Process inhibitor for readily polymerizable acrylate monomer

The present invention provides an improvement in methods for preparing and processing readily polymerizable acrylate monomers. The improvement comprises employing phenyl-para-benzoquinone, 2,5-di-phenyl-para-benzoquinone, and mixtures thereof as process inhibitors. The process inhibitors are present in a concentration of about 50 to 3000 ppm, preferably about 250 to 2000 ppm, and most preferably about 500 ppm.

BACKGROUND OF THE INVENTION 
Readily polymerizable acrylate monomers are important chemicals of 
commerce. The current production of methyl methacrylate and related 
monomers is approximately 1.3 billion pounds per year. These monomers are 
most often prepared by methods involving a distillation step at elevated 
temperature and/or reduced pressure. 
Acrylic acid and derivatives thereof are currently preferably prepared by 
the oxidation of propylene. The process involves a two-step oxidation, 
followed by an extraction, and vacuum distillation. Acrylic acid can also 
be produced by the reaction of acetylene, carbon monoxide, and water in 
the presence of nickel carbonyl. This method also involves the removal of 
product by distillation. A similar process which additionally includes 
methyl or ethyl alcohol as a reactant is employed for the preparation of 
methyl or ethyl acrylate, respectively. Again, distillation is employed to 
purify and collect the product. 
A widely employed process for the preparation of methacrylic acid involves 
the acid hydrolysis of acetone cyanohydrin. Methyl methacrylate is 
prepared by a similar process involving the acid methanolysis of acetone 
cyanohydrin. Each of these methods likewise employs vacuum distillation at 
elevated temperatures (e.g., about 100.degree. C.) in order to recover the 
product. 
In the above-described processes for the preparation of acrylate monomers, 
like all other preparations of polymerizable monomers, care must be 
exercised to remove the products from the reaction mixture and to inhibit 
the monomer before uncontrolled polymerization can ensue. The reactivity 
of these acrylate monomers necessitates the use of a relatively large 
amount of process inhibitor in order to prevent polymerization. Currently 
used process inhibitors include hydroquinone, the methyl ether of 
hydroquinone, and p-benzoquinone. A compound which has been proposed for 
use as a process inhibitor is n-nitroso-phenylhydroxylamine. However, this 
compound is expensive and is reported to be extremely toxic. 
When investigating the usefulness of a compound as a process inhibitor for 
acrylate monomer, a number of characteristics are evaluated. The compound 
must exhibit an effectiveness as a polymerization inhibitor equal to or 
greater than the currently employed or proposed process inhibitors. The 
compounds should exhibit a lower environmental impact and toxicity than 
currently employed and proposed process inhibitors. A process inhibitor 
which is proposed for use in acrylate monomer systems should be soluble in 
acrylate monomers, such as methyl methacrylate, or in suitable carrier 
solvents at levels which are high enough to allow the preparation of 
commercially useful stock solutions. Also, the process inhibitor must be a 
compound which does not entrain with the acrylate monomer during the final 
purification of the monomer. 
It has now been found that all of these characteristics are surprisingly 
and unexpectedly exhibited by phenyl-para-benzoquinone, 
diphenyl-para-benzoquinone, and mixtures thereof. 
Japanese Kokai Tokkyo Koho No. 81 86,123 discloses the use of 
2-phenyl-1,4-benzoquinone as a polymerization inhibitor for aromatic vinyl 
compounds. The use of 2,5-diphenyl-p-benzoquinone as a polymerization 
inhibitor for a polyester resin system diluted with styrene is disclosed 
in Proceedings of the 23rd Annual Technical Conference of the Reinforced 
Plastics/Composites Division of the Society of the Plastics Industry 
(Plastics Ind., Inc.: New York) 1968. The use of diphenyl-p-benzoquinone 
in admixture with another inhibitor to inhibit the crosslinking of 
polyolefins is disclosed in British Patent Specification No. 1,077,634. 
The use of diphenyl-p-benzoquinone as an inhibitor in the curing of 
unsaturated polyester is disclosed in U.S. Pat. No. 3,026,286. None of 
these prior art references recognizes the unique characteristics of 
phenyl-para-benzoquinone and/or diphenyl-para-benzoquinone which make 
these compounds, either alone or in mixtures, desirable as process 
inhibitors for acrylate monomers. 
SUMMARY OF THE INVENTION 
The present invention provides an improvement in processes for the 
preparation of readily polymerizable acrylate monomers. The improvement 
comprises employing a process inhibitor comprising 
phenyl-para-benzoquinone, 2,5-diphenyl-para-benzoquinone, or a mixture 
thereof.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention relates to the use of phenyl-para-benzoquinone, 
2,5-diphenyl-para-benzoquinone, or a mixture thereof as a process 
inhibitor in the preparation of readily polymerizable acrylate monomers. 
As used herein, the term "process inhibitor" refers to a polymerization 
inhibitor which is employed during the preparation and processing of the 
monomer. Process inhibitors can be distinguished from product inhibitors, 
which are combined with the monomer in order to inhibit polymerization 
during storage and handling. 
According to the process of the present invention, 
phenyl-para-benzoquinone, 2,5-diphenyl-para-benzoquinone, or a mixture 
thereof is employed as a process inhibitor during the preparation and 
processing of certain monomeric substances. These benzoquinone derivatives 
are well known chemical compounds whose preparation is well known in the 
art. Phenyl-para-benzoquinone and diphenyl-para-benzoquinone are available 
commercially from a number of sources. 
While phenyl-para-benzoquinone appears to be more effective as a process 
inhibitor, and is therefore preferred for use in the process of the 
present invention, either of the above-identified compounds or mixtures 
thereof may be used in the process of the present invention. Furthermore, 
additional inhibitors may also be used in combination with the 
above-identified compounds. These additional inhibitors may be other known 
process inhibitors or may be product inhibitors. Such additional 
inhibitors may include para-benzoquinone, hydroquinone, tert-butyl 
catechol, diphenylamine, the methyl ether of hydroquinone, etc. 
The present process is applicable to readily polymerizable acrylate 
monomers. The term "acrylate monomer" is intended to include acrylic acid, 
methacrylic acid, and the many various esters thereof. While the following 
is not intended to be an exhaustive listing of such compounds, the esters 
of acrylic acid can include n-alkyl esters, such as methyl acrylate, ethyl 
acrylate, propyl acrylate, butyl acrylate, pentyl acrylate, hexyl 
acrylate, heptyl acrylate, octyl acrylate, decyl acrylate, dodecyl 
acrylate, hexadecyl acrylate, etc.; secondary and branched-chain alkyl 
esters, such as isopropyl acrylate, isobutyl acrylate, sec-butyl acrylate, 
2-methylbutyl acrylate, 3-methylbutyl acrylate, 2-ethylbutyl acrylate, 
1,3-dimethylbutyl acrylate, 2-ethylhexyl acrylate, 1-methylheptyl 
acrylate, etc.; esters of olefinic alcohols, such as allyl acrylate, 
2-methylallyl acrylate, 2-butenyl acrylate, 3,7-dimethyl-2,6-octadienyl 
acrylate, furfuryl acrylate, etc.; amino alkyl esters, such as 
2-(dimethylamino)ethyl acrylate, 2-(diethylamino)ethyl acrylate, 
2-(dibutylamino)ethyl acrylate, 2-morpholinoethyl acrylate, etc.; esters 
of ether alcohols, such as 2-methoxyethyl acrylate, 2-ethoxyethyl 
acrylate, 2-isopropoxyethyl acrylate, 2-butoxyethyl acrylate, 
2-(2-ethylhexoxy)ethyl acrylate, 2-phenoxyethyl acrylate, 2-benzyloxyethyl 
acrylate, 2-(2-methoxyethoxy)ethyl acrylate, 2-(2-ethoxyethoxy)-ethyl 
acrylate, 2-(2-phenoxyethoxy)ethyl acrylate, tetrahydrofurfuryl acrylate, 
etc.; cycloalkyl esters, such as cyclohexyl acrylate, 2-methylcyclohexyl 
acrylate, 3,3,5-trimethylcyclohexyl acrylate, 4-cyclohexylcyclohexyl 
acrylate, etc.; esters of halogenated alcohols such as 2-bromoethyl 
acrylate, 2-chloroethyl acrylate, 3-bromopropyl acrylate, 
2,3-dibromopropyl acrylate, 1-bromoisopropyl acrylate, 3-chloropropyl 
acrylate, 1,3-dichloroisopropyl acrylate, etc.; nitroalkyl esters, such as 
2-nitroethyl acrylate, 2-nitropropyl acrylate, 2-nitrobutyl acrylate, 
2-methyl-2-nitropropyl acrylate, 2,2-dinitropropyl acrylate, etc.; glycol 
diacrylates, such as ethylene glycol diacrylate, propylene glycol 
diacrylate, 1,3-propanediol diacrylate, 1,4-butanediol diacrylate, 
diethylene glycol diacrylate, triethylene glycol diacrylate, dipropylene 
glycol diacrylate, 2,5-hexanediol diacrylate, 2,2-diethyl-1,3-propanediol 
diacrylate, etc. 
The esters of methacrylic acid similarly are legion. Examples of such 
esters include methyl methacrylate, ethyl methacrylate, propyl 
methacrylate, isopropyl methacrylate, butyl methacrylate, isobutyl 
methacrylate, sec-butyl methacrylate, tert-butyl methacrylate, hexyl 
methacrylate, octyl methacrylate, isooctyl methacrylate, 2-ethylhexyl 
methacrylate, decyl methacrylate, vinyl methacrylate, allyl methacrylate, 
oleyl methacrylate, cyclohexyl methacrylate, 1-methylcyclohexyl 
methacrylate, 3,3,5-trimethylcyclohexyl methacrylate, phenyl methacrylate, 
benzyl methacrylate, 1,2-propanediol dimethacrylate, 1,3-butanediol 
dimethacrylate, 1,4-butanediol dimethacrylate, 2,5-dimethyl-1,6-hexanediol 
dimethacrylate, glycidyl methacrylate, 2,3-epoxybutyl methacrylate, 
2-hydroxyethyl methacrylate, methoxymethyl methacrylate, ethoxymethyl 
methacrylate, 2-ethoxyethoxymethyl methacrylate, benzyloxymethyl 
methacrylate, 1-ethoxyethyl methacrylate, tetrahydrofuryl methacrylate, 
dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate, 
tert-butylaminoethyl methacrylate, 3-diethylaminopropyl methacrylate, 
cyanomethyl methacrylate, 4-thiocyanatobutyl methacrylate, 
2-ethylenephosphitopropyl methacrylate, dimethylphosphinomethyl 
methacrylate, 2-cyanoethyl methacrylate, chloromethyl methacrylate, 
1,3-dichloro-2-propyl methacrylate, 4-bromophenyl methacrylate, 
2-bromoethyl methacrylate, 2,3-dibromopropyl methacrylate, 2-iodoethyl 
methacrylate, etc. 
Acrylate monomers which are preferred for use in the process of the present 
invention include acrylic acid, acrylic acid esters having 1-8 carbon 
atoms in the alcohol moiety thereof, methacrylic acid, methacrylic acid 
esters having 1-8 carbon atoms in the alcohol moiety thereof, and mixtures 
of the foregoing. Such compounds are represented by formula I below, 
wherein R represents hydrogen or methyl and R' represents hydrogen or an 
alcohol derivative having 1-8 carbon atoms. 
##STR1## 
Acrylate monomers represented by Formula I above which are especially 
preferred for use in the process of the present invention include acrylic 
acid, methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl 
acrylate, methacrylic acid, methyl methacrylate, ethyl methacrylate, butyl 
methacrylate, 2-ethylhexyl methacrylate, and mixtures of the foregoing. 
The acrylate monomer which is most preferred for use in the process of the 
present invention is methyl methacrylate. While portions of the present 
specification may refer specifically to methyl methacrylate as an 
illustrative member of this class of compounds, it is to be understood 
that this specification applies to all members of the described class of 
readily polymerizable acrylate monomers. 
As stated above, during the latter stages of current processes for the 
production of acrylate monomers, the crude monomer is typically subjected 
to distillation (usually at elevated temperatures and/or reduced 
pressures) in order to remove excess reactants and other impurities from 
the desired product. In accordance with the process of the present 
invention, phenyl-para-benzoquinone, diphenyl-para-benzoquinone, and 
mixtures thereof are employed as process inhibitors during the preparation 
of the monomer and especially during the distillation step, which is when 
polymerization is most likely to occur. 
The process inhibitor can be supplied to the process in a variety of ways. 
Preferably, a separate inhibitor stream is provided directly to each 
reaction vessel and/or distillation column in which polymerization is 
likely to occur. For example, in the production of methyl methacrylate by 
the acetone cyanohydrin process, acetone cyanohydrin and concentrated 
sulfuric acids are pumped into a cooled hydrolysis kettle to form the 
intermediate methacrylamide sulfate. The stream leaving the hydrolysis 
kettle is dehydrated and cooled, after which it goes into an 
esterification kettle where it is reacted continuously with methanol. An 
inhibitor is advantageously added to this esterification kettle, and also 
to the subsequent acid stripping column and rectifier column. Regardless 
of the manner in which the process inhibitor is provided to the reaction 
system, the inhibitor is typically removed from the system in the final 
distillation wherein crude monomer is taken overhead and the remaining 
impurities and the process inhibitor are taken off as bottoms from the 
column. Alternative and/or additional locations for the introduction of 
process inhibitor and adaptations to systems for the production of other 
acrylate monomers will be apparent to the person of ordinary skill in the 
art. 
The process inhibitor is provided to the reaction system in an amount which 
is sufficient to effect the inhibition of polymerization. Typically, the 
process inhibitor will be present in an amount of about 50 to 3,000 ppm, 
based upon the weight of process inhibitor per total weight of reactants 
and products present in the reaction system. Preferably, the process 
inhibitor is present in a concentration of about 250 to 2,000 ppm, with a 
concentration o about 500 to 1,000 (e.g., about 500) ppm being especially 
preferred. 
The process inhibitor can be provided to the acrylate monomer preparation 
process either directly or as a stock solution. Phenyl-para-benzoquinone, 
diphenyl-para-benzoquinone, and mixtures thereof are sufficiently soluble 
in suitable carrier solvents to allow the preparation of such a stock 
solution. Suitable carrier solvents include lower aliphatic alcohols, 
water, and, preferably, methyl methacrylate itself. In the preferred 
embodiment wherein methyl methacrylate is employed as the carrier solvent, 
the stock solution commonly comprises about 5 to 10% by weight of 
inhibitor in the solution. The use of stock solutions of process 
inhibitors is well known in the art and is a wide-spread practice. 
The invention will be further illustrated by the following example although 
it will be understood that this example is included merely for purposes of 
illustration and is not intended to limit the scope of the invention. 
EXAMPLE 
This example illustrates the improved inhibiting effect provided by 
phenyl-para-benzoquinone and 2,5-diphenyl-para-benzoquinone. 
In each run of the present example, the indicated process inhibitor was 
provided in a concentration of 500 ppm to methyl methacrylate monomer 
which had been distilled to remove storage inhibitor. 
The inhibitors which were employed were phenyl-para-benzoquinone, available 
commercially from Eastman Kodak Organic Chemicals; 
2,5-diphenyl-p-benzoquinone, available commercially from Eastman Kodak 
Organic Chemicals; p-benzoquinone, available commercially from Eastman 
Chemical Products, Inc.; and n-nitroso-phenyl-hydroxylamine, available 
commercially from Mallinckrodt. 
The inhibited methyl methacrylate was refluxed at 98.degree. C. for the 
indicated periods of time. At the end of each of the indicated periods of 
time, one milliliter of solution was removed, weighed, and contacted with 
methanol. Any polymer present in the solution sample was allowed to 
precipitate over a period of time of one hour. The solution was then 
centrifuged and filtered so as to remove any precipitated polymer. The 
precipitated polymer was dried and weighed. The results are given in Table 
I. In Table I, a Percent Polymer Formation of 0.00 indicates that a clear 
solution with no signs of polymer formation was obtained; a Percent 
Polymer Formation of &lt;0.02 signifies that a hazy solution was obtained, 
but that the Percent Polymer Formation was too low for detection (i.e., 
&lt;0.02%). 
TABLE I 
______________________________________ 
Percent Polymer Formation 
(Hours Refluxed) 
Inhibitor 2 4 8 24 32 48 
______________________________________ 
Phenyl-p-Benzo- 
0.00 &lt;0.02 0.02 0.11 0.19 0.24 
quinone 
2,5-Diphenyl-p-Benzo- 
&lt;0.02 0.04 0.09 0.32 0.33 -- 
quinone 
p-Benzoquinone 
0.20 0.25 0.35 1.2 1.3 -- 
n-Nitroso-Phenyl- 
0.00 0.00 0.00 0.33 0.50 1.3 
hydroxyl Amine 
______________________________________ 
The results of Table I indicate the superior effectiveness of 
phenyl-para-benzoquinone and 2,5-diphenyl-para-benzoquinone as process 
inhibitors for acrylate monomers, such as methyl methacrylate. The samples 
in which phenyl-para-benzoquinone was employed as a process inhibitor 
showed little, if any, polymer formation through eight hours of reflux. 
From eight hours to 48 hours, only minimal amounts of polymer were formed. 
Likewise, 2,5-diphenyl-p-benzoquinone showed minimal polymer formation 
through 32 hours of reflux. In contrast, a currently employed process 
inhibitor, p-benzoquinone, showed significant polymer formation even after 
two hours of reflux with a sharp increase in polymer formation observed at 
relatively longer reflux times. Also in contrast to the process inhibitors 
of the present invention, n-nitroso-phenyl-hydroxylamine, a proposed 
commercial inhibitor, demonstrated undesirable levels of polymer formation 
at 32 to 48 hours of reflux. 
Thus, the process inhibitors of the present invention demonstrate an 
advantageous effectiveness as polymerization inhibitors. In addition, the 
process inhibitors employed in the process of the present invention 
demonstrate significantly reduced environmental and toxic effects when 
compared to the proposed commercial inhibitor 
n-nitroso-phenyl-hydroxylamine. 
The process inhibitors of the present invention are therefore far more safe 
and effective than current commercially employed or proposed acrylate 
monomer process inhibitors. 
The invention has been described in detail with particular reference to 
preferred embodiments thereof, but it will be understood that variations 
and modifications can be effected within the spirit and scope of the 
invention.