There is provided a silyl ketene acetal mixture which has a reduced susceptibility to oxidation on exposure to ambient air. The silyl ketene acetal mixture comprises a silyl ketene acetal to which is added a phenolic compound, the phenolic compound being present in an amount sufficient to be effective as an oxidation inhibitor.

BACKGROUND OF THE INVENTION 
This invention relates to silyl ketene acetals which are stabilized against 
oxidation with ambient oxygen. More specifically, this invention relates 
to silyl ketene acetals in which a minor portion of a phenolic compound 
has been added as an oxidation inhibitor. 
The first reference to preparation of silyl ketene acetals (SKA) was in the 
late-1950's by Petrov et al., J. Gen. Chem. (USSR), 29(1959), pp. 
2896-2899. Silyl ketene acetals are characterized by the backbone 
structure, 
##STR1## 
These organosilane intermediates are of interest because of the ability to 
further react the SKA's to other intermediates which would be difficult to 
synthesize by other means. A very recent application is the use of SKA's 
as acrylate polymerization initiators. This concept known as Group 
Transfer Polymerization (GTP) was developed by DuPont and is disclosed in 
three recent U.S. patents--U.S. Pat. No. 4,414,372, Farnham et al., issued 
Nov. 8, 1983; U.S. Pat. No. 4,417,034, Webster, issued Nov. 22, 1983; and 
U.S. Pat. No. 4,508,880, Webster, issued Apr. 2, 1985. 
Rutbottom and Marreno, Syn. Comm., 11(6) (1981), pg. 505-511, discloses 
that meta-chloroperbenzoic acid (MCPBA) oxidizes SKA's to form 
alpha-hydroxy esters. Tamao and Maeda, Tetrahedron Letter, 27: 1 (1986), 
pp. 65-68, report that vinyl alkoxysilanes undergo a similar oxidative 
rearrangement with MCPBA. 
Phenolic compounds, primarily phenolic compounds with a hindered structure, 
are known to be polymerization inhibitors for vinylic materials such as 
acrylates and methacrylates. Inhibitors such as 4-methoxyphenol, 
2,6-di-t-butyl-4-methylphenol, and 2,4-di-methyl-6-t-butylphenol are known 
to be effective in preventing radical chain polymerization of 
alpha,beta-unsaturated esters. 2,4-di-methyl-6-t-butylphenol, also known 
as butylated hydroxytoluene or BHT, is also known as an antioxidant for 
food, animal and vegetable oils, synthetic rubber, plastics, and soaps. 
None of the above references demonstrate or suggest the mixture of an SKA 
and a phenolic compound as disclosed by the instant invention. 
The objective of the instant invention is to provide a silyl ketene 
material with a significantly reduced susceptibility toward oxidation on 
contact with ambient air. 
DESCRIPTION OF THE INVENTION 
In accordance with the instant invention there is provided a silyl ketene 
acetal mixture which has a reduced susceptibility to oxidation on exposure 
to ambient air, the composition of this mixture being described herein. 
What is described therefore, is a silyl ketene acetal mixture, said silyl 
ketene acetal having reduced susceptibility to oxidation, said mixture 
comprising 
(A) a silyl ketene acetal, the silyl ketene acetal being present as a major 
portion; and 
(B) a phenolic compound, said phenolic compound being present as a minor 
portion and being present in an amount sufficient to be effective as an 
oxidation inhibitor. 
Generically, a silyl ketene acetal is a compound with the structural 
backbone, 
##STR2## 
Any SKA can be stabilized in this invention. Substituent groups of the SKA 
are not critical to the invention so long as these groups are not reactive 
with the phenolic compound. Preferred silyl ketene acetals may be selected 
from a group consisting of 
##STR3## 
wherein 
each R is independently selected from a group consisting of alkyl groups 
containing 1 to 4 carbon atoms, alkoxy groups containing 1 to 4 carbon 
atoms, aryl groups, and alkaryl groups; wherein a is 0, 1, 2, or 3; n is 
0, 1, 2, 3, 4, 5, or 6; m is 0, 1, 2, 3, 4, 5, or 6; and p is 0, 1, 2, or 
3; 
wherein Z is selected from a group consisting of 
EQU --Y, (i) 
wherein Y is selected from a group consisting of C.sub.1-20 alkyl, alkenyl, 
or alkadienyl; C.sub.6-20 cycloalkyl, aryl, alkaryl, or aralkyl; any of 
said group containing one or more ether oxygen atoms, tertiary amino 
groups, amido groups, thio groups, siloxy groups, or carbonyl groups 
within aliphatic segments thereof; and any of such group containing one or 
more functional substituents that are unreactive under silylating 
conditions, 
EQU --W, (ii) 
wherein W is selected from a group consisting of C.sub.1-20 alkyl, alkenyl, 
or alkadienyl; C.sub.6-20 cycloalkyl, aryl, alkaryl, or aralkyl; any of 
said group being terminated by trialkylsilyl groups, tertiary amino 
groups, isocyanato groups, perhalo groups, amido groups, thio groups, 
cyano groups, phosphonate groups, siloxy groups, or carbonyl groups 
thereof; and any of such group containing one or more functional 
substituents that are unreactive under silylating conditions, 
EQU --SiR.sup.i.sub.3, (iii) 
wherein each R.sup.i is independently selected from the group consisting of 
alkyl radicals containing 1 to 4 carbon atoms, alkoxy groups containing 1 
to 4 carbon atoms, and aryl groups, 
##STR4## 
wherein G.sup.1 and G.sup.2 are independently selected from the group 
consisting of (a) alkyl radicals containing 1 to 4 carbon atoms, aryl 
groups, and alkoxy groups containing 1 to 4 carbon atoms, (b) 
trialkylsilyl, and (c) alkyltrialkylsilyloxy; 
wherein Q is selected from a group consisting of C.sub.1-20 alkylene, 
alkenylene, or alkadienylene; C.sub.6-20 cycloalkylene, arylene, 
alkarylene, or aralkylene; any of said group containing one or more ether 
oxygen atoms, tertiary amino groups, amido groups, thio groups, siloxy 
groups, or carbonyl groups within aliphatic segments thereof; and any of 
such group containing one or more functional substituents that are 
unreactive under silylating conditions; Silyl ketene acetals of the 
formula, 
##STR5## 
may be such materials as 
##STR6## 
or like materials. 
The inventor has unexpectedly found that silyl ketene acetals undergo 
auto-oxidation on exposure to ambient air to form alpha-siloxy ester of 
the general structure, 
##STR7## 
This oxidation was found to occur very rapidly, consuming as much as 5 
weight percent of the desired SKA in one day. In the application of SKA's 
as an acrylate polymerization initiator, described supra, the presence of 
these alpha-siloxy esters severely compromises the purity of the SKA's and 
their effectiveness as polymerization initiators. 
The phenolic compound can be represented by the general formula, 
##STR8## 
The phenolic compound can be selected from a group consisting of phenol 
and substituted phenols. The substituted phenols can be such materials as 
hydroquinone, 4-methoxyphenol, 2,6-di-t-butyl-4-methylphenol, 
2,2'-methylene-bis-(6-t-butyl-p-cresol), 
2,2'-thiobis-(6-t-butyl-p-cresol), 1,1,3-tris(2-methyl-4-hydroxy-5-t-butyl 
phenylbutane), tetrakis(methylene(3,5-di-t-butyl-4-hydroxy 
cinnamate))methane, and like materials. These phenolic compounds are 
available commercially. Many of these substituted phenols are listed in 
Chemical and Process Technology Encyclopedia (1974), pp. 131-135. 
For the purposes of the instant invention, the term "in an amount 
sufficient to be effective as an oxidation inhibitor" means that amount of 
the phenolic compound that will inhibit the oxidation of a silyl ketene 
acetal so that the initial siloxy ester concentration is not increased 
more than four-fold upon storage of 30 days in contact with ambient air or 
that the absolute content of the siloxy ester of the desired silyl ketene 
acetal is a maximum of about 10 weight percent. The substituted phenols 
are somewhat more effective as an oxidation inhibitor for SKA's than is 
phenol. The substituted phenols are a more preferred oxidation inhibitor 
than phenol. For the substituted phenols, alone, the term "in an amount 
sufficient to be effective as an oxidation inhibitor" means that amount of 
a substituted phenol that will inhibit the oxidation of a silyl ketene 
acetal so that the siloxy ester concentration is not increased more than 
about 50 to 75 percent upon extended storage of 20 days. Preferred 
phenolic compounds are selected from a group consisting of hydroquinone, 
4-methoxyphenol, and 2,6-di-t-butyl-4-methylphenol. 
It has been found in the instant invention that a molar concentration of a 
phenolic compound, relative to the SKA, of greater than about 100 parts 
per million is effective in inhibiting the formation of the undesirable 
siloxy ester. The inventor of the instant invention believes that 
inhibitor concentrations of less than 100 ppm can be effective, however, 
with less assurance of reliability. A preferred concentration of oxidation 
inhibitor is 500 ppm on a molar basis relative to the silyl ketene acetal. 
Higher concentrations of inhibitors are effective, however the inventor 
believes that concentrations of greater than 10,000 parts per million are 
not economically practical. 
The following examples are presented to aid in the understanding of the 
instant invention by those skilled in the art. These examples are to be 
illustrative and are not to be construed as limiting the instant invention 
as delineated in the claims.

EXAMPLE 1 
(Not within the scope of the instant invention) 
A freshly distilled sample of a silyl ketene acetal (SKA) was exposed to 
ambient air to study the effect upon product quality. The SKA evaluated 
was 
##STR9## 
A stock mixture or master batch of material was made from the SKA and 
treated toluene. The toluene was added at about 3.5 weight percent to 
serve as an internal standard for subsequent gas chromatographic analyses. 
The toluene was treated by being distilled from calcium hydride. The 
toluene so treated was then stored over molecular sieves. 
Approximately 1 gram (g) of the SKA was placed in a 8 milliliter glass 
vial. The vials were sealed with an appropriate cap. This volume of SKA 
left an air space above the liquid of approximately 80 percent of the 
volume of the vial. A sample of the freshly distilled SKA was analyzed, 
and will be designated as Sample A. Six other samples were prepared in the 
above manner, and these samples are designated as Samples B, C, D, E, F, 
and G. These samples were stored sealed under ambient conditions. The 
samples were held for various periods of time, and analyzed by gas 
chromatographic analysis. The most significant change noted in product 
quality was the formation of the siloxy ester species, 
##STR10## 
The starting SKA had a purity of 98.7 weight percent by gas chromatographic 
analysis. Table 1 is a summary of the results of analysis of the samples 
taken at various times. The analysis of the siloxy ester species, above, 
are reported in weight percent, designated as "%SiO". Time of sampling is 
reported in hours and days, and is designated "Time". 
TABLE 1 
______________________________________ 
Sample Time % SiO 
______________________________________ 
A 0 hours 0.26 
B 0.5 0.50 
C 1 0.64 
D 3 1.24 
E 5 2.39 
F 24 4.56 
G 4 days 2.73 
C 25 1.97 
E 25 4.21 
______________________________________ 
These above results demonstrate that silyl ketene acetals will oxidize upon 
contact with ambient air in the vapor space in a liquid storage vessel. 
EXAMPLE 2 
A study was made to evaluate the effect of the addition of a phenolic 
material as an inhibitor to the formation of the siloxy ester material 
shown to be generated upon exposure to ambient air in Example 1. 
Experimental and analytical procedures similar to those utilized in 
Example 1 were applied to this study. 
The silyl ketene acetal was the same species as utilized in Example 1. This 
SKA had analysis of 97.6 weight percent SKA, as determined by gas 
chromatographic analysis. The stock SKA solution had an analysis of 94.8 
percent SKA, 2.8 percent toluene, and 0.60 percent siloxy ester, all in 
weight percent. This stock solution was designated as Sample H. 
The phenolic material evaluated was 2,6-di-t-butyl-4-methylphenol, 
hereafter referred to as BHT. Several samples were prepared by mixing BHT 
with the SKA stock solution at concentrations of approximately 6700, 3300, 
1600, 750, and 130 parts per million, respectively, on a molar basis 
relative to the SKA. These samples are designated as Samples I, J, K, L, 
and M, respectively. The various samples were analyzed by a gas 
chromatographic technique at various times after being placed into the 
glass vials in contact with ambient air. 
Table 2 is a summary of the results. The various samples are identified by 
content of BHT, expressed as parts per million (ppm) relative to the SKA, 
and designated as "ppm BHT"; by weight percent siloxy ester content, 
designated as "%SiO"; and by the time after addition of samples to the 
glass vials, expressed in days, designated "Time". 
TABLE 2 
______________________________________ 
Sample Time ppm BHT % SiO 
______________________________________ 
H 0 0 0.60 
I 3 6700 0.42 
J 3 3300 0.41 
K 3 1600 0.33 
L 3 750 0.43 
M 3 130 0.43 
H 3 0 0.83 
H 24 0 3.56 
M 24 130 0.69 
______________________________________ 
These above results demonstrate that 2,6-di-t-butyl-4-methylphenol is an 
effective inhibitor to the oxidation of a silyl ketene acetal. 
EXAMPLE 3 
A study was undertaken to evaluate the effectiveness of several phenolic 
compounds as oxidation inhibitors for the silyl ketene acetal, 
##STR11## 
The phenolic compounds evaluated were phenol, 4-methoxyphenol (MEHQ), and 
hydroquinone (HQ). 
The experimental and analytical procedures are similar to those utilized in 
Examples 1 and 2. The stock solution of this SKA in toluene had an 
analysis by gas chromatographic analysis of 86.9 weight percent SKA, 8.2 
percent toluene, and 2.7 percent siloxy ester. Table 3 is a summary of the 
various samples, mixed with above inhibitors and analyzed after various 
times of exposure to ambient air in a sealed state. The stock SKA solution 
in toluene is designated as Sample N. The remaining samples are designated 
as Samples O, P, Q, R, S, and T, respectively. The results in Table 3 are 
reported by inhibitor used, designated as "Inhib"; concentration of 
inhibitor in SKA in ppm on a molar basis, designated as "ppm"; time in 
hours after exposure to ambient air, designated as "Time"; and siloxy 
ester content of the SKA solution in weight percent, designated as "%SiO". 
TABLE 3 
______________________________________ 
Sample Inhib ppm Time % SiO 
______________________________________ 
N None 0 0 2.7 
N None 0 24 19.6 
N None 0 68 34.2 
O Phenol 180 24 3.7 
O Phenol 180 67 5.3 
P Phenol 360 24 4.9 
P Phenol 360 67 8.6 
Q MEHQ 160 24 2.3 
Q MEHQ 160 67 2.5 
R MEHQ 310 24 2.4 
R MEHQ 310 65 2.5 
S HQ 150 24 2.3 
S HQ 150 65 2.5 
T HQ 300 24 2.1 
T HQ 300 65 2.5 
______________________________________ 
These above results demonstrate the oxidation inhibiting effect of phenolic 
compounds mixed with silyl ketene acetals when exposed to ambient air in a 
sealed situation. 
EXAMPLE 4 
Another test was performed with the SKA species of Example 3, and 
2,6-di-t-butyl-4-methylphenol (BHT). 
The stock solution of the SKA in toluene had an analysis by gas 
chromatography of 87.0 weight percent SKA, 7.3 percent toluene, and 2.2 
percent siloxy ester. Two samples were evaluated, the first with no BHT, 
the second sample with 170 ppm BHT. These samples are designated as 
Samples U and V, respectively. 
Table 4 is a summary of the results. The samples are identified by content 
of BHT, expressed as parts per million (ppm) relative to the SKA, and 
designated as "ppm BHT"; by weight percent siloxy ester content, 
designated as "%SiO"; and by the time after addition of samples to the 
glass vials, expressed in hours and days, designated "Time". 
TABLE 4 
______________________________________ 
Sample Time ppm BHT % SiO 
______________________________________ 
U 0 hours 0 2.2 
U 1 0 2.7 
U 16 0 7.0 
U 25 0 8.3 
V 0 hours 170 2.2 
V 24 170 2.1 
V 25 days 170 2.1 
______________________________________ 
These above results further demonstrate the oxidation inhibiting effect of 
phenolic compounds mixed with silyl ketene acetals when exposed to ambient 
air in a sealed situation.