Polymerization of vinyltrimethylsilane caused by ethyllithium in a hydrocarbon medium

POLYMERIZATION OF VINYLTRIMETHYLSILANE CAUSED BY ETHYLLITHIUM IN A HYDROCARBON MEDIUM* 1~. S. NAMETKIN, S. G. DURGAR'YAN a n d V. S. KHOTIMSKII A. V. Topchiev Petrochemical Synthesis Institute, U.S.S.R. Academy of Sciences

(Received 7 October 1968)

I~ ~ARLIER studies we showed that vinyltrialkyl(aryl)silanes could be polymerized in the presence of metallic lithium and alkyllRhium [1, 2], and the possibilRy of preparing high molecular weight polyvinyltrimethylsilane in this way was shown in reference [3]. The present paper gives the results of further investigations carried out with a view to elucidating the general mechanism and special features of vinyltrimethylsilane polymerization in the presence of alkyllithium initiators. Vinyltrimethylsilane (VT~S) was selected for investigation as being the simplest and most readily available representative of compounds in this category. Cyclohexane was used as solvent, and ethyllithium was the initiator. EXPERIMENTAL PROCEDURE AND SUBSTANCES USED IN THE POLYMERIZATION VTMS was obtained using the same method as in [4]. The synthesized VTMS was subjected to double-distillation over metallic sodium and a middle fraction boiling at 54.5 ° was selected. After being purified in this way the monomer was treated with LiH and with LiC~H5 powder, after which it was reeondensed into a Schlenk vessel, where it was kept under dry purified argon over metallic sodium. The purity of the VTMS was verified by gas-liquid chromatography (GCHT-18 and F-11 "Perkin Elmer" chromatographs) using phases of different polarity (Carbowax 1540 on teflon 10 : 90 and PPMS-4 -- 15% on Inzensk brick, granulation 0.25-0-5 ram); it was also verified by means of the physical constants. I n conducting the analysis the temperature of the chromategraph column was 40°; gas-carrier (helium) rate 50 ml/min. The chromatographic analysis showed that She compound purified by the method described above conSained a single component. The purity of the V T M S amounted to 99"9Yo (n D 25 1.3875, d15 0.6865). The cyclohexane was treated with several portions of concentrated sulphuric acid until it stopped turning dark. The solvent was distilled on a column in a current of inert gas after being earefuly washed with water and dried over calcium chloride, phosphorus pentoxide and metallic sodium. A middle fraction boiling at 81 ° was treated with eShyllithium and kept in the form of a solution conSaining ethyllithium in a Schlenk vessel under dry and purified argon. Chromatographic analysis of the solvent showed that cyelohexane purified in this way contains practically no impurities. Immediately before the polymerization She cyclohexane and VTMS were treated wish LiC2H5 and degassed on a vacuum manifold. * Vysokomol. soyed. A l l : No. 9, 2067-2072, 1969. 2360

Polymerization of viayltrimethylsilane

2361

E t h y l l i t h i u m was synthesized from metallic lithium and ethylbromide in n-pentane solution using the Kocheshkov method [5]. The polymerization was conducted with a cyclohexane solution prepared from ethyllithimn freshly recrystallized from n-pentane. The concentration of organolithium compounds was determined b y acidometry [5]. Kinetic studies of the polymerization process were carried out b y the dilatometric method, i n v a c u o . The dilatometer capacity was usually 7.5 ml and the g r a d u a t e d capillary diameter was 1.8-2 mm. Variations in the level of the liquid in the dilatometer were measured to within ±0.005 ml. The dilatometer was charged with the monomer and solvent b y recondensation i n v a c u o (1 × 10-atom) from graduated measuring tanks on the manifold of the vacuum apparatus. The initiator solution of known concentration was metered from a Schlenk vessel i n v a c u o into the dilatometer cooled with liquid nitrogen. Before being charged with the reactants the dilatometer was carefully heated for several hours ir~ v a c u o (1 × 10 a ram) at 250-300 ° to remove traces of water and impttrities from the wails. W h e n charged the dilatometer was disconnected from the vacuum apparatus and thermostatted. The thermostat temperature was maintained to within ± 0 . 0 5 ° . To determine the yield of polymer a t the end of the experiment the dilatometer was opened and the contents were quantitatively transferred to a Petri dish and dried, first in air and then i n v a c u o , to constant weight. DISCUSSION OF RESULTS T h e r e s u l t s o f s o m e o f t h e e x p e r i m e n t s i n t h e p o l y m e r i z a t i o n o f VTI~IS i n i t i a t e d b y e t h y l l i t h i u m h a v e b e e n t a b u l a t e d (see b e l o w ) a n d c h a r a c t e r i s t i c c u r v e s o f V T ~ S p o l y m e r i z a t i o n a r e s h o w n i n F i g . 1. The polymerization of VT~S initiated by ethyllithium in cyclohexane s o l u t i o n p r o c e e d s a t a m o d e r a t e r a t e u n d e r h o m o g e n e o u s c o n d i t i o n s a n d is characterized by S-shaped kinetic curves with well-defined induction periods. VTMS

POLYMERIZATION

OF

n,

CYCLOHEXA2~'E SOLUTION,

INITIATED

Concentration, mole/1.

Concentration, Yield, [t/] (25 °, mole/1. mole/1.. wt.~oo i eyelohexane) •min monomerl LiC2H~

BY

i

monomeri LiC2Hs '0.0045 0-O06 0.009 0.010 0.012 [ 0"015

I

9U, Yield, [t/] (25 °, eyclomole/1. • wt. % ' hexane) • nfin

i

l

1.4 1.4 1.4 1.4 1-4 1"4

LiC2H5 AT 25 °

100 100 100 100 100 100

0"303 0.214 0'175 0.155 0-013

0.00147 0.00182 0.00201 0.00269 0.0030 0.0035

1.4 1.4 1.4 0.85 2.04

' 0'020 0,0225 0,027 0.027 0.027

100 100 100

0.050 0.087 0.100

I00

--

100

--

0.0045 0.0050 0.0061 0"0015 0.0142

The appreciable induction period and the S-shaped kinetic curves probably indicates that VT~S reacts slowly with ethyllithium and that the propagation o f t h e p o l y v i n y l t r i m e t h y l s i l a n e ( P V T ~ S ) c h a i n s is m u c h m o r e r a p i d . A study of the depletion of ethyllithium during the polymerization of VTMS i n c y c l o h e x a n e s o l u t i o n [6] s h o w e d t h a t t h e i n i t i a l a m o u n t o f e t h y l l i t h i u m is continuously reduced during the process.

2362

N.S.

NAM~Tr:IN et

al.

The reaction of ethyllithium with VTi~S b y the scheme

/

CH3

LiC2H~-?CH2 = CH--Si--CH3 -~C2I-Is--CH2--CHsLi

\

I

CH3

Si(CHs)8

is probably the act of initiation of the polymerization reaction. The possibility of VT/ffS reacting with lithium alkyls was indicated b y Cason and Brooks who studied the addition of organolithium reactants to vinylsilanes [7, 8].

tog [~] -0"5

!

tOO

d

•~ 60

-1"0 •

6

8

7

100

3o0 T i m e , rain FIG. I

J

5o0

-1"5

I o °°°

I 2

cog M/z

FIG. 2

FIG. 1. Kinetics of VTMS polymerization in cyclohexane solution at 25° in the presence of ethyllithium. Initial monomer concentration 1.4 mole/1. (1-8) and 2"08 mole/1. (9). Initial ethyllithium concentrations (mole/1.): 1--0.0045, 2--0.006, 3--0.009, 4--0-01, 5--0.011, 6--0.015, 7--0"025, 8--0.027, 9--0.027. 2. Intrinsic viscosity of polymer vs. initial molar ratio of monomer to initiator in the polymerization of VTMS in cyclohexane solution in the presence of ethyllithium at 25°.

FIG.

The polymerization of VTI~[S in cyclohexane solution continues uninterruptedly until all the monomer has been expended; at the same time the molecular weight increases continuously with the degree of conversion, and the intrinsic viscosity depends on the ratio M/I, where iV[ is the amount of monomer polymerized, in moles, and I is the amount of initiator, in moles. ° Figure 2 shows the change in the intrinsic viscosity of P V T ~ S in relation to changes in the 1~/I ratio plotted in logarithmic coordinates; Fig. 3 shows the change in molecular weight with the degree of conversion. Seeing that the polymerization of V T ~ S proceeds until all the monomer has been expended, and considering the fact that the molecular weight of PVTMS rises during the polymerization process, and that there is proportionality between the initial M/I ratio and the intrinsic viscosity of the resulting polymer, it appears that the polymerization takes place through "living" lithiumpolysiliconhydrocarbon polymers that are stable and capable of independent ex-

Polymerization of vinyltrimethylsilane

2363

istence, i.e. polymers of the type:

1

H~c--sli--CH3 L

(~Ha

J n ,

so that the reaction proceeds through the so-called "living chains" mechanism [9]. Figure 4a shows the polymerization rate for VTi~IS plotted against monomer concentration over the range 0.8 to 2 mole/1, at 25 ° with an initial initiator concentration of 0.009 mole/l. According to this curve the rate of VTMS polymerization under these conditions is proportional to the monomer concentration, i.e. the reaction is first order in respect to monomer. Mw,lv -s 800

a00

20O

I

I

I

I

I

20

~o

6o

80

Io0

Deopeeof conversion, ~. % FIG. 3. Molecular weight of polymer vs. degree of conversion in VTMS polymerizatiom in eyclohexane solution in the presence of ethyllithium at 25 °. Concentration of (CHa)aSiCH = =CH2-- 3 mole/1, concentration of LiC2Hs-- 0.0045 mole/1.

Figure 4b depicts the rate of VTI~S polymerization plotted against the square of the monomer concentration with an initiator concentration of 0.027 mole/1. This curve shows that in the region of relatively high ethyllithium concentrations (0.027 mole/1.) and over the range of monomer concentrations from 0.8 to 2.04 mole/1, the polymerization of VTI~IS is a second order reaction with respect to monomer. The different orders of the polymerization rate with respect to monomer for the region where the initiator concentration varies, are probably due to the effect that the initiation stage has on the polymerization rate; the effect in question is particularly marked when there are high ethyllithium concentrations. The dependence of the rate of VTI~S polymerizavion on the ethyllithium concentration (the temperature and the monomer concentration being constant) was investigated with initial ethyllithium concentrations in the region of 0.002 to 0.006 mole/1. This is illustrated in Fig. 4c. With low ethyllithium concentra-

2364

N . S. N ~ E m x r ~

et al.

tions the rate of VTMS polymerization rises with increase in the concentration of the initial organolithium compound, although the curve of the polymerization rate vs. ethyllithium concentration deviates from direct proportionality. The polymerization rate increases up to ethyllithium concentrations of 0.027-0.028 mole/l. With initiator concentrations in the region of 0.03 to 0.04 mole/1, the polymerization rate remains practically constant with increase in the initiator concentration, but the rate is reduced on increasing the initiator concentration still further.

w,m~,mole/l..min 0.2 a 0"! 1"0

[M],

w ,mole ~

, fgz

mole/L C

w,mole/L.m[n 0.15I 2"0

O'G

O2

O.Ol

0,03 0.05 mole/L

[LiCzHs] ,

1

2

3

[M] 2, moleZ/l, z

/-/

F I c . 4. R a t e of VTMS polymerization in relation to: a - - m o n o m e r concentration in cyclohexane during polymerization in the presence od ethyllithium at 25 °. LiC2H5 concentration --0.009 mole/l,; b - - s q u a r e of monomer concentration m cyclohexane during polymerization in the presence of ethyllithium at 25 °, LiC~I-I~ concentration--0.027 mole/l; c--initial ethyllithium concentration in the polymerization of VTMS in cyclohexane solution a t 25 °. (CH3)3SiCH =CHz concentration-- 1.4 mole/l.

The nonlinear nature of the curve of polymerization versus initial initiator concentration, and also the independence of the polymerization rate when the concentration of the organolithium compound exceeds a certain limit was observed by other authors studying the polymerization of styrene, isoprene and butadiene in the presence of organolithium compounds in a hydrocarbon medium [10-14]. As in the case of hydrocarbon monomers, the complex dependence of the rate of VT1W_.Spolymerization on the initial ethyllithium concentration is probably due to the well-known fact of the existence of organolithium compounds in an associated form in hydrocarbon media [15-18]; it may be accounted for by change in the activity of organolithium compounds accompanying changes

Polymerization of vinyltrimethylsilane

2365

in the degree of their association. The concentration of active centres will therefore be determined by the dissociation of the associated organolithium compounds present in the system. Analysis of the mechanism of VT)SS polymerization in cyclohexane solutions initiated by ethyllithium, leads us to conclude that the main features of the polymerization of vinylsilanes are similar to those of the polymerization of hydrocarbon monomers through the so-called "anionic" scheme. Note that the hydrocarbon analogue of VTI~S, 3,3-dimethylbutene-1 (neohexane) (CH2:CHC(CH3)3) is practically inert towards metallorganic catalysts [19] and polymerizes mainly through a cationic mechanism. According to our data and also according to Kanazashi [20] VT~S does not polymerize through a cationic mechanism. "We know that the selective capacity of monomers in respect to polymerization by one of two ionic mechanisms is related to the distribution of the electron density at the double bond. Polymerization through an anionic scheme is characteristic of asymmetrically substituted olefins with a substituent that reduces the electron density at the double bond, while polymerization through a cationic mechanism is more probable in the case of monomers having increased electron density at the double bond, i.e. monomers with an electron donor substituent. The group (Ctta)3C has a positive inductive effect. On the basis of the simple inductive effect of (CH3)a~ groups (where M : C , Si) the electron donor properties of this group in the compound CH2:CH2M (CIIa)a should be strengthened by the replacement of C by Si owing to the higher electron-positivity of the latter [21]. The behaviour of the compounds Ctt2-~CtIC(CH3) a and CH2:CHSi(CH3) a in ionic polymerization reactions indicates that the electron density distribution at the double bond in VT~S differs from that in neohexane, that is to say the (CHa)fii group has not only a ÷I-effect but also electron acceptor properties, the effect of which results in polarization of the multiple bond in VT)IS as fol. lows: (CH3)3Si--CH = CH The mechanism of electron displacements in the vinylsilane molecule that results in this type of electron density distribution at the double bond is probably related to the possibility of u-electrons of the vinyl group interacting with vacant 3d-orbitals of the silicon atom. CONCLUSIONS

(1) Some of the kinetic relations of the polymerization of vinyltrimethylsilane in cyclohexane solutions have been studied in the presence of ethyllithium. (2) I t has been shown that in the presence of ethyllithium the polymeriza-

2366

N. S. NAMETKI~ et al.

tion o f v i n y l t r i m e t h y l s i l a n e in c y c l o h e x a n e solutions proceeds until all the m o n o m e r has been expended. The molecular weight o f the p o l y m e r s increases with the degree o f conversion o f the m o n o m e r a n d their intrinsic viscosity d e p e n d s on the ratio o f the m o n o m e r a n d initiator concentrations. The kinetic relations show t h a t the p o l y m e r i z a t i o n proceeds t h r o u g h the so-called "living c h a i n s " mechanism. (3) I t is suggested t h a t the p o l y m e r i z a b i l i t y of v i n y l t r i m e t h y l s i l a n e in the presence of alkyllithium initiators is due to the favorable a s y m m e t r y of the double b o n d owing to the effect o f the Si a t o m . Translated by R. J. A. I-IENDR¥

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10.

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