Kinetics of thermal depolymerization of trimethylsiloxy end-blocked polydimethylsiloxane and polydimethylsiloxane-N-phenylsilazane copolymer

The Science of the Total Environment, 73 (1988) 71~85 Elsevier Science Publishers B.V., A m s t e r d a m - - Printed in The N e t h e r l a n d s

71

KINETICS OF THERMAL D E P O L Y M E R I Z A T I O N OF TRIMETHYLSILOXY E N D - B L O C K E D P O L Y D I M E T H Y L S I L O X A N E AND POLYDIMETHYLSILOXANE-N-PHENYLSILAZANE COPOLYMER

M. ZELDIN, D.W. KANG, G.P. RAJENDRAN, B. QIAN and S.J. CHOI

Department of Chemistry, Indiana University Purdue University at Indianapolis, 1125 E. 38th Street, P.O. Box 647, Indianapolis, IN 46223 (U.S.A)

ABSTRACT The t h e r m a l degradation of Me3SiO end-blocked polydimethylsiloxane (eb.PDMS) and polydimethylsiloxane-N-phenylsilazane (eb-PDMS-NPhSz) copolymer was studied. For both polymers, relative degree of polymerization (DP/DP0) as a function of conversion (C = 1 - W/Wo)data were obtained. For eb-PDMS with three different molecular weights, the results were consistent with a m e c h a n i s m involving a rate-determining random siloxane bond cleavage initiation step followed by a rapid and complete depropagation of the active f r a g m e n t s evolving volatile cyclic oligomers. Rate c o n s t a n t s for initiation were obtained at four t e m p e r a t u r e s from plots of D-P ' vs. time for eb-PDMS of Mn = 6.83 × 104. An A r r h e n i u s activation energy of ~ 80 kcal m o l - ' was determined and is consistent with a SiOSi scission t r a n s i t i o n state. The degradation of eb-PDMS-NPhSz appears to follow the same depolymerization process evolving cyclic oligomers. A l t h o u g h DP/DP 0 vs. C data s u g g e s t a random cleavage-complete depolymerization mechanism, an A r r h e n i u s plot suggests a more complex degradation mechanism. The role of impurities as degradation catalysts is discussed.

INTRODUCTION

The thermal degradation of linear polydimethylsiloxanes (PDMS) under vacuum yields volatile cyclic oligomers [1-4]. Under non-equilibrium conditions in a "catalyst-free" environment it has been shown that the degradation products of trimethylsilyl end-blocked or hydroxyl terminated PDMS are principally cyclic trimer, (Me2SiO)3, with lesser quantities of the larger ring compounds [5-7]. For the latter material thermal gravimetry and molecular weight measurements confirm that the polymer degrades in a stepwise fashion from the chain ends (A) [4]. For the end-blocked material it has been suggested that the low activation energy for depolymerization (i.e. 43kcalmol ' by isothermal TGA) relative to the siloxane bond energy (i.e. 108kcalmo1-1) precludes initiation by random scission of the siloxane bond, but is consistent with a mechanism involving an intramolecular cyclic (loop) transition state (B) accompanied by siloxane bond rearrangement [5]. Substituent effects appear to support this hypothesis.

0048-9697/88/$03.50

© 1988 Elsevier Science Publishers B.V.

72 R2

R2

R2

R2

- - x / ~ r Si --O-- Si - ?,~ Si - O-Si

R2

R2

R2

R~SLO~Si O ~ S i - - O - S i ~ .

R2

~ / x - - - - Si O - - S i R2

(A)

--O-Si R2

R2

(B)

Data from the aforementioned studies do not preclude an unzipping mechanism, initiated either at the chain ends or by a random chain cleavage. Furthermore, no distinction has been made in previous studies between partial and complete unzipping upon initiation. These basic mechanisms have been treated theoretically. Appropriate equations are available to test each process and, where appropriate, to deduce relevant reaction parameters. We have recently shown that the thermal depolymerization of eb-PDMS follows closely a theoretical expression which relates the degree of polymerization of the residue polymer as a function of conversion fraction for a degradation mechanism involving a rate-determining random scission initiation step followed by a complete depropagation of the kinetically active fragments [6]. We have suggested that the previously determined relatively low activation energy might be explained by a mechanism involving siloxane bond rupture catalyzed by trace quantities of impurities which were either introduced inadvertantly during synthesis or leached from the reaction vessel walls during thermolysis. In the present study we wish to report further evidence of a random cleavage initiation mechanism for both end-blocked PDMS and endblocked PDMS copolymer containing N-phenylsilazane units (PDMS-NPhSz). Additionally, an Arrhenius activation energy was determined for the homoand copolymer from initiation rate constants as a function of temperature. THEORY Equations based on MacCallum's theoretical kinetics treatment of thermal depolymerization are given below [8]. In this treatment, depolymerization mechanisms are divided into two groups categorized by the nature of the initiation reaction; i.e. random scission along the polymer molecule backbone or initiation at the polymer chain ends. Each category is subdivided into (i) initiation followed by partial unzipping only and (ii) initiation followed by complete unzipping only. Relationship (1) is generally applied for any polymer degradation W = N(DP)m

(1)

where W is the weight of the polymer at time t, N is the number of molecules in the sample at time t, DP is the number-average degree of polymerization of the sample at time t, and m is the repeat-group molecular weight. Differentiation of (1) with respect to t gives 1 dW m dt -

N d(DP) ~ + ~

dN d--7

(2)

73 For random-cleavage initiation followed by an incomplete unzipping process and assuming that the depolymerization follows first-order kinetics with respect to sample weight. dN/dt

=

kW/m

-dW/dt

=

kWZ

(3) (4)

where Z is defined as the average zip length. Combination of (2)-(4) followed by integration gives DP DP0

-

f(1 - C) C + f

(5)

where f = Z/DP0 and C is the fraction of conversion (1 - W / W o ) . For random initiation followed by complete unzipping, Z is defined as some multiple of DP (6), where b is a parameter related to the polydispersity of the polymer. Z = b(DP)

(6)

Combining (3), (4), and (6) into (2) and integrating gives -

=

(7)

For terminal initiation followed by partial unzipping - dN/dt

=

0

(8)

From (2), (4), and (8) and integrating one gets DP DP0

-

1

-

C

(9)

For terminal initiation followed by complete unzipping -dN/dt

=

kN

(10)

Combining (1), (2), (4), and (10) and integrating gives DP DP0

-

1

(11)

Consideration of an intramolecular loop mechanism in which only small loops of constant average size are formed and concommitantly removed from the polymer, Eqns (4), where Z is the average number of monomer units in the loop, and (8) apply. Incorporation into (2) and integration gives (9). Figure 1 gives representative theoretical curves for (5), (7), (9), and (11) as a plot of the fractional number-average degree of polymerization as a function of conversion. Line (I) corresponds to degradation by chain-end initiation followed by a complete unzipping process. Since initiation is the rate-determining step, no change in molecular weight with conversion is expected. If

74

"5

(iV) Conversion

fraction

Fig. 1. Representative theoretical plots of 11 (I), 7 (II), 9 (III) and 5 (IV).

c h a i n - e n d i n i t i a t i o n is followed by i n c o m p l e t e unzipping, a m o l e c u l a r w e i g h t c h a n g e p r o p o r t i o n a l to c o n v e r s i o n (III) is obtained. An i n t r a m o l e c u l a r smallloop m e c h a n i s m affords the s a m e r e l a t i o n s h i p . A m e c h a n i s m i n v o l v i n g r a n d o m scission followed by c o m p l e t e u n z i p p i n g r e s u l t s in a r e l a t i v e l y slow d e c r e a s e in m o l e c u l a r w e i g h t w i t h c o n v e r s i o n (II), w h e r e a s r a n d o m i n i t i a t i o n followed b y i n c o m p l e t e u n z i p p i n g r e s u l t s in a r a p i d d e c r e a s e in m o l e c u l a r w e i g h t w i t h c o n v e r s i o n (IV). In the l a t t e r two cases a family of c u r v e s c a n be g e n e r a t e d d e p e n d e n t on b a n d Z, r e s p e c t i v e l y (Fig. 2). I n t h e case of a r a n d o m s c i s s i o n - c o m p l e t e u n z i p p i n g m e c h a n i s m a timed e p e n d e n t e q u a t i o n (12) c a n be derived by s u b s t i t u t i o n of (3), (4) a n d (6) into (2).

D~_

~o

0 0

C

1

Fig. 2. Theoretical plots of 7 for various values of b, and 5 for various values of Z/DP0.

75 - d(DP)

dt

-

k(b

- 1) (DP) 2

(12)

Thus the time rate of change of the average degree of polymerization is second order in DP and integrates to the linear expression 1/DP =

1/DP0 - k(b - 1)t

(13)

Plots of DP/DP0 vs. C (i.e. 1 - W/Wo), where DP is obtained from membrane osmometry or exclusion chromatography, can be used to distinguish between mechanistic types; and, for mechanism (II), plots of (DP) -1 vs. t at different temperatures enable evaluation of k, the rate of initiation (scission) and activation energy. EXPERIMENTAL Infrared spectra were obtained using a P e r k i n - E l m e r 710B grating spectrometer. P r o t o n magnetic resonance spectra were obtained in CC14 or CDC13 solution with CH2 C12 as an internal reference on a Varian EM3930 (60 MHz) spectrometer. Mass spectra were observed using a Finnigan MAT model 4500 equipped with a gas chromatograph. Molecular weight measurements were determined either by exclusion c h r o m a t o g r a p h y operating with a Waters R401 differential r e f r a c t o m e t e r and two AS1103/k ultragel columns or membrane osmometry (Knauer) using a H e w l e t t - P a c k a r d 0-8 non-aqueous membrane.

Synthesis of trimethylsilyl end-block polydimethylsiloxane (PDMS) In a typical experiment, distilled w at er was placed in an ice bath and a mixture of freshly distilled Me2SiC12 (125ml, Silar Laboratories) and diethyl ether (125 ml) was added dropwise for 2 h with stirring. After reaction the ether layer was removed, washed several times with dilute aqueous NaHCO3 solution, then several times with distilled water, and dried over reagent-grade anhydrous MgSO4 overnight. The solution was filtered and the solvent was removed by distillation. The product was a clear fluid. The reaction flask was purged with oxygen-free nitrogen. Fou r drops of Me4NOH in methanol (20%) was added to the fluid and the mixture was heated to 100-110°C. The viscosity of the fluid increased with time and the react i on was stopped at a desired viscosity by addition of toluene (20 ml) followed by product separation with addition of methanol. The resulting fluid (hydroxyl-terminated PDMS) was isolated and dried under vacuum at 40-50°C for at least 40 h. End blocking was achieved by refluxing the fluid with a mixture of excess 1,1,1,3,3,3-hexamethyldisilazane (96ml, Aldrich Chemicals) and Me3SiC1 (20ml) in toluene. The polymer was isolated by precipitation from toluene solution with distilled methanol and purified by filtration of a toluene solution t h r o u g h an activated carbon bed followed by several precipitations from toluene with methanol. The

76 polymers were dried under vacuum at 60°C. Polymers with Mn of 68300, 110000 and 240000 were prepared for degradation studies.

Synthesis of N-phenylcyclotrisiloxazane (DNPh) D NPhwas prepared by modification of methods described in the literature [9]. In a typical experiment 1,5-dichlorotrisiloxane (0.066 mol), prepared by established methods [10], triphenylamine (13g) and DMF (70ml) were placed in a three-necked round-bottom flask with a thermometer, reflux condenser and nitrogen inlet tube. Distilled aniline (0.066 mol) was added dropwise for 40 min with stirring under a N2 stream at 0°C. After addition the reaction mixture was warmed to 40°C for several minutes, then reacted for 8 h at 25°C. After filtering the solution, the filtrate was stored in dry ice overnight. The solid precipitate was dissolved in ether and the insoluble residue was removed by filtration. The filtrate was evaporated to dryness to give a white solid (Yield 20%, m.p. 72-73°C lit., 80°C [9]). Ir (Nujol, KBr, cm '): 1690w, 1590m, 1460 s, 1375s, 1300w, 1255 s, 1225m, 1165w, 1020s, br, 910m, 890m, 805s, 755w, 695m, 605m, 590w, 510w, 470w; mass spectrum: (CI/CH4), m/e 298; (EI, 70eV), m/e 297 (27%, M ÷), 282 (100%, M CH~), 207 (61%, M (CH3)~SiO2), 190 (25%), 133 (28%), 73% (95%).

Synthesis of Me3SiO end-blocked poly(dimethylsiloxane-N-phenylsilazane) (PDMS-NPhSz) In a typical experiment D3 (0.11mol), D NPh (0.022mol) and toluene (26ml) were placed in a three-necked round-bottom flask equipped with a thermometer, reflux condenser and a nitrogen inlet tube. Air was removed under vacuum and replaced by dry nitrogen. Potassium silanolate (0.4ml), prepared from (MeSiO)4 and KOH [11] (2.8mgK/ml catalyst solution), and distilled DMSO (0.8ml) were added. The mixture was heated to 90-110°C for 4h with stirring. Volatile materials were removed by heating to 130°C under vacuum for 5 h. The residue was dried under vacuum at 55°C for 24 h. End-blocking of the fluid product was achieved by refluxing with excess 1,1,1,3,3,3-hexamethyldisilazane (75ml, Aldrich) and trimethylchlorosilane (10ml, Dow Corning) in toluene for 60h. The excess volatile compounds were removed under vacuum at 60°C. The copolymer could not be purified by procedures described for end-blocked PDMS owing to the sensitivity of the Si-NPh bonds in the copolymer toward protonic solvents. Mn, 20,500; (polydispersity (gpc), 1.22); 'H-NMR (CC14, C6H,2; ppm): 7.98 m, 0.77 s, 0.57 s, 0.32 s, - 0.63 s; Ir (Nujol, KBr, cm 1): 1590w, 1480 m, 1410m, 1260 s, 1220w, 1060s, br, 980sh, 910w, 860m, 800s, 700m [11]; Anal. (Theoretical, %): C, 34.7; H, 8.06; Si, 36.0; N, 0.74; Found: C, 35.9; H, 8.8; Si, 35.4; N, 0.61.

Thermal degradation study Degradation studies were carried out in a tube furnace with a metal tube

77 heater (850 W/120 V, Acra Electric Corp.) controlled to _+1.5°C by a proportional electronic thermocouple regulator (Love Controls model 72) with a digital temperature readout (Omega digital thermometer 199T). Two types of experiments have been carried out: the single-oven method in which weight loss was obtained periodically on a single sample to about 80% conversion and residue samples were extracted for molecular weight measurements; and the multiple-oven technique in which a number of samples in different ovens with a common temperature control were simultaneously depolymerized and weight loss data were obtained by removal of each sample at different conversion points. The same results were obtained regardless of technique. In a typical single-oven experiment, up to I g of end-block PDMS was placed in a Pyrex ampule (11 × I cm) mounted vertically in the middle of the tube furnace. The sample was evacuated (10 3torr), then heated to temperature. The reaction was quenched by removing the oven. The weight of residue provided a measure of conversion and a small sample of residue (ca. 20rag) was removed for molecular weight determination. The experiment was continued until conversion exceeded 70%. Volatile products were collected in dry ice-acetone and/or liquid-nitrogen traps and analyzed by comparison with the infrared spectra and gas chromatographs of authentic compounds. At all temperatures examined, the residues remained fluid for the duration of the experiment. The distribution of volatile products by GC/MS were: (PDMS): D3, 51%; Dr, 20%; D0+, 28%; other, ~ 1%; PDMS-NPhSz: D3, 50%; D~Ph, 7%; D4, 15%; D~Ph, 2%; D ~ , 20%; ~-'5+nNeh,~ 5% and trace amounts of analine. Depolymerization data for eb-PDMS and eb-PDMS-NPhSz are summarized in Figs 3-6. Kinetic data for eb-PDMS of Mn = 68,300 from 450 to 510°C are given in Table 1 and for eb-PDMS-NPhSz of M n = 20,500 from 447 to 493°C are given in Table 2.

1

.

.

M n = 6 . 8 3 x 104 . . . .

b=1.3 . .

. + 450°C 475°C

DP

0 490eC

O

C

Fig. 3. P l o t o f D P / D P 0 vs. C f o r e b - P D M S o f M n = 6.83 × 104 f r o m 450 t o 510°C.

78 M n =1.10 x l O 5

b=1.4

1

.

.

.

.

.

o' 4 5 0 o'C X 4 8 0 °C ~" 5 0 5 ° C

t

O

i

i

i

o

i

c

Fig. 4. P l o t o f D P / D P 0 vs. C for e b - P D M S o f M n = 1.10 x 105 from 450 to 505°C.

RESULTS AND DISCUSSION

When Me3SiO end-blocked PDMS is heated above 400°C, depolymerization occurs giving principally volatile cyclic oligomers (e.g. D3, 51%, D4, 20%, D~s, 28%). Small quantities of impurities play an important role in catalyzing the decomposition of PDMS at elevated temperatures [12-15]. Thus the polymer was carefully purified (i.e. precipitations from toluene with methanol and M,q ~ 2 . 4 0 x 105

b =1.~5

11

t •

o

450 C 475 C

o

0

0

C

Fig. 5. P l o t o f D P / D P 0 vs. C for e b - P D M S of Mn = 2.40 × 105 from 450 to 500°C.

79 --

Mn = 2 . 0 5 x 1 0

4

b=1.2

1.0

0.8

0.6

DPo 0.4

0.~

O.C 0.0

0,2

0.4

0.6

0.8

1.0

C

Fig. 6. Plot of DP/DP0 vs. C for eb-PDMS-NPhSz of Mn = 2.05 × 104 from 447 to 493°C. removal of extraneous materials by filtering through an activated carbon bed). The residue during depolymerization remained a colorless liquid and conversion to volatile materials was almost quantitative (i.e. 99+ %). For a polymer of molecular weight 6.83 x 104 (DP0,923), measurement of the change in degree of polymerization with conversion fraction (Fig. 3) corresponds closely to the theoretical expression (7) for a degradation mechanism involving a rate-determining random chain cleavage process which is followed by a rapid and complete depolymerization of the kinetically active fragments. Similar results were found for homopolymers o f M n = 1.10 x 105 and 2.40 × 105 (Figs. 4 and 5, respectively). The results are inconsistent with a chain-end initiated process [Fig. 1; (I), (III)], a random cleavage-partial unzipping process (IV), or a loop mechanism (III) where only small volatile oligomers are formed and volatilized. It has been shown that expression (7) depends on the type and breadth of the molecular weight distribution as well as the overall kinetic reaction order. For example, b = 1 corresponds to a process whereby the rate of weight loss is first order in polymer weight as predicted for a monodisperse polymer. In contrast, higher values of b (e.g., 1.5, 2.0) reflect broader and more complicated distributions (e.g., coupling, exponential, respectively). The values of b in this study appear to increase with Mn and suggest a complicated distribution, perhaps converging on an exponential form. An understanding of the significance of b in PDMS depolymerization awaits further studies correlating b with M~ and distribution. eb-PDMS-NPhSz, owing to the hydrolytic sensitivity of the silazane bond (Si-N~Si), could not be purified by precipitation methods. Degradation of the copolymer (M, = 2.05 x 104) above 440°C resulted in depolymerization with evolution of cyclic oligomers with and without the phenylsilazane moiety (e.g. D3, 50%; D NPh, 7; D4, 15%; D Neh, 2%; D~ s, 20%; ~5+n~Ph,~ 5% and irace quantities

1.08 1.12 1.14 1.15 1.17 1.20 1.27 1.30

4.46 x 10 -8 0.96

0 300 720 2900 6200 9050 12100 13200

k (min -1) c.c b 3.50 x 10 -7 0.98

1.08 1.12 1.13 1.17 1.23

~--~ 1 ( x 1 0 3)

. + 1.5°C. b Correlation coefficient from a linear least-squares analysis.

0 190 550 910 1300

t(min)

~

t(min)

1 ( x 1 0 -3 )

475°C

450°C

0 120 440 830 1070

t(min)

490°C 1 ( x 1 0 -3 )

6.54 x 10 7 0.96

1.08 1.11 1.13 1.21 1.31

~

Kinetic data for the t h e r m a l depolymerization of eb-PDMS (M n = 68300) at 450 °, 475 °, 490 ° and 510°C a

TABLE 1

0 60 120 180 270

t (min)

510°C

4.12 x 10 -6 0.97

1.08 1.11 1.16 1.25 1.41

~--~-1 (x 10 3)

2.14 × 10 6 0.90

k ( m i n -1) c.c. b

1.11 × 10 _5 0.92

3.61 3.84 3.92 4.07 4.88 5.11

D-P -1 (× 10 ~)

b Correlation coefficient from a linear least s q u a r e s anlaysis.

a + 1.5oc.

3.61 3.69 3.76 4.24 4.22

0 230 640 970 1615

0 160 260 460 560 660

t (min)

~

t (min)

1 (× 10 3)

459°C

447°C

0 90 210 270

t (min)

475°C

1.89 x 10 5 0.92

3.61 3.64 4.09 4.67

D-P-~ (× 10 3)

0 80 140 200 260

t (min)

493°C

Kinetic data for the t h e r m a l depolymerization of eb-PDMS-N-phenylsilazane (Mn = 20500) at 447 °, 459 °, 475 ° and 493°C a

TABLE 2

1.97 × 10 5 0.90

3.61 3.70 3.89 4.60 4.44

1(×10-3)

82

of analine). Degradation data (DP/DP0 vs. C) for the copolymer are summarized in Table 2 and Fig. 6. Although there is considerably more scatter in the data points with respect to the theoretical curve (b = 1.20) compared with the homopolymer experiments, the results appear to be consistent with a random chain cleavage-complete unzipping mechanism. It should be noted, however, that the copolymer discolors (pale yellow to amber) during the course of degradation and, depending on the temperature, 5-10% of the initial copolymer weight remains as a solid glassy brown residue. Further consideration of the theoretical model leads to Eqn. (13) which expresses the kinetics of the random scission-complete unzipping mechanism. Kinetics data for eb-PDMS (J~n = 6.83 × 104) in the temperature range 45(~ 510°C are summarized in Table 1. Sample plots of DP- 1 as a function of time at 450 °, 475 °, 490 ° and 510°C are given in Fig. 7. Specific rate constants for random cleavage initiation are obtained from the slopes of the lines. The correlation coefficients for linearity are at least 0.96. The activation energy (Ea) determined from the Arrhenius plot (Fig. 8) is ~ 8 0 k c a l m o l 1 (correlation coefficient, 0.99), which is consistent with the energy requirements for scission of the Si O bond (BE ~ 108 kcal mol-1 [16]). The activation energy from these experiments is significantly larger than the value reported by Thomas and Kendrick (i.e. 43kcalmol 1) [5] from isothermal TGA experiments for the depolymerization of eb-PDMS and estimates of E, from the first-order thermal

1.5 1.4 1.31.2' 1.1 0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.2

0.4

0.6

0.8

1.0

1.2

}.4

0,2

0,4

0.6

0,8

1.0

1.2

1.4

2. 0

,4.0

6.0

8.0 10.0 12.0 Time (rnin) x l O - 3

14.0

1.3 12 1,1 ---1 DP 1.4 1.3 1.2 1,1

1.4 1,3 1.2 1.1

Fig. 7. P l o t of D--P 1 vs. t i m e for d e p o l y m e r i z a t i o n of eb-PDMS at 450 °, 475 °, 490 ° and 510°C.

83 -12

-13

-14 _n k -15

-16

-17 1.2~,

1.30

1.32 1.34 T ( K)-I x 103

1.36

1.38

Fig. 8. Arrhenius plot for the thermal degradation of eb-PDMS. decomposition of D4 in the pyrolysis of D5 (i.e. 61kcal mol-1) [17]. The entropy of activation for the decomposition of eb-PDMS can be estimated from the intercept in Fig. 8. Ifa unimolecular process is assumed, AS ~ is 22 e.u. The positive value is indicative of a degradation involving a chain bond rupture rate-determining step as opposed to a cyclic concerted mechanism which would require a highly ordered transition state and thus a negative value of AS t . Degradation kinetics for eb-PDMS-NPhSz (Table 2) were carried out at four temperatures to 50-70% conversion. Correlation of the data with Eqn. (13) was less than satisfactory if compared with eb-PDMS. Nevertheless, rate constants were estimated. The Arrhenius plot (Fig. 8) suggests changes in degradation mechanism with temperature, perhaps as a result of varying amounts and/or types of catalytic species being released in different temperature regions. -9

-10

-11 Ln k -12

-13

i

i

1.30

1,32

.

r

1.34 1.36 T ( ° K ) -~ x l O 3

i

r

1.38

1.40

Fig. 9. Arrhenius plot for the thermal degradation of eb-PDMS-NPhSz.

84

[i~,1) (C)

J

L Me'~"(~t)'~Me [D]

T h e a b o v e r e s u l t s i l l u s t r a t e t h e i m p o r t a n c e of i m p u r i t i e s in t h e d e g r a d a t i o n p r o c e s s . A l t h o u g h p o l y s i l o x a n e s s h o w l i t t l e i n c l i n a t i o n to form s t a b l e c o m p l e x e s w i t h d o n o r o r a c c e p t o r r e a g e n t s , t h e i m p o r t a n c e of m e t a l - c o n t a i n i n g e l e c t r o p h i l i c [i.e. [18, 19] FeC13/(CH3CO)20, ZnBr2/PBr3], n u c l e o p h i l i c (i.e. [20] SO~ ), a n d s t r o n g l y a c i d i c [21] r e a g e n t s in c a t a l y t i c c l e a v a g e of t h e s i l o x y b o n d is w e l l d o c u m e n t e d . T h u s t r a n s i t i o n s t a t e s / i n t e r m e d i a t e s (C, D), i n v o l v i n g a w e a k e n i n g of t h e s i l o x a n e bond, w h i c h is p r e s u m a b l y s t a b i l i z e d by (p~d) ~ b o n d i n g , t h r o u g h l o c a l i z a t i o n of t h e e l e c t r o n s on o x y g e n by c a t a l y s t p a r t i c i p a t i o n is r e a s o n a b l e a n d c o n s i s t e n t w i t h t h e k i n e t i c r e s u l t s . M o r e o v e r , C a n d D w i l l r e s u l t in i n c r e a s e d s t e r i c c r o w d i n g a b o u t t h e s i l o x a n e u n i t t h e r e b y f a v o r i n g s c i s s i o n . S t u d i e s of w a l l effects, v e s s e l c o m p o s i t i o n , a n d d o p i n g w i t h c a t a l y s t s on t h e m e c h a n i s m a n d a c t i v a t i o n e n e r g y a r e c u r r e n t l y u n d e r way. ACKNOWLEDGEMENT A c k n o w l e d g m e n t is m a d e to t h e D o n o r s of T h e P e t r o l e u m R e s e a r c h F u n d , a d m i n i s t e r e d by t h e A m e r i c a n C h e m i c a l S o c i e t y , for p a r t i a l s u p p o r t of t h i s r e s e a r c h a n d t h e D o w C o r n i n g C o r p o r a t i o n for a f e l l o w s h i p g r a n t . D W K a n d S J C e x p r e s s s i n c e r e s t g r a t i t u d e to D a n k o o k U n i v e r s i t y a n d t h e M i n i s t r y of E d u c a t i o n ( K o r e a ) w h o p r o v i d e d f i n a n c i a l s u p p o r t for s a b b a t i c a l l e a v e to c a r r y out this research. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

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