecular attacks of the chains by l i v i n g centers can be neglected i n the f i r s t stage of polymerization ( 5 , 7 ) . The nature of the active propagating species as well as their r e l a t i v e proportion i s _

0097-*156/85/0286-Ό023$06.00/0 © 1985 American Chemical Society

In Ring-Opening Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

RING-OPENING POLYMERIZATION

24


not well known i n the investigated systems C5,8^,9). For instance, the dependence of the rate of polymerization of D3 on the concentration of active centers i s complex i n the case of a benzene/ THF (50/50) medium, with Li+ as counterion 05). This behavior i s obviously connected with the presence of associations of the lithium s i l a n o l a t e s . Thus we thought that use of cryptands for the anionic polymerization of cyclosiloxanes might suppress the aggregates and simplify the mechanism (10,11,12). These macrobicyclic ligands (I) discovered by Lehn (13) form extremely stable cation inclusion complexes, called cryptâtes. In the nomenclature system used herein, Structure I would be designated [211] when m=2 and n=p=l.

CH -CH [0-CH -CH ] 2

Ν

2

2

2

m

CH -CH [ 0-CH -CH ] 2

2

2

2

CH -CH [0-CH -CH ] 2

2

2

2

n

p

These complexes lead to a considerable increase of the i n t e r i o n i c distance i n the ion pairs and i t has been shown that such ligands have a marked activating effect on anionic polymerizations (14,15, 16). Moreover, the aggregates are destroyed and simple kinetic results have been obtained i n the case of propylene sulfide and ethylene oxide polymerizations (12). Thus, i n the present paper we present our data concerning the anionic polymerization of cyclosiloxanes, namely D3 and D4, with cryptâtes as counterions, i n benzene or toluene solution. Anionic Polymerization of Do, It i s known that catalysts involving bases generally do not polymerize D3 at moderate temperatures, except i n the presence of donor solvents (3). Preliminary experiments have shown however that polydimethylsiloxane (PDMS) of very high molecular weight was obtained i n 60% y i e l d after five minutes at room temperature using KOH complexed with [222] i n benzene (14_, 16). However our k i n e t i c investigations were performed using homogeneous conditions with lithium cryptâtes as counterions. Living PDMS can be prepared by adding the [211] cryptand to benzene solutions of D3 after reaction with n-butyllithium, at 25°C, as can be seen from the results of Table I. The propagation reaction i s much faster with [211] than with THF. Kinetics of the propagation reactions were followed by d i l a t o metry under high vacuum, at 20°C, at various l i v i n g ends concentrations, with nearly the same value of [M] (0.5 mole. 1~*) (10,11,17). Some experiments were performed i n toluene instead of benzene and in some cases, t e r t i a r y butyllithium was used as i n i t i a t o r instead of n-butyllithium. The results were nearly the same within the experimental errors. The values of the apparent propagation rate constant kp - R /[M]x[C] were determined for each Q

p



1

a

0.86 1.26

91 59 98

0.4 4 7

1.5

0

0

8.9

8.9

6.75

0.35

0.59 >

0.39 >

)Benzene/THF

a

a

* 50/50 (v/v).

0.80

1.50

91

2

1.9

0.80

10.8

13.1

0.83

5

theor.

χ ΙΟ"

^

(%)

Yield

-1.40

Time (hr.)

Propagation

-100

[211/[Li+]

1

1

4

(mole.l' )

[C] χ 10

2.7

(mole.l" )

[D3]

1.14

-

-

-

1.10

χ ΙΟ"

5

osm.

Table I. Polymerization of D3 I n i t i a t e d by n-BuLi, i n Benzene at Room Temperature ( I n i t i a t i o n time: 16 h. at 25°C. Termination reagent: trimethylchlorosilane)


5

0.91

1.20

1.10

χ 1(Γ

GPC

RING-OPENING

26

POLYMERIZATION

experiment (Rp = rate of polymerization, [M] and [C]: monomer and l i v i n g ends concentrations). On plotting R /[M] versus [C] (Figure 1), a straight l i n e passing throught the o r i g i n i s obtained, showing that the reaction order in active centers i s equal to 1 for a concentration range between 3.7 χ 10~3 and 7.5 χ 10~5 m o l e . l " . This result can be interpreted either by assuming that only one type of active species i s present - presumably cryptated ion pairs - or by assuming that i f there are different types of active centers, they might have the same r e a c t i v i t y . In order to elucidate this point, v i s c o s i t y measurements of l i v i n g and deactivated PDMA solutions were performed i n toluene, with L i + [211] as counterion. As no s i g n i f i c a n t change was observed, i t can be deduced that the f r a c t i o n of aggregates i s n e g l i g i b l e ( 3)

(1)

x

where k j and kp are the rate constants of formation and of propa gation of D , respectively, Κχ i s the molar c y c l i z a t i o n equilibrium constant and [D ] eq. i s the concentration of D i n equilibrium. The rates of consumption of D4 and formation of c y c l i c oligomers ϋχ are given by k i n e t i c equations (2) and (3): x

X

x

x

x

d[D ] 4

[C]

(2)

- ( dx " p x [ D ] ) [C]

(3)

D

k

- ( V I 4 ] ~ d4) dt d[D ] x

k

k

x

dt This r e l a t i o n has been proposed by Shinohara (21) and applied by Yamashita et a l . (22,23) to the anionic polymerization of ε-caprol actone. If the rate of formation of D4 i s assumed to be negligible compared to i t s rate of polymerization at the beginning of the reaction, Equation (2) becomes: d[D ] 4

(4) dt


RING-OPENING

30

POLYMERIZATION


weight %

Figure 3.

Conversion of D4 and formation of D5, D , and D7 as a function of time: [C] = 2.76 χ 10"* mole.l" ; [M] * 1.06 mole.l" . 6

1

Q

1


BOILEAU

2.

Anionic Polymerization of Cyclosiloxanes

31

Thus knowing kp4, i t i s possible to evaluate k 4 from Equation (1) by determining [D4] equation for each experiment. Concerning the formation of by-product cyclosiloxanes, two types of approximations can be made. The f i r s t one consists i n assuming that the rate of polymerization of D i s negligible compared to i t s rate of formation at the beginning of the reaction. Thus Equation (3) becomes: d

x

d[D ] x

k

- dx

[C]

(5)


dt and k p can be evaluated from Equation (1) by determining [D ]eq. On the other hand, integration of Equation (3) gives: X

x

[°χ]

k

dx

k

px

=

Ο

k

" e" Px[C]t)

(6)

if [D ] = 0 If the product kp [C]t i s smaller than 1 (

1

W

3

x

0.006

>

)

4700

17

6.5

1.2

280

1

0.4

0.07

357

1

0.1

0.06

88

1

1.1

a

) Found i n toluene at 20°C with Li+ + [211]

b

) Found i n the KOH-catalyzed (22).

11.8

(11,17).

solvolysis of siloxanes i n methanol

c) Found i n the ring opening of ϋχ by potassium phenyldimethylsilanolate i n heptane-dioxane, v/v (95:5) at 30°C ( 9 ) .


Anionic Polymerization of Cyclosiloxanes

2. BOILEAU

33

Molecular Weight Distributions of PDMS +

Gel permeation chromatography of PDMS prepared from D4 with L i + [211] as counterion, i n toluene, shows a bimodal d i s t r i b u t i o n . The r a t i o of the percentage of low molecular weight polymers over the percentage of high molecular weight PDMS i s nearly constant during the course of the polymerization. Its value i s rather important: l/4

Anionic Polymerization of Cyclosiloxanes with Cryptates as Counterions

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