Inhibiting polymerization of ethylenically unsaturated monomers

Certain substituted diaryl amines are used to inhibit the polymerization of ethylenically unsaturated monomers; for example, unsaturated carboxylic acids and derivatives thereof.

This invention relates to the use of certain substituted diaryl amines to 
inhibit the polymerization of ethylenically unsaturated monomers. 
Accordingly, the present invention provides a method for inhibiting 
polymerization of a polymerizable ethylenically unsaturated monomer which 
comprises admixing therewith a polymerization inhibiting-effective amount 
of a compound or mixture of compounds of formula (1) 
EQU R--X--Ar.sub.1 --NH--Ar.sub.2 --Y--H (I) 
wherein 
Ar.sub.1 and Ar.sub.2, independently, are substituted or unsubstituted 
aromatic ring systems, 
X and Y, independently, are --NH--, --S-- or --O--, and 
R is hydrogen, substituted or unsubstituted alkyl, substituted or 
unsubstituted cycloalkyl, substituted or unsubstituted aryl, substituted 
or unsubstituted aralkyl or substituted or unsubstituted arylcycloalkyl. 
The present invention also provides novel compositions comprising a 
polymerizable ethylenically unsaturated monomer and a polymerization 
inhibiting-effective amount of a compound of formula (I). 
In the compounds of formula (I) Ar.sub.1 and Ar.sub.2 are, preferably, 
phenylene, naphthylene, anthrylene or phenanthrylene, which may be 
substituted or unsubstituted. More preferably, Ar.sub.1 and Ar.sub.2 are, 
independently, substituted or unsubstituted phenylene or naphthylene, most 
preferably substituted or unsubstituted phenylene. 
Any substituent on Ar.sub.1 and Ar.sub.2 is preferably selected from those 
which donate electron density to the aromatic nucleus to which they are 
attached. 
More preferably, any substituent on Ar.sub.1 or Ar.sub.2 is C.sub.1-10 
alkyl, phenyl, naphthyl, phenyl C.sub.1-10 alkyl, naphthyl C.sub.1-10 
alkyl, C.sub.1-10 alkoxy, hydroxy, C.sub.1-10 alkylamino, amino, thiol, 
C.sub.1-10 alkylthio, C.sub.5-10 cycloalkyl, C.sub.5-10 cycloalkyloxy or 
C.sub.5-10 cycloalkylamino. 
R is preferably hydrogen; substituted or unsubstituted C.sub.1-10 alkyl; 
substituted or unsubstituted C.sub.5-10 cycloalkyl; substituted or 
unsubstituted phenyl, naphthyl, anthryl or phenanthryl; substituted or 
unsubstituted phenyl C.sub.1-10 alkyl or naphthyl C.sub.1-10 alkyl; or 
substituted phenyl C.sub.5-10 cycloalkyl or naphthyl C.sub.5-10 
cycloalkyl; with substituted or unsubstituted phenyl or naphthyl, 
especially substituted or unsubstituted phenyl, being the more preferred 
aryl significances. 
Any substituent on R is preferably C.sub.1-10 alkyl, phenyl, naphthyl, 
phenyl C.sub.1-10 alkyl, naphthyl C.sub.1-10 alkyl, C.sub.1-10 alkoxy, 
hydroxy, C.sub.1-10 alkylamino, amino, thiol, C.sub.1-10 alkylthio, 
C.sub.5-10 cycloalkyl, C.sub.5-10 cycloalkyloxy or C.sub.5-10 
cycloalkylamino. 
Any alkyl or cycloalkyl moiety in R or in a substituent on Ar.sub.1 or 
Ar.sub.2, more preferably contains 1 to 4 and 5 to 8 carbon atoms, 
respectively. 
Preferably, Ar.sub.1, Ar.sub.2 and R are, independently, unsubstituted or 
mono- or disubstituted. 
X and Y are preferably, independently, --NH-- or --O--. More preferably at 
least one of X and Y is --NH--. 
Preferably, the groups R--X-- and H--Y-- are para or ortho, more preferably 
para, with respect to the group --NH-- in the center of formula (I). 
A preferred class of compounds of formula (I) is that represented by 
formula (I') 
EQU R--NH--Ar.sub.1 '--NH--Ar.sub.2 '--Y'--H (I') 
in which 
R is as defined above, 
Ar.sub.1 ' and Ar.sub.2 ' are, independently, substituted or unsubstituted 
phenylene or naphthylene, and 
Y' is --NH-- or --O--. 
A first more preferred subclass of compounds of formula (I') is of formula 
(I'a) 
EQU R.sub.a --NH--Ar.sub.1 '--NH--Ar.sub.2 '--Y'--H (I'a) 
in which Ar.sub.1 ', Ar.sub.2 ' and Y' are as defined above and R.sub.a is 
substituted or unsubstituted phenyl. 
A second more preferred subclass of compounds of formula (I') is of formula 
(I'b) 
EQU H.sub.2 N--Ar.sub.1 '--NH--Ar.sub.2 '--Y'--H (I'b) 
in which Ar.sub.1 ', Ar.sub.2 ' and Y' are as defined above. 
In each of formulae (I'a) and (I'b), Ar.sub.1 ' and Ar.sub.2 ' are 
preferably substituted or unsubstituted phenylene, more preferably mono- 
or di- C.sub.1-4 alkylphenylene or phenylene. 
The most preferred compounds of formula (I'a) are those of the formula 
(I"a). 
##STR1## 
wherein R.sub.1 is hydrogen or methyl, especially hydrogen. 
The most preferred compounds of formula (I'b) are those of formula (I"b) 
##STR2## 
in which R.sub.1 and Y' are as defined above, with Y' preferably being 
--NH--. 
The compounds of formula (I) are either known or can be produced by known 
processes from known starting materials. 
The ethylenically unsaturated monomers which can be treated in accordance 
with the present invention include, but are not limited to, unsaturated 
carboxylic acids and derivatives thereof, such as esters, anhydrides, 
amides and halides, as well as unsaturated nitriles. Also included are 
polymerizable vinyl compounds, e.g. vinyl halides, vinyl acetate, vinyl 
alcohol and aromatic vinyl compounds, such as styrene. 
Representative of the unsaturated carboxylic acids and derivatives thereof 
are C.sub.3-18 mono-, di- and tricarboxylic acids, such as acrylic, 
methacrylic, angelic, crotonic and oleic acids and their corresponding 
anhydrides, chlorides, amides and unsubstituted and substituted C.sub.1-18 
alkyl and alkenyl esters, such as methyl, ethyl, butyl, 2-ethylhexyl, 
lauryl, stearyl and dimethylaminoethyl esters, as well as acrylonitrile 
and methacrylonitrile. Preferred monomers of this type are acrylic and 
methacrylic acid, and the anhydrides, chlorides and C.sub.1-18 alkyl 
esters thereof, especially the esters. 
Representative of the vinyl compounds are vinyl chloride, vinyl acetate, 
vinyl alcohol, styrene, alphamethyl styrene, vinyl toluene and vinyl 
naphthalene, especially styrene. 
The compound of formula (I) may be mixed with the monomer at any stage 
where it is desired to inhibit polymerization, e.g. during manufacture, 
purification, transportation or storage. Of particular interest is the 
inhibition of polymerization during distillation of the monomer and for 
this purpose the compound of formula (I), or a mixture thereof, may be 
introduced into the distillation pot, the distillation column, the 
condensers or ancillary equipment. Usually it is mixed with the monomer in 
the distillation pot. 
It may also be advantageous to add some of the inhibitor of this invention 
to the monomer after completion of distillation in order to prevent 
polymerization during transportation and/or storage. 
The compound of formula (I) may be added directly to the monomer without 
any preliminary conditioning. Preferably, however, it is first dissolved 
in a suitable solvent which will not interfere in any subsequent treatment 
of the monomer. An especially suitable solvent is the monomer itself. 
However, other solvents may include components used in the preparation of 
the monomer, such as alcoholic or olefinic components, and other inert 
solvents appropriate to the processing step at which the application will 
occur. 
The amount of polymerization inhibitor will depend on the particular 
monomer and the conditions under which polymerization is to be inhibited. 
It will be well within the skill of the art to determine, without undue 
experimentation, what amounts are suitable for a given monomer and set of 
conditions, especially temperature. While, as indicated hereinafter, an 
amount of 10 parts per million (0.001%) based on the weight of the monomer 
has been found to be effective, it is contemplated that in normal useage 
higher concentrations will be employed. In general, the amount of compound 
of formula (I) can range from as little as 0.0005% by weight, based on the 
weight of the monomer, up to the solubility limit of the compound, e.g. 
about 20% in methyl methacrylate. The preferred range is about 0.001 to 
5%, more preferably 0.01 to 0.5%, by weight. 
When it is desired to polymerize the monomer, any of the inhibitor which is 
still in contact therewith can be separated by distillation or by other 
conventional methods, e.g. solvent extraction. On the other hand, small 
amounts of the inhibitor may not require removal but may instead be 
counteracted by the use of an effective amount of polymerization initiator 
.

The following describe testing which has been carried out for the purpose 
of evaluating the polymerization inhibition according to the present 
invention. 
Inhibition Testing 
The inhibitor to be evaluated was dissolved in freshly distilled 
(inhibitor-free) methyl methacrylate monomer to a concentration of 10 ppm 
(0.001%), by weight. Samples (10 ml each) of the resulting test solution 
were pipetted into individual test tubes which were then sparged with 
nitrogen to displace dissolved oxygen. Each sample was sealed under a 
nitrogen blanket with a closure containing a thermocouple enclosed in a 
glass sheath (containing silicon oil) which protruded into the test 
solution. A control sample was prepared in the identical manner, except 
that no inhibitor was added to the methyl methacrylate. 
The tubes containing the test solution and the control were then placed in 
a thermostatically controlled oil bath preheated to a test temperature of 
90.degree. C. The thermocouples were connected to an Esterline Angus 
datalogger programmed to collect and record time and temperature data at 
fifteen minute intervals. 
When polymerization occurred, the heat given off caused a rise in 
temperature which was sensed by the thermocouple and recorded for each 
test sample and the control. 
The foregoing procedurre was repeated for each inhibitor to be evaluated, a 
control being included with the testing of each inhibitor. 
From the recordings the times of occurrence of maximum temperature 
(exotherm) were averaged for each inhibitor and for the controls. The 
average times, or raw inhibition times (RIT), are given in Table 1 below. 
By subtracting the average RIT of the controls from the average RIT of 
each tested inhibitor the average inhibition time (IT) for each inhibitor 
was calculated and is also given in Table 1, wherein compounds (A), (B) 
and (C) are known inhibitors and compounds (1), (2), (3) and (4) are 
inhibitors according to the present invention. 
TABLE 1 
__________________________________________________________________________ 
Inhibitor Concentration 
RIT (hrs) 
IT (hrs) 
__________________________________________________________________________ 
Uninhibited (control) 0 12.2 0.0 
(A) 
Hydroquinone 10 ppm 34.2 21.9 
(B) 
##STR3## 10 ppm 37.1 24.9 
(C) 
diphenylamine 10 ppm 21.0 8.8 
(1) 
##STR4## 10 ppm 80.1 67.9 
(2) 
##STR5## 10 ppm 40.1 27.9 
(3) 
##STR6## 10 ppm 50.4 38.2 
(4) 
##STR7## 10 ppm 154.0 141.8 
__________________________________________________________________________ 
*Approximately 5%, by weight, being in oxidized form having the formula 
##STR8## 
Distillation Testing 
A 500 ml resin flask, equipped with a 5-plate Oldershaw column, a 
multi-ratio distillation head, a high efficiency brine-cooled condenser, a 
mechanical stirrer, a heating mantle slaved to a constant temperature 
controller and vacuum equipment was charged with 250 ml of pre-distilled 
(inhibitor-free) methyl methacrylate monomer and an amount of inhibitor 
(1) identified in Table 1 to give a concentration of 0.001%, by weight. 
The monomer was then distilled under vacuum at a temperature of 78.degree. 
C. and a reflux ratio of 5:1 until no further distillate collected in the 
receiver. A sample of the collected distillate was then assayed 
spectrophotometrically at 329 nm to an accuracy of about 0.2 ppm. 
The foregoing was repeated two more times with fresh charges of methyl 
methacrylate and the results were averaged, giving the following results: 
% Monomer recovered by distillation--92.4 
% Monomer lost to equipment--1.3 
% Pot residue--6.3 
Distillation Time (in minutes)--295 
% Inhibitor co-distilled--0 
Under otherwise identical conditions a sample of inhibitor-free methyl 
methacrylate showed polymerization after one hour to an extent requiring 
discontinuation of the distillation. 
The inhibitors according to this invention have been defined above by 
formulae which represent their reduced forms. However, as indicated for 
compound (1) in Table 1, many of these compounds may be present in minor 
proportions in their corresponding oxidized forms. It has been determined 
that for compound (1) the oxidized form, per se, does have polymerization 
inhibiting activity but that is lower than that of the form corresponding 
to formula (I). Accordingly, the amounts of compound of formula (I) 
specified herein are intended to include the optional presence of a minor 
proportion, up to 20% of the total weight of the inhibitor, of the 
oxidized form, i.e. of the formula (II) 
EQU R--X--Ar.sub.1 --N.dbd.Ar.sub.2 .dbd.Y (II) 
wherein Ar.sub.1, Ar.sub.2, R, X and Y are as defined above.