Dimer reduction

The dimer content in alpha, beta- ethylenically unsaturated ketones may be reduced by treating the ketone with a peroxide of the formula ##STR1## wherein R.sub.4 is a C.sub.1-6 branched or straight chain alkyl radical or a C.sub.6-10 aromatic radical and n is 1 or 2. The treated ketone has a faster rate of polymerization with other ethylenically unsaturated monomers.

FIELD OF THE INVENTION 
The present invention relates to a process for treating solutions of ketone 
monomers which contain an ethylenic unsaturation alpha, beta to the 
carbonyl group. More particularly the present invention relates to a 
process for reducing the dimer content in such ketones. 
BACKGROUND OF THE INVENTION 
Ketones with an ethylenic unsatuation alpha beta to the carbonyl group have 
a number of uses. The utility of these monomers is based on the activation 
of the carbonyl group by ultraviolet radiation. 
Such ketones may be used in photoresist applications such as making 
microcircuits. For example methyl isopropenyl ketone 
(3-methyl-3-butene-2-one) or MIPK is useful in photoresist applications as 
disclosed in: 
CA 98:82425c of Japanese Kokai 57,159,045; 
CA 100:219136r of Japanese Kokai 58,39,779., and 
CA 101:46307n of Japanese Kokai 58,52,634. 
Such ketone monomers are also useful in the manufacture of photodegradable 
plastics as disclosed in Canadian Patent 1,000,000 granted Nov. 16, 1976 
to James E. Guillet and Harvey Troth. 
Specific ketone monomers are useful in other applications. For example MIPK 
may be used in the production of isoprene as disclosed in U.S. Pat. No. 
3,009,005 issued Nov. 14, 1961. 
Unforunately, these ketones tend to form dimers. It is believed that the 
monomers undergo cyclodimerization. For example MIPK would be expected to 
form a 2,3-dihydropyran derivative. These dimers are an impurity and 
should be removed. Furthermore, in cases where the ketone is subjected to 
free radical polymerization, the dimers act as inhibitors. While it is 
possible to overcome the inhibiting effect of the dimer by using 
additional initiator this results in a lower molecular weight polymer 
which is undesirable and also more expensive. 
U.S. Pat. No. 2,851,497 teaches that such dimers may be converted to the 
monomer by treating the dimer in a vapour phase at a temperature of at 
least 375.degree., preferably 420.degree. to 520.degree. C. While this may 
be an effective way of reducing or removing dimer, it is energy and 
capital intensive, and possibly dangerous. 
The present invention seeks to provide a simple method to "reduce" such 
dimers. In accordance with the invention the dimer is not physically 
removed from the monomer but is believed to be converted to a different 
innocuous species. The decrease in dimer content may be monitored using 
conventional analytical procedures, such as chromatography. 
SUMMARY OF THE INVENTION 
The present invention provides a method for reducing the dimer content in a 
liquid mixture consisting essentially of: 
one or more monomers of the formula 
##STR2## 
wherein R.sub.1 is a hydrogen atom or a C.sub.1-4 alkyl radical; 
R.sub.2 is a hydrogen atom or a C.sub.1-4 alkyl radical; and 
R.sub.3 is a C.sub.1-4 alkyl radical or a C.sub.6-10 aromatic radical; 
and 
dimers thereof; 
which comprises contacting said mixture with at least one or more peroxides 
selected from the group consisting of: 
(i) benzoyl peroxide; and 
(ii) peroxides of the formula 
##STR3## 
wherein R.sub.4 is a C.sub.1-6 branched or straight chain alkyl radical 
or a C.sub.6-10 aromatic radical and n is 1 or 2, at a temperature and for 
a period of time so that the weight percent of a residual dimer in said 
mixture is reduced by at least 20 percent. 
The present invention also provides a process comprising homopolymerizing 
the treated ketone or copolymerizing the treated ketone with one or more 
copolymerizable monomers.

DETAILED DESCRIPTION 
The ketones which may be treated in accordance with the present invention 
are of the formula 
##STR4## 
wherein: R.sub.1 is a hydrogen atom or a C.sub.1-4 alkyl radical; 
R.sub.2 is a hydrogen atom or a C.sub.1-4 alkyl 
radical; and 
R.sub.3 is a C.sub.1-4 alkyl radical or a C.sub.6-10 aromatic radical. 
Some ketones which come within the above formula are methyl vinyl ketone 
(MVK), ethyl vinyl ketone, propyl vinyl ketone, isopropyl vinyl ketone, 
butyl vinyl ketone, methyl isopropenyl ketone (MIPK), ethyl isopropenyl 
ketone, phenyl vinyl ketone and phenyl isopropenyl ketone. Preferred 
ketones are methyl vinyl ketone and methyl isopropenyl ketone. Generally 
the ketones contain, in addition to the dimer, up to a maximum 4 to 5 
preferably less than 2 weight percent of other ingredients such as 
inhibitors and other ketones. On aging at a temperature above the freezing 
point of the ketone, preferably above about 0.degree. C. the dimer is 
formed. When received the dimer content may range anywhere from a minimum 
of 0.3 to up to 18 weight percent. Typically the dimer concentration is 
greater than 3 weight percent and thus should be reduced by at least 20 
percent based on the weight of dimer in the ketone before treatment 
preferably to as low as practically possible before use. 
The peroxide or peroxy esters useful in accordance with the present 
invention are benzoyl peroxide and peroxides of the formula: 
##STR5## 
wherein R.sub.4 is a C.sub.1-6 branched or straight chain alkyl radical or 
a C.sub.6-10 aromatic radical; and n is 1 or 2. 
Useful peroxides include benzoyl peroxide and 2,5-dimethyl-2,5-bis(benzoyl 
peroxy) hexane. A particularly useful peroxide is benzoyl peroxide. These 
peroxides may contain up to 20 or 25 weight percent water. Care should be 
taken if the peroxides are used at low temperatures to avoid phase 
separation and/or ice formation. 
In accordance with the present invention the peroxide is added to the 
ketone and the resulting mixture is aged at a temperature and for a time 
so that the residual dimer content in said ketone mixture is reduced by at 
least 20 percent. Preferable the residual dimer content in the ketone is 
reduced to 2 or more preferably 1 weight percent or less. Desirably the 
residual dimer content is reduced to less than 0.5 weight percent of said 
ketone. 
Typically the peroxide is used in an amount of at least 0.5 weight percent 
of the mixture being treated (e.g. ketone, comonomers, diluent). 
Preferably, the amount of peroxide used is from 0.5 to 8 weight percent of 
said mixture; most preferably from about 1 to about 4 weight percent of 
said mixture. As the reaction is between the dimer and the peroxide, the 
rate of reaction will depend on the concentration of dimer and peroxide. 
If the concentrations of dimer and/or peroxide become very low the rate of 
reaction becomes prohibitively slow. For mixtures of pure ketone 
containing up to about 18 percent dimer 2 percent peroxide based on the 
weight of the composition being treated is an effective peroxide 
concentration. If, for example the dimer concentration were reduced with 
another monomer or diluent a corresponding increase in the peroxide 
concentration would be required. A limiting amount of peroxide, is when 
the peroxide concentration is such that the subsequent polymerization of 
the ketone cannot be controlled or when there is a danger or 
polymerization during treatment. At typical dimer concentrations from 
about 2 to 18 percent of the ketone, the ketone concentration should be 
at least 25 weight percent, more preferably about 60, most preferably 
about 75 weight percent of the monomer mixture being treated. 
The temperature of the aging process may be from 0 to 45, preferably from 0 
to 21 (room temperature) .degree. C. The time of treatment may be from 0.5 
up to 50 hours; preferably less than 6 hours. Preferably the time of 
treatment is from about 3 to about 6 hours. The amount of peroxide, 
temperature and duration of treatment are all variables which will affect 
the degree of dimer removal from the ketone monomer. Appropriate 
conditions may be established by simple routine testing. It should be 
noted that the temperature may be varied during the treatment. For example 
the peroxide ketone mixture could be prepared at about room temperature 
(e.g. 20.degree.-25.degree. C.) and aged for 1 to 3 hours. Or the mixtures 
may be aged 1 hour, at room temperature (21.degree. C.) then cooled to 
0.degree. to 5.degree. C. and held for 3 to 6 hours. 
After the ketone is treated as described above, it may subsequently be 
polymerized. The ketone may be homopolymerized or it may be copolymerized 
with an ethylenically unsaturated monomer. Suitable copolymerizable 
monomers include C.sub.8-10 vinyl aromatic monomers which are 
unsubstituted or substituted by a C.sub.1-4 alkyl radical or a chlorine 
atom; C.sub.2-6 alkenyl nitriles; C.sub.3-6 ethylenically unsaturated 
carboxylic acids; C.sub.1-8 alkyl and hydroxyalkyl esters of C.sub.3-6 
ethylenically unsaturated carboxylic acids; and anhydrides of C.sub.3-6 
ethylenically unsaturated di-carboxylic acids. 
Useful vinyl aromatic monomers include styrene and alpha methylstyrene. 
Useful nitriles include acrylonitrile and methacrylonitrile. Useful 
unsaturated carboxylic acids include acrylic acid, methacrylic acid; 
fumaric acid and itaconic acid. Suitable esters of ethylenically 
unsaturated carboxylic acids include methyl acrylate, ethyl acrylate, 
butyl acrylate, methyl methacrylate, ethyl methacrylate, and hydroxyethyl 
acrylate. 
Suitable anhydrides of C.sub.3-6 ethylenically unsaturated di-carboxylic 
acids include maleic anhydride. 
The processes for polymerizing ethylenically unsaturated monomers are well 
known in the art and include thermal and free radical polymerization. The 
polymerization may be carried out in an aqueous emulsion in an organic 
solution, or in bulk. There is quite a comprehensive discussion of such 
polymerizations in the chapter "Polymerization of Alpha-beta-Unsaturated 
Carboxyl Compounds," (David M. Wiles) in the text Structure and Mechanism 
in Vinyl Polymerization T. Tsuruta and K. F. O'Driscoll, Marcel Dekker, 
Inc. New York, the text of which is herein incorporated by reference. It 
should be noted that the peroxides used to treat the ketones, may be also 
used to polymerize them. Thus, the ketones may be treated with all or a 
portion of the above disclosed peroxides and subsequently polymerized. It 
is also possible to treat the ketone with a mixture of one or more of the 
above peroxides and one or more additional non reactive peroxides such as 
t-butyl benzoyl peroxide or lauryl peroxide. Thus, it is possible to use 
mixed initiator systems to polymerize the ketone. 
The following examples are intented to illustrate, but not limit, the 
invention. In the examples unless otherwise specified parts are parts by 
weight (e.g. grams). 
EXAMPLE I 
Styrene monomer in a glass vial was free radically polymerized at 
88.degree. C. with 0.0;0.25;0.5., and 1 weight percent of methyl 
isopropenyl ketone dimer which did not contain any inhibitor. The 
initiator was a mixed initiator comprising per 100 parts by weight of 
monomer 0.24 parts by weight of benzoyl peroxide and 0.06 parts by weight 
of tertiary butyl perbenzoate. The conversion of the styrene at 6 hours 
was determined. (e.g. the vials were opened; cooled to stop polymerization 
and the solids contents determined). A plot was made of conversion against 
MIPK dimer concentration. The results are shown in FIG. 1. FIG. 1 shows 
that MIPK dimer acts as a strong free radical polymerization inhibitor. 
EXAMPLE II 
A series of polymerizations were made of a 
90:10 styrene/MIPK monomer and dimer mixture and a 
95:5 styrene/MIPK monomer and dimer mixture. 
In both cases the initiator was a mixture of benzoyl peroxide and tertiary 
butyl perbenzoate. The weight percent of initiator is set forth in Table 
I. 
TABLE 1 
______________________________________ 
S/MIPK S/MIPK 
95:5 90:10 
tert butyl tert butyl 
Benzoyl 
per Benzoyl per 
Peroxide 
benzoate Total Peroxide 
benzoate 
Total 
______________________________________ 
0.24 0.06 0.30 0.24 0.06 0.30 
0.36 0.09 0.45 0.40 0.10 0.50 
0.48 0.12 0.60 0.56 0.14 0.70 
0.60 0.15 0.75 0.72 0.18 0.90 
0.88 0.22 1.10 
1.04 0.26 1.30 
1.20 0.30 1.50 
1.36 0.34 1.70 
______________________________________ 
The monomer mixtures were polymerized in glass vials at 88.degree. C. At 6 
hours the polymerizations were cooled to stop polymerization and the 
solids content in the vial was determined. The solids content was the 
conversion at 6 hours. A plot was made of the conversion at 6 hours 
against the total initiator concentration. This plot is shown in FIG. 2. 
This data shows that 3 and 5 times as much initiator is required to obtain 
100 percent conversion in systems with 5 and 10 weight percent MIPK than 
in pure styrene. 
The data from FIG. 2 was used to calculate the initiator required for 100 
percent conversion at various amounts of MIPK (e.g. 0, 5, and 10) 
This data was plotted in FIG. 3. 
The MIPK used in the polymerization was analyzed and found to contain 8 
percent dimer. 
From this data the following empirical equation was derived. 
EQU I=0.015 [M.sub.2 ][D.sub.2 ]+0.282 
wherein 
I is the required initiator concentration to achieve about 100 percent 
conversion in 6 hours,: 
M.sub.2 is the weight percent (based on the total monomers) of MIPK in the 
monomer mixture; 
D.sub.2 is the weight percent of dimer in MIPK. 
From this equation the amount of initiator is not significantly increased 
for a polymer containing 5-10 weight percent MIPK if the dimer 
concentration is 2 weight percent or less. Obviously if the concentration 
of ketone, such as MIPK, used in the polymerization is greater then there 
should be a lower dimer content in the ketone. Thus, one should seek at 
least about a 20 percent reduction in dimer in the ketone in order obtain 
a reasonable rate of reaction without increased initiator concentration. 
Preferably the dimer concentration should be less than 2 desirably less 
than 1, most preferably less than 0.5 weight percent of ketone monomer. 
EXAMPLE IIIA 
A series of vial copolymerizations of 95:5 styrene/MIPK carried out. The 
MIPK contained about 8 percent dimer. In one case all the styrene/MIPK and 
initiator (Benzoyl peroxide) were aged for various times before 
polymerization at 88.degree. C. In the other cases mixtures of MIPK and 
part of the benzoyl peroxide initiator were aged then mixed with a 
comparably aged solution of styrene and remaining initiator immediately 
prior to polymerization. The aging was at 21.degree. C. for 1 hour then 
for the specified time at 5.degree. C. At 3 and 6 hours the solids of the 
polymerization system was determined. The data is set forth in Table II. 
For the different treatments conversion is plotted as a function of time 
in FIG. 4. 
TABLE II 
______________________________________ 
COPOLYMERIZATION OF STYRENE/MIPK AFTER 
VARIOUS PERIODS AND TYPES OF AGING. 
POLYMERIZATION 
RECIPES (1) (2) (3) (4) (5) (6) 
______________________________________ 
[A] Styrene 95.0 95.0 95.0 -- -- -- 
MIPK 5.0 5.0 5.0 -- -- -- 
Benzoyl Peroxide 
0.3 .03 0.3 -- -- -- 
[B] MIPK -- -- -- 5.0 5.0 5.0 
Benzoyl Peroxide 
-- -- -- 0.1 0.1 0.1 
[C] Styrene -- -- -- 95.0 95.0 95.0 
Benzoyl Peroxide 
-- -- -- 0.2 0.2 0.2 
Aging Time Before 
Polymerization 
At 21.degree. C. (hrs.) 
1.0 1.0 1.0 1.0 1.0 1.0 
At 5.degree. C. (hrs.) 
0.0 4.5 23.0 0.0 4.5 23.0 
Solids After 
Polymerization (wt. %) 
3 Hrs. 32.3 35.6 31.9 39.2 46.4 40.4 
6 Hrs. 47.4 47.7 46.4 74.3 84.7 87.0 
Polymerization Temperature: 
88.degree. C. 
Polymerization Time: 6.0 Hrs. 
______________________________________ 
The three solutions, [A], [B]and [C], were aged as shown above. 
The polymerization vials were then charged according to the above amounts. 
EXAMPLE IIIB 
A monomer mixture was prepared comprising: 
Styrene: 25 
MIPK: 75 (containing about 8 percent dimer) 
Benzoyl Peroxide: 1 
The mixture was divided into two samples. One sample was aged for 2 hours 
at 22.degree.. The second sample was aged 2 hours at 22.degree. C. and 21 
hours at 5.degree. C. The samples were polymerized at 85.degree. C. for 6 
hours. The 6 hour solids for the samples aged at 22.degree. C. only, was 
21.6%, the 6 hour solids for the sample at 22.degree. C. and 5.degree. C. 
was 36.8% 
From this data it can be seen that the process is most effective when aging 
the ketone in the presence of the peroxide and no further diluent or other 
monomers. 
EXAMPLE IV 
A sample MIPK and various amounts of benzoyl peroxide were aged for 1 hour 
at 22.degree. C. and various times at 3.degree. C. The samples were 
analyzed at various times at day 1, 2, and 3 for dimer content. 
The results are set forth in Table III. 
TABLE III 
______________________________________ 
THE EFFECT OF AGING MIPK/ 
BENZOYL PEROXIDE ON MIPK COMPOSITION 
Sample Number 1 2 3 
MIPK Lot 1051-1063 (Wt. %) 
100.00 99.0 98.0 
Benzoyl Peroxide (Wt. %) 
0.0 1.0 2.0 
______________________________________ 
Day 1 Results 
Time Aged 
22.degree. C. (Hrs.) 
1.0 1.0 1.0 
3.degree. C. (Hrs.) 
0.0 1.5 3.0 
MIPK (%) 84.5 83.3 85.0 
MIPK Dimer (%) 8.3 2.3 0.6 
MEK (%) 3.2 2.9 3.0 
EVK (%) 2.6 2.5 2.5 
Other (%) 1.4 9.0 8.9 
Day 2 Results 
Time Aged 
22.degree. C. (Hrs.) 
1.0 1.0 1.0 
3.degree. C. (Hrs.) 
24.0 26.5 29.0 
MIPK (%) 82.0 84.0 86.0 
MIPK Dimer (%) 10.9 0.4 0.2 
MEK (%) 2.8 3.1 3.0 
EVK (%) 2.5 2.5 2.5 
Other (%) 1.8 10.0 8.3 
Day 3 Results 
Time Aged 
22.degree. C. (Hrs.) 
1.0 1.0 1.0 
3.degree. C. (Hrs.) 
46.0 48.0 50.0 
MIPK (%) 82.1 85.6 84.1 
MIPK Dimer (%) 10.9 0.3 0.2 
MEK (%) 2.8 2.9 2.9 
EVK (%) 2.5 2.5 2.5 
Other (%) 1.7 8.7 10.3 
Solids (wt. %) 0.0 0.34 0.77 
______________________________________ 
These results show that at 1 or 2 percent benzoyl peroxide aging at about 
49-51 hours produces substantially similar reductions in dimer. 
Significant reductions in dimer content may be obtained in from 2.5 to 3 
hours at 3.degree. C. using 1 to 2 weight percent benzoyl peroxide (based 
on the MIPK). Clearly significant reductions in dimer level are achievable 
in accordance with the present invention. 
EXAMPLE V 
A series of polymerizations of 95 parts of styrene and 5 parts of various 
ketone monomers was carried out. In the A series the ketone monomer and 
0.1 parts of initiator were aged at 3.degree. C. This solution was mixed 
with a solution of styrene and 0.2 part of initiator, which had been aged 
at 3.degree. C. for the same amount of time. The two aged solutions were 
mixed immediately prior to polymerization, and polymerized. In the B 
series of experiments the ketone, styrene and initiator were mixed 
together and aged at 3.degree. C. for the specified period of time, then 
polymerized. The solids at various times was measured. Additionally the 
dimer content in the ketone was measured before and after aging, of the 
peroxide in the ketone monomer only. 
The ketone, initiator, aging time, 6 hour solids and dimer content are set 
forth in Table IV. 
EXAMPLE VI 
In a manner similar to example V mixtures of 90:5:5 Styrene:ACN:MIPK were 
polymerized. The initiator was Benzoyl peroxide. The samples were aged as 
described in Example V for 20 hours. The polymerization temperature was 
85.degree. C. The 6 hours solids was 100 percent for the sample prepared 
by aging the MIPK and 0.1 parts of peroxide. The 6 hour solids for aging 
the entire reaction mixture was 66.4 percent. The dimer content before 
aging with peroxide was 10.4 and 0.7 after aging. 
TABLE IV 
__________________________________________________________________________ 
A B A B A B A B A B A B A B 
__________________________________________________________________________ 
Initiator 
t-butyl perbenzoate 
0.3 
0.3 0.06 
0.06 
Lauroyl peroxide 0.3 
0.3 
Azobisobutyronitrile 0.3 
0.3 
t-butyl hydroperoxide 0.3 0.3 
benzoyl peroxide 0.24 
0.24 
0.3 
0.3 
0.3 
0.3 
ketone-MIPK 
X X X X X X X X X X X X 
Methyl vinyl ketone X X 
(MVK) 
Polymerization 
95 95 75 75 75 75 105 105 85 85 85 85 85 85 
Temp. .degree.C. 
Aging time (hrs.) 
19 19 19 19 19 19 21 21 19 19 20 20 20 20 
6 hrs. solids 
44.9 
44.2 
28.4 
29.2 
61.9 
60.2 
56.5 
51.8 
62.0 
45.0 
94.0 
50.6 
82.2 
65 
Dimer Content 
before aging 
10.4 
10.4 
10.4 
10.4 
10.4 
10.4 
10.4 
10.4 
10.4 
10.4 
10.4 
10.4 
5.8 
5.8 
after aging 
10.1 9.3 9.9 10.5 0.8 0.8 0.03 
__________________________________________________________________________ 
X = means the ketone is present. 
EXAMPLE VII 
In a manner similar to Example V a mixture of 95:10:5 styrene:methyl 
methacrylate:MIPK was polymerized. The initiator was benzoyl peroxide. The 
samples were aged as described in example V for 21 hours. The 
polymerization temperature was 85.degree. C. The 6 hours solids for the 
sample prepared by aging the MIPK and 0.1 parts benzoyl peroxide before 
polymerization was 93.4 percent. The 6 hours solids for the sample 
prepared by aging the entire reaction mixture before polymerization was 
57.4 percent. The initial dimer concentration was 10.4 percent and the 
dimer concentration in the aged solution of MIPK and benzoyl peroxide was 
0.7 percent. Examples V, VI and VII show that free radical initiators 
other than those which contain an aromatic nucleus do not "reduce" the 
dimer content in alpha beta ethylenically unsaturated ketones. The 
experiments also confirms the conclusion reached in Experiment III. 
Additionally, the experiments show that the process may be used with a 
number of copolymerizable monomers such as methyl methacrylate and 
acrylonitrile. 
EXAMPLES VIII 
A 2 weight percent solution of 2,5-dimethyl-2,5-bis(benzyl peroxy) hexane 
[Lupersol 118 (trademark)]was prepared in methyl isopropenyl ketone 
(MIPK). The sample was aged 20 hours at room temperature. The dimer 
content of MIPK at zero and at 20 hours was measured. At zero the dimer 
content was 10.4 percent. At 20 hours the dimer content was 7.7 percent. 
This shows about a 20 percent reduction in dimer content as a result of 
the treatment.