Ab Initio Study of Anionic Polymerization: Initiation Reactions of

Hassan K. Khartabil , Marilia T. C. Martins-Costa , Philippe C. Gros , Yves Fort and Manuel F. Ruiz-López. The Journal of Physical Chemistry B 2009 1...
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J , Phys, Chcm, 1994,98, 9410-9416

9470

Ab Initio Study of Anionic Polymerization: Initiation Reactions of Carbosilane Polymer AMtomo Tachiband and Tasuku Yano Division of Molecular Engineering, Faculty of Engineering, Kyoto University, Kyoto 606-01,Japan Received: April 8, 1994; In Final Form: July 7, 1994@

An ab initio quantum chemical study is performed for some initiation reactions of anionic polymerization of SiHZmCHz. As initiator reagent, lithium alkoxide has better selectivity than the alkyl metal: formation of CH30-SiH2-CH2-Li is more feasible than CH~O-CHZ-S~HZ-L~, and other reaction paths are found to be subsidiary.

I. Introduction Silicon carbide (Sic) has been widely accepted as a promising material of both high-temperature structural and electronic performance.' Recent topics on the manufacturing tool of S i c have included synthesizing ultrafine S i c powders by plasma chemical vapor deposition* (CVD) and laser-driven CVD.3,4 Radicals or ions should play an important role as activated species in the CVD reaction system. Recently, in the low-power regime of the CVD system, notable new chemical processes have been found.5 We have studied the CVD reaction mechanism of Si-C bond formation using ab initio quantum chemical technique and found that the silicon-carbon bond is rather hard to form whatever the active species is.637 We shall now examine in this paper theoretically a putative reaction mechanism of anionic polymerization in order to make the Si-C bond directly in the form of polycarbo~ilane.~~~ The idea is to utilize the highly polarizable nature of the Si=C Anionic polymerization is an important industrial technique.l 2 In particular, living polymerization is most promising. This acknowledges12the reaction field under nonpolar solvent, where the carbanion active site has a long lifetime, keeping the growing point of polymerization intact against possible rearrangement elements. Quantum chemical study in this kingdom including solvent effects has just started using the MNDO Hamilt~nian,'~ but no fully ab initio works have been performed as far as the authors are aware. In this paper, we studied model initiation reactions and their competitive ones as follows: SiH,=CH,

1

+

X-Li 2: X = C H 3 3: X = CH3O

-

X-SiH,-CH,-Li 4: X = C H 3 6: X = C H 3 0

-

(Ia,b)

X-CH,-SiH,-Li 5: X = C H 3 7: X = C H 3 0 (IW)

-

SiH,=CH-Li

+

10 X-H

-

(IIIa,b)

8: X = C H 3 9: X = C H , O CH,=SiH-Li 11 :SiH-CH,-Li 12

+ X-H (IVa,b)

+ X-H (Va,b)

@Abstractpublished in Advance ACS Abstracts, August 15, 1994.

0022-365419412098-9470$04.50/0

TABLE 1: Total Energied (au) and Zero-Point Energleg (au) MP2suecies SiHyCH2 CH3-Li CH3-0-Li CH3-SiHZ-CHZ-Li SY n anti

(full)

CISD(full)

ZPE(MP2)

ZPE(scaled)

-329.299 04 -329.334 96 0.041 39 0.038 90 -47.193 66 -47.213 62 0.034 71 0.032 62 -122.291 55 -122,310 54 0.042 72 0.040 16 -376.592 14 -376.641 32 0.081 13 0.076 26 -276.593 27 -376.642 50 0.081 75 0.076 85

CH3-CH2-SiHz-Li sYn anti

-376.584 29 -376.637 44 0.084 90 0.079 80 -376.587 18 -376.640 16 0.085 15 0.080 04

CH3-0-SiHz-CHz-Li SY n

anti

CH3-0-CH2-SiH2-Li SY n anti

CHq CH,-OH SiHZ=CH-Li CHZ=SiH-Li SiH-CH2-Li TSa TSb

-451.688 54 -451.735 93 0.088 80 0.083 47 -45 1.660 45 -451.707 96 0.088 02 0.082 73 -45 1.626 35 -451.599 07 -40.369 85 -115.38984 -336.154 42 -336.173 33 -336.18445 -376.487 94 -451.582 64

-451.679 19 -45 1.652 25 -40.392 58 -115.41446 -336.185 94 -336.208 11 -336.218 75 -376.539 16 -451.630 84

0.090 32 0.089 60 0.046 60 0.053 03 0.030 92 0.034 11 0.034 24 0.078 24 0.086 82

0.084 90 0.084 23 0.043 80 0.049 85 0.029 06 0.032 06 0.032 18 0.073 54 0.081 61

Calculated with the 6-31G** basis set using MP2(fu11)/6-31G** optimized geometries. At the MP2 level; ZPEs (scaled) were scaled by 0.94.

where reactions a,b refer to those reactions with X = CH3 (IVa), CH3O (I-Vb). Reactions I and I1 are initiation reactions that form a silicon-carbon chain. In these reactions the monomer is assumed to be SiH2=CH2, which represents the substrate that has an unsaturated silicon-carbon bond. Methyl lithium (X = CH3) is a typical reagent of alkylmetal that gives a carbanion. Lithium methoxide (X = CH30) is a model of the alkoxide. Products of I and I1 are carbanions and silanions, respectively. Reaction I11 is abstraction of hydrogen attached to carbon. Reaction IV is abstracLionof hydrogen attached to silicon. The initiation reaction of anionic polymerization is either addition or hydrogen abstraction. The last reaction, V, is formation of silylene. The individual character of the initiation reactions will be clarified and compared with each other. It will be found that the principal addition reaction of the Si=C bond has no significant deep well for the reactant complex, in contrast to the formation of stable pentacoordinatedSi complex of reactants for the substitution reaction of the Si-R bond. 14-23 11. Method of Calculation

Molecular orbital calculations were performed using the GAUSSIAN92 program package.24 Geometries of all compounds and transition states (TSs) were optimized to minimize the energy of the second-order Moller-Plesset perturbation t h e ~ r y ,which ~ ~ , ~includes ~ correlation for all electrons, using 0 1994 American Chemical Society

J. Phys. Chem., Vol. 98, No. 38, 1994 9471

Initiation Reactions of Carbosilane Polymer

TABLE 2: Vibrational Frequencies (cm-I) species

vibrational frequencies 464(b1), 479@2), 758(a2), 794(b1), 862(b2), 963(a1), 1015(al), 1455(a1), 2376(a1), 2400(b2), 3247(a1), 3352(b2) 486(e), 617(al), 1142(al), 1520(e), 3089(a1), 3187(e) 167(e), 837(a1), 1224(e), 1286(a1), 1547(a1), 1563(e), 3017(al), 3078(e) 14i(a”), 190(a’), 268(a”), 294(a’), 391(a”), 453(a”), 587(a’), 608(a”), 637(a’), 725(a’), 745(a’), 784(a’), 910(a”), 930(a’), 947(a”), 992(a’), 1322(a’), 1443(a’), 1527(a’), 1532(a”), 2264(a‘), 2272(a”), 3042(a’), 3124(a”), 3165(a’), 3209(a’), 3240(a”) 68(a”), 113(a’), 178(a”), 246(a’), 388(a”), 498(a”), 588(a’), 671(a’), 715(a”), 735(a’), 803(a’), 851(a’), 939(a”), 961(a”), 976(a’), 1003(a’), 1345(a’), 1439(a’), 1518(a”), 1520(a’), 2224(a”), 2231(a’), 3112(a’), 3121(a’), 3184(a”), 3223(a’), 3226(a”)

SiHz=CHz CH3-Li CH3-0-Li syn

anti CH3-CH2-SiH2-Li sYn

104i(a”), 108(a’), 252(a”), 311(a”), 312(a’), 447(a’), 613(a”), 632(a‘), 768(a’), 787(a”), 1000(a’), lOll(a’), 1011(a”), 1087(a’), 1295(a’), 1313(a”), 1457(a’), 1515(a’), 1554(a’), 1563(a”), 2207(a’), 2208(a”), 3100(a’), 3132(a’), 3180(a”), 3190(a’), 3202(a”) 84(a”), 121(a’), 239(a”), 281(a’), 339(a”), 452(a’), 583(a”), 646(a‘), 769(a’), 808(a”), 995(a’), 1010(a’), 1032(a”), 1084(a’), 1304(a’), 1311(a”), 1460(a’), 1512(a’), 1561(a’), 1561(a”), 2201(a’), 2204(a”), 3110(a’), 3114(a’), 3167(a”), 3198(a’), 3221(a”)

anti

CH3-0-Si H2-CH2-Li 93(a”), 105(a”), 155(a”), 219(a’), 401(a’), 452(a”), 462(a’), 505(a”), 542(a’), 640(a’), 701(a’), 772(a”), 824(a’), 925(a”), 935(a’), 1019(a’), sw 1130(a’), 1210(a”), 1230(a’), 1447(a’), 1527(a’), 1557(a”), 1570(a’), 2275(a’), 2276(a”), 31 10(a’), 3188(a’), 3199(a”), 3227(a’), 3269(a”) anti 55(a”), 98(a’), 100(a”), 140(a”), 213(a’), 312(a’), 399(a”), 588(a’), 624(a”), 750(a’), 790(a’), 828(a”), 851(a’), 946(a”), 986(a’), 1026(a’), 1134(a’), 1213(a”), 1236(a’), 1443(a’), 1534(a’), 1564(a”), 1567(a’), 2213(a”), 2229(a’), 3082(a’), 3129(a’), 3156(a”), 3204(a”), 3210(a’) CH3-0-CHz-SiH2-Li 96(a”), 107(a”), 229(a”), 260(a’), 306(a”), 330(a’), 439(a’), 511(a’), 596(a”), 674(a’), 740(a’), 866(a”), 893(a’), 1017(a’), 1134(a’), 1203(a”), sYn 1220(a’), 1253(a”), 1328(a’), 1510(a’), 1532(a’), 1543(a”), 1571(a’), 2227(a’), 2230(a”), 3102(a’), 3108(a’), 3175(a”), 3199(a”), 3236(a’) anti 88(a”), 96(a”), 105(a’), 239(a’), 251(a”), 343(a”), 373(a’),458(a’), 598(a”), 691(a’), 787(a’), 916(a”), 971(a’), 1004(a’), 1171(a’), 1205(a”), 1230(a’), 1271(a”), 1355(a’), 1511(a’), 1538(a’), 1547(a”), 1573(a’), 2234(a’), 2240(a”), 3026(a’), 3063(a’), 3075(a”), 3131(a”), 3226(a’) 1406(t2), 1627(e), 3135(a1), 3282(t2) CH4 CH,-OH 347(a”), 1083(a’), 1117(a’), 1208(a”), 1409(a’), 1539(a’), 1564(a”), 1579(a’), 3097(a’), 3170(a”), 3246(a’), 3914(a’) 78(a’), 218(a”), 510(a”), 577(a’), 692(a’), 814(a”), 858(a’), 988(a’), 1056(a’), 2272(a’), 2289(a’), 3215(a’) SiH2=CH-Li CHySiH-Li 134(a’), 187(a”), 473(a’), 632(a’), 735(a”), 796(a”), 903(a’), 918(a’), 1468(a’), 2222(a’), 3202(a’), 3299(a’) :SiH-CH*-Li 250(a), 420(a), 510(a), 718(a), 735(a), 837(a), 872(a), 1154(a), 1501(a), 1723(a), 3048(a), 3256(a) 349i(a’), 51(a”), 135(a”), 279(a’), 305(a’), 372(a”), 41 l(a’), 444(a”), 493(a’), 506(a”), 645(a’), 777(a”), 858(a’), 883(a’), 927(a”), 961(a’), TSa 1090(a’),1447(a’), 1477(a’), 1511(a”), 2348(a’), 2378(a”), 3038(a’), 3142(a”), 3233(a’), 3287(a’), 3333(a”) TSb 450i(a’), 70(a”), 104(a”), 150(a”), 21 l(a‘), 324(a’), 369(a’), 388(a”), 405(a’), 515(a”), 692(a’), 825(a”), 837(a’), 955(a”), 962(a’), 1016(a’), 1194(a’),1216(a’),1219(a”), 1458(a’), 1522(a’), 1550(a”), 1560(a’), 2343(a’), 2370(a”) 3042(a’), 3111(a”), 3153(a’), 3213(a’), 3321(a”)

TABLE 3: Relative Energiee reaction MP2(full) 0.0 SiH,=CH, CHLi ~H~-&HZ-CH~-L~ -59.4 SYn -59.8 anti $4.2 TSa CH3-CHz-SiHz-Li -52.3 SYn -56.2 anti -21.8 SiHZ=CH-Li CI& -33.7 CHz=SiH-Li + CI& -38.7 :SiH-CH*-Li CI& 0.0 SiHz=CHz CH30Li CH3-O-SiH2-CHz-Li -58.7 SY n -41.5 anti $6.6 TSb CH~-O--CHZ-S~HZ-L~ -18.8 SYn -2.1 anti +29.0 -SiHz=CH-Li + CH3-OH +19.0 Li-SiH=CHz CH3-OH +12.1 :SiH-CHZ-Li -t CH3-OH

H

-

1.466

--

\

1.711

/

+

‘H

H

---

I(Cd

H

._

2(C3v)

--

+

+ +

+

CISD(ful1)+ QC 0.0

-55.2 -55.6 +7.2 -50.6 -54.4 -20.5 -34.7 -39.4 0.0

-54.0 -36.9 +10.8 -17.5 -1.0 +28.5 +16.2 +9.6

Calculated with the 6-31G** basis set using MP2(fu11)/6-31G** optimized geometries, including ZPE(sca1ed) corrections. +0.70 C

$/

L

1.382

O

1.606

Li + o m

HIH 3(C3V)

Figure 1. MP2(fu11)/6-31G**optimized structuresof 1,2,and 3. Bond

lengths are in angstroms. Bond angles and dihedral angles are in degrees. The atomic charges of heavy atoms are in italics. The symmetry of each compound is entered in parentheses. the standard 6-31G** basis set (denoted as MP2(full)/ 6-31G**27). The harmonic frequencies of all structures were calculated at the MP2 level, and zero-point energies (ZPE) are scaled by 0.94.27 The electron energies of the single- and double-substituted configuration interactionsz8(CISD) including unlinked cluster quadruple correctionsz9~30 (QC) were calculated (denoted as CISD+QC(full)/6-3 lG**//MP2(ful1)/6-3lG**27).

The atomic charge was calculated using Mulliken population at the MP2(fu11)/6-31G** level. Realistic three-dimensional charge distribution was further examined using electrostatic potential, being found to confirm the Mulliken population analysis for the highly polarizable ionic nature of the reactants. 111. Results and Discussions Optimized structures are shown in Figures 1-4. Their total energies, zero-point energies (DES), and vibrational frequencies are listed in Tables 1 and 2. Optimized Structures. In Figure 1 are shown optimized structures of the monomer and the initiators. The bond length of Si-C in SiH2=CH2 (1) is 1.711 A, as has already been reported.31 Our result is in agreement with other results at the

Tachibana and Yano

9472 J. Phys. Chem., Vol. 98,No. 38, 1994

He1.091 4

0

//:

9111,0 1

/

112.3\

-0.61 H6

.+H7 1.089

H5* ''08'

111.6

110.7

\

2.500

\2.002

1.

anti-4f Cs)

Lil

c0.20

anfi-5(Cs)

*

Q(C4.C3-Si2.H7)=53.3 O(Lil -Siz-C3-H9)=57.3

O(Si2-C3.C4-H&59.4

H6

I

+0.33

H6

\ 1.087

1.087 +0.30

1.479

1.768

C 1.484

Lil

1.790

107.0 71.9

1.092

-0.66 1.086

syn-7( CS) O(04.C3-Si2-H7)=127.7 O(Lil-Siz-C3-H10)=117.5 O(C3-O4.C5-H1 1)=60.Z

syn-6( CS) @(Si3-O4-C5-H1 )=60.2 Q(Lil-C2-Si3.H10)-l 11.7 Q(04-Si3-C2-H7)=1 13.7

H6

\

1.088 108.2

\Lil +0.38 anti-6( Cs)

@(Si~O4-C5-Hl1)=60.6 O(Lil -C2-Si3-Hlo)=58.4 O(04-Si3-C2-H8)=61 .6

syn-7( CS) Q(H4-C3-Si2-H8)=54.1 Q(Lil-Si2-C3-Hlo)=58.4 O(C3-04-C5-H1,)=60.5

Lil +0.20

Figure 2. MP2(fu11)/6-31G** optimized structures of syn- and anfi-4, 5, 6, and 7. For details, see caption of Figure 1.

CISD/TZ2P and MP2/6-3 lG* level^.^*^^^ This compound is highly polarized, where the atomic charges of Si and C in 1 are + O S 1 and -0.54, respectively. This ionic character will be

further discussed in the Reaction Mechanism subsection. The C-Li bond length in CH3-Li (2) is 2.003 A. This bond length is in good agreement with other r e s ~ l t s . The ~ ~ ,C-Li ~ ~ bond is

J. Phys. Chem., Vol. 98, No. 38, 1994 9473

Initiation Reactions of Carbosilane Polymer

+0.39

Lil

1.084

H4

lo(&) +0.27

2.445

1.084

\

H5

11(CS)

1.085

+0.27

h

106.5

-0.01 c 3

1.420

107.3

0.962

Hl 9( CS) @(H&-Oz-H1)=61.4

Figure 3. MP2(fu11)/6-31G**optimized structures of 8, 9, 10, 11, and 12. For details, see caption of Figure 1.

TSb( Cs) 0(04-C3-Si2-H7)=1 11.6 O(Lil-Si&-H1~)=l 01.5 @(C~04-CyH11)=60.3

Figure 4. MP2(fu11)/6-31G**optimized structures of TSa and TSb. Arrows show the eigenvector associated with the imaginary frequency. For details, see caption of Figure 1. ionic, as expected, where the atomic charges of C and Li are -0.61 and +0.41, respectively. The bond length of C-0 and 0-Li in CH30-Li (3) are 1.382 and 1.606 A, respectively, in agreement with other results at the level of HF/6-311+G**.36 The atomic charges of 0 and Li are -0.84 and +0.63, respectively, reflecting the ionic character of the 0-Li bond in 3 as well as the C-Li bond in 2. In Figure 2 are shown optimized structures of the products 4-7. We have obtained a pair of optimized geometries of CH3-SiH2-CH2-Li (4). The first is the anti form. In this conformation, the methyl group and lithium atom are staggered.

The second is the syn form, in which the methyl group and lithium atom are eclipsed. Comparing electron energies of these two geometries, the anti form is more stable than the syn form by 0.7 kcdmol. We performed frequency analyses of these two geometries. The anti form has no imaginary frequency numbers. On the other hand, the syn form has an imaginary frequency wavenumber, 14i cm-', as shown in Table 2. The vibrational vector of this imaginary frequency showed a rotation of the Si-C 0 bond, changing into the anti form. The unique optimized geometry of the product of reaction Ia is the anti form. The bond length of C-Li in this structure is 2.002 A.

9474 J. Phys. Chem., Vol. 98, No. 38, 1994

.-

-

CH3Li+SiH2CH2*' 0.0

\\

Tachibana and Yano TS

TS

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r

'I

\

,,

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CH30LitSiH2CH2

I

0.0

L

I 1

I

'\ \

I

anti -1.o

\

1

I

,,

CH30CH2SiHzLi

1,

,

I

,,

+10.8

/'

I \

,

I

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\

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\

,

I

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syn

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CH3CH2SiH2Li

,

\

'l1

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\,

,

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1.

syn -50.6

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I

, I

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anti -54.4

anti -36.9

I \

,,

I I

I I -

anti -55.6

SY n

-55.2

SYn -54.0

CH3SiH2CH2Li

CH30SiH2CH2Li

CH3Li+SiH2CH2 SiH2=CHLi + CH3OH

+

, CH2=SiHLi I' +28.5CH30H

0.0 :SiH-CHZLi + CHI

(d) Figure 5. Energy diagrams of reactions in kcal/mol: (a) Ia and IIa; (b) lb and IIb; (c) IIIa, ma, and Va; (d) IIIb, IVb, and Vb. Relative energies are calculated by using total energies at the CISD(full)+QC/6-31G**//MP2(fu11)/6-3 1G**level corrected with ZPEs. -39.4

This length agrees with that of the ionic bond of carbon-lithium in 2. Furthermore, the atomic charges of C and Li are -0.77 and f0.43, respectively. These atoms are so charged that the C-Li bond is ionic. We obtained optimized geometries of syn and anti forms of CH3-CH2-SiH2-Li (5). Comparison of electron energies and frequency analyses of these two forms gave us the same results that are given in the case of 4. The syn form is 1.8 kcal/mol above the anti form. It had one frequency mode with a negative eigenvalue (the equivalent frequency wavenumber is 104i cm-', as shown in Table 2), which showed rotation of the Si-C u bond and transformation into the anti form. The absolute minimum of 5 is therefore the anti form. The Si-Li bond length in 5 is 2.504 A. This is in good agreement with the length of the silicon-lithium ionic bond in SiH3Li at the HF/ 6-31G* But the Si atom in 5 is positively charged, +0.48. This is an important characteristic of the species having a silicon atom attached to a lithium atom. In the case of CH30-SiHz-CH2-Li (6), as well as compound 4 and 5, we obtained two optimized geometries. We compared the electron energies and vibrational modes. The results differed from those of the last two species. We did not see an imaginary frequency number. Both of these geometries are local minima on potential energy surfaces. As the syn form is more stable than the anti form by 17.6 kcal/mol, we defined the absolute minimum structure of 6 as the syn form. In this structure the bond angle of Si-C-Li is 81.6". This value is small because of the electric force by the positively charged Li

atom and the negatively charged 0 atom. The C-Li bond length is 2.093 A, which is in good agreement with the carbonlithium bond length in 2. The atomic charges of C and Li are -0.66 and f0.30, respectively. The C and Li are ionically bound. We found two local minima of the potential energy surface (7). The syn form was more stable of CH30-CH2-SiH2-Li than the anti form, by 17.1 kcdmol. The bond angle of Li-Si-C is 7 1 .go, whose small value is caused by the electric attractive force of the charged Li and 0 atom. As in compound 5 , the Si atom in 7 is ositively charged, +0.20. The Si-Li bond length is 2.452 This is in good agreement with the result in the case of 5. In Figure 3 are shown optimized structures of the products 8-12. The Si-C bond lengths in 10, 11, and 12 are 1.715, 1.747, and 1.792 A, respectively. The Si-C bond length in 12 is longer than that of the other two, so that the Si-C bond in 12 is a saturated bond and this species is SH-CHz-Li. Comparing electron energies of these three compounds, 12 is the most stable. If an initiator is CH3-Li, the unique optimized geometry of the product is the anti form. Using CH30-Li, the product has two local minima, and the syn form is more stable. The relation of atomic charges and Si-C bond lengths shows that electric field affects bond length. The more charged Si and C are, the shorter the Si-C bond length is. Reaction Pathways. We examined the existence of barriers of reactions Ia,b and IIa,b. First, we optimized reactant

1.

J. Phys. Chem., Vol. 98, No. 38, I994 9475

Initiation Reactions of Carbosilane Polymer

H

H

"\

C-0Hfi

Li

"\

C-0Hfi

Li

(c)

Figure 6. Electrostatic potentials: (a) S i H z e H 2 ; (b) CH3Li; (c) CH3OLi, calculated at the HF/6-3 1G** level using MW6-31G** optimized structures. Negative area is shaded.

Figure 7. (a) LUMO of siH2-H~; (b) HOMO of CH3Li; (c) HOMO of CH30Li, calculated at the HF/6-31G** level using MP2/6-31G** optimized structures. One phase of the orbital is shaded.

supermolecules of Ia and Ib starting with various geometries with long distances in between SiH2=CH2 and X-Li and found that the optimized structures were the same as syn-4 and syn-6. Thus, we come to conclude that there is no barrier in reactions Ia and Ib. On the contrary, we obtained optimized geometries of saddlepoints of reactions IIa,b (TSa, TSb) as shown in Figure 4. The transit vector (the vibrational frequency wavenumber is 3491' cm-', as shown in Table 2) in TSa (see Figure 4) shows that the C-C bond is binding and that the CH2 group and the SiH2 group are transforming from a planar structure to a tetrahedral one. The Li atom is moving from the CH3 group to the SiH2 group; so this is the transition state of reaction IIa. In this transition state the bond angles of H-C-Si and C-Si-H were 121.3' and 117.4', respectively. The shape of the SiH2 group is similar to the tetrahedral structure and that of the CH2 group is similar to the planar one. In this reaction the SiH2 group re-forms before the CH2 group does. The C-Li length is 1.970 which is equivalent to the carbon-lithium bond length in 2. But the CH3-CH2 distance is 2.510 A. This is much longer than the equilibrium bond length, 1.527 A, for CH3-CH2 in compound 5. This transition state may lie at the beginning of the reaction path. Such an early transition state of addition to an unsaturated bond has already been reported.38 As the vibrational mode with a negative force constant (the frequency wavenumber is 450i cm-l, as shown in Table 2) in

TSb shows that the CH3O-Li bond is breaking and the CH3O-C and SiH2-Li bonds are forming, this is the transition state of reaction IIb. The bond angles of H-C-Si and C-Si-H were 120.9' and 114.5'. The structures of the SiH2 group and the CH2 group resemble those of the tetrahedral SiH2 group and the planar CH2 group of TSa, respectively. The length of CH30-C is 2.073 A. This length is much longer than that of the product. This transition state is also considered at the reactant side of the reaction path.38 Reaction Mechanism. Relative energies of reactions I-Va,b are shown in Table 3. The energy diagrams are presented in Figure 5. The enthalpies of reactions Ia, IIa, Ib, and IIb are -55.6, -54.4, -54.0, and -17.5 kcallmol, respectively. With regard to reaction selectivity, the alkoxide is superior to the alkylmetal, because the energy difference between Ia and IIa is small, 7.2 kcallmol, and that between Ib and IIb is large, 36.5 kcdmol. This difference shows that the C atom in CH3Li prefers Si as well as C, and the 0 atom in CH3-0-Li likes Si more than C. This is in close agreement with the theoretical result by Luke et al.16939 The electrostatic potentials of 1,2,and 3 are shown in Figure 6. The Si side in 1,as well as the Li side in 2 and 3, is positive. The C side in 1, the CH3 side in 2,and the CH3O side in 3 are negative. This negatively charged group of 2 or 3 should be attractive to the positively charged Si atom in 1, so that reaction Ia and Ib easily occur at the initial stage of the reaction.

A,

9416 J. Phys. Chem., Vol. 98, No. 38, I994

The highest occupied molecular orbitals (HOMO) of 2 and 3, as well as the lowest unoccupied molecular orbital (LUMO) of 1, are shown in Figure 7. The HOMO of 2 is that of the CH3 anion, because of the CH3-Li bond being ionic, as discussed before. Note that the LUMO of 1 is the n* orbital, which is dominant at the Si atom. This suggests that electrons are carried from the HOMO of the initiators to the Si atom in the monomer, not to the C atom. In reactions Ia,b the electrons are carried from the HOMO of 2 or 3 to the LUMO of 1, so that the carbon atom or the oxygen atom in 2 or 3 is easily bound to the silicon atom in 1, because of a good overlap of orbitals. Such reactions should easily occur with less (actually no) barriers. In order to get the normal sequence -(-Si-C-Lin the polymer, it is necessary to inhibit reaction V and, if possible, reactions 11-IV altogether. In this viewpoint, the alkoxide is better than the alkylmetal as the initiator. Indeed, if using the alkoxide, the reaction path Ib is selected exclusively, leaving the other reaction paths IIb, IIIb, IVb, and Vb subsidiary. In the propagation reaction, if we simulate the living terminal site as CH3-Li, reaction path Ia is selected exclusively by the same reasoning.

IV. Conclusion We have examined some initiation reactions of anionic polymerization of SiH2=CH2 using ab initio molecular orbital techniques. As an initiator reagent, lithium alkoxide has better selectivity than the alkyl metal: the formation of CH30-SiH2-CH2-Li is more feasible than CH30-CH2-SiH2-Li, and the other reaction paths are found to be subsidiary. The resultant CH2-Li+ terminal should further attack other monomers and lead to living polymerization of carbosilane. We have thus studied a new ab initio theoretical branch of silicon chemistry in the mechanism of anionic polymerization that is of technological importance. Since the actual reaction system is not in vacuo, solvent effects should play an important role, which is our next target under consideration.

Acknowledgment. This work was supported by a Grantin-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan, for which the authors express their gratitude. We wish to thank the Computer Center of the Institute for Molecular Science and the Data Processing Center of Kyoto University, for their generous permission to use the HITAC M-680 and S-820 and FACOM M-780/30, VP400E, and VP-200 computers, respectively. References and Notes (1) Marshall, R. C., Faust, J. W., Jr.; Ryan, C. E., Eds. Silicon Carbide 1973; Univ. of South Carolina Press: Columbia, SC, 1974. (2) Hollabaugh, C. M.; Hull, D. E.; Newkirk, L. R.; Petrovic, J. J. J . Mater. Sci. 1983, 18, 3190.

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