Imidazole agent for retarding the polymerization of aziridine compounds

Imidazoles in dissolved form act as an agent for retarding the gelation of the polymerization of aziridine compounds initiated with sulfonium salts. The composition is particularly useful in dentistry applications, such as in the preparation of impressions and provisional denture parts.

In order to produce precise impressions, especially in dentistry, working 
models particularly for dental technology, and of temparary dental crowns 
and bridges, aziridine-containing substances are polymerized as described, 
for example in U.S. Pat. Nos. 3,453,242 and 4,093,555. As described in 
these patents, the aziridine-containing compounds are used together with 
suitable fillers, dyestuffs, and further adjuvants. 
For several applications, especially in dentistry and dental technology, it 
is significant that the processing period of the initiated mixture and the 
termination of the polymerization reaction can be determined within 
relatively narrow limits. This is the case, for example, in the 
preparation of a jaw impression. The processing period of the initiated 
mixture must be sufficiently long that the material may be placed on an 
impression tray and can then be brought into the patient's mouth. 
Thereafter, the polymerization should be terminated as soon as possible in 
order that the impression can be removed after a short time without 
undergoing any dimensional change with the required time being tolerable 
for the dentist and the patient. The situation is similar in the case of 
an intraoral preparation of temporary dental crowns and bridges, and also 
in the preparation of a jaw model by the dental technician. 
As disclosed in U.S. Pat. No. 4,167,618, it has been known to use as 
polymerization initiators for these aziridine compounds sulfonium salts 
containing in the .beta.-position to the central sulfur atom an 
electron-attracting group and a non-nucleophilic anion. 
The processing period (potlife) and setting period (curing time) of the 
mixtures can be influenced within relatively wide limits with the use of 
sulfonium salts as polymerization initiators owing to the chemical 
constitution of the sulfonium molecule. 
When a less active sulfonium initiator is employed, the available 
processing time will be sufficiently long, but setting will take place so 
slowly that when used for taking the impression, especially in the dental 
medicine area, an intolerable delay of curing of the rubber-elastic 
impression composition will occur. An active sulfonium salt effects rapid 
termination of the rubber formation so that an impression, for example, 
can be taken out of the mouth after a short time without any deformation 
of the impression occurring. However, then the processing period is so 
short that there is the risk, particularly at warm temperatures such as in 
the summer, that the impression will be introduced into the mouth when 
already in the reaction phase and thus when the composition already 
exhibits a certain resilience, or displacements in the gum region are to 
be expected. 
In the selection of the sulfonium salt, additional requirements have to be 
taken into account besides the desired temporal course of the 
polymerization including the availability of the starting components to 
attain the desired substitution, possibilities of a suitable control of 
the reaction, thermal and hydrolytic stability of the product, and the 
solubility of the sulfonium salt in the selected aziridine compound. 
Frequently, it is not possible to find a sulfonium salt that effects the 
desired course of the polymerization without major deviations. It is a 
further disadvantage that the course of the polymerization can be 
influenced only to a small extent by variations in the quantity of the 
sulfonium salt initiator. 
It has been known that amines can retard the polymerization of aziridine 
compounds. However, the common organic amines (e.g., tributylamine, 
benzylamine and triethanolamine) either do not show any effect whatsoever, 
or retard curing so much that they cannot be employed. 
Therefore, the problem underlying the invention is the provision of 
compounds that retard gelling of the polymerization of aziridine compounds 
initiated with sulfonium salts without substantially extending the curing 
period. 
In this regard, and in accordance with the present invention, it has been 
found that imidazoles prolong the processing period of aziridine compounds 
mixed with sulfonium initiators without substantially infuencing the 
curing thereof. 
Therefore, the invention is directed to a composition comprising at least 
one aziridine compound which is polymerizable with sulfonium salts, said 
composition containing at least one imidazole compound in dissolved form. 
The invention further relates to the use of imidazoles capable of 
dissolving in aziridine compounds as agents for retarding gelation of the 
polymerization of aziridine compounds initiated with sulfonium salts. 
Depending on the concentration of the imidazole derivative, the attainable 
retardation of curing ranges from about 15 seconds to several minutes. The 
addition of imidazoles to the aziridine-containing polymerizable 
compositions according to the invention permits the use of a highly 
reactive sulfonium initiator (preferably having a nitrile substituent 
located in the .beta.-position with respect to the central sulfur atom), 
thus simultaneously providing sufficient processing time and rapid curing 
of the composition. 
In principle, imidazole itself and all 1-substituted imidazole derivatives 
are suited for the purpose of the invention, provided they are soluble in 
the particular aziridine compound(s) employed. Imidazoles more sparingly 
soluble in the aziridine compounds employed may be dissolved by the 
addition of plasticizers such as phthalic acid esters, as solubilizers. 
Preferably, 1-substituted imidazoles are employed. 
For example, the imidazoles used according to the invention may have the 
general formula 
##STR1## 
wherein R represents: (a) C.sub.1 -C.sub.18 alkyl 
(b) substituted C.sub.1 -C.sub.18 alkyl 
(c) C.sub.3 -C.sub.12 cycloalkyl 
(d) substituted C.sub.3 -C.sub.12 cycloalkyl 
(e) C.sub.2 -C.sub.18 alkenyl 
(f) substituted C.sub.2 -C.sub.18 alkenyl 
(g) substituted phenyl 
(h) H 
The alkyl group in (a) and (b) preferably has from 1 to 12 carbon atoms, 
the cycloalkyl group in (c) and (d) has from 3 to 6, especially 5 or 6, 
carbon atoms, and the alkenyl group in (e) and (f) has from 2 to 12 carbon 
atoms, vinyl and allyl being preferred alkenyl groups. 
Substituents for the groups (b), (d), (f) and (g) include ester groups, 
e.g., C.sub.1 -C.sub.18 alkoxycarbonyl groups; amidocarbonyl groups, e.g., 
C.sub.1 -C.sub.18 alkylamidocarbonyl groups or aralkylamidocarbonyl groups 
such as benzylamidocarbonyl; acylamido groups, e.g., C.sub.2 -C.sub.18 
alkanoylamido or benzoylamido groups; acyloxy groups, e.g., C.sub.2 
-C.sub.18 alkanoyloxy or benzoyloxy groups; and ether groups, preferably 
with 1 to 18 carbon atoms, e.g., C.sub.1 -C.sub.18 alkoxy groups. Also, 
optionally substituted phenyl moieties and 1-imidazolyl groups are 
suitable as substituents for the (b), (d), (f) and (g) groups. 
Examples of suitable imidazole compounds are 1-methylimidazole, 
1-(n-butyl)imidazole, 1-decylimidazole, 1-laurylimidazole, 
1,.omega.-bis(1-imidazolyl)-C.sub.1 -C.sub.10 alkanes such as 
1,2-bis(1-imidazolyl)ethane and 1,10-bis(1-imidazolyl)decane, 
11-(1-imidazolyl)undecanoic acid benzylamide, 1-cyclohexylimidazole, 
1-benzylimidazole, 1-(2-ethoxyethyl)imidazole, 
1-(4-methoxyphenyl)imidazole, and 
1-[3-(2-ethylhexanoyl)amidopropyl]-imidazole. 
The imidazoles are employed in an amount of from about 0.01 to 3%, 
preferably 0.05 to 2% by weight, based on the weight of the aziridine 
compounds. 
The imidazoles used according to the invention are prepared by means of 
known processes described, for example, in J. Chem. Soc., 1963, 2197 and 
Helv. Chim. Acta, 42, 1845 (1959). 
The compounds disclosed in U.S. Pat. Nos. 3,453,242 and 4,093,555 can be 
employed as the aziridine compounds, e.g., polyethers with terminal 
aziridino groups and bisphenol-A derivatives with terminal aziridino 
groups. As sulfonium initiators, those disclosed in U.S. Pat. No. 
4,167,618 are suitable, the sulfonium salts containing nitrile or ester 
groups in the .beta.-position with respect to the sulfur atom being 
preferred. The amounts thereof range from about 1 to 8% by weight for 
aziridine compounds as described in U.S. Pat. No. 4,093,444; for other 
aziridine compounds the amount employed ranges from about 2 to 20% by 
weight, based on the weight of the aziridine compounds. 
The aziridine compounds mixed with the imidazoles used according to the 
invention are used for producing precise impressions and working models, 
and also primarily in dental impression compositions on the basis of 
polyethers having terminal aziridino groups. The preparation of such 
aziridino polyethers is described in U.S. Pat. No. 3,453,242. Fillers, 
plasticizers, dyestuffs, taste-improving additives and other customary 
dental adjuvants are added to said aziridino polyethers in the preparation 
of compositions used for impressions. 
Structurally related compounds, such as N-substituted pyrazoles and 
triazoles, do not exhibit the effect desirable for the purposes of the 
invention (that is, retardation of the beginning of gelling upon the 
addition of the sulfonium salt to the aziridine compound).

The following Examples are given merely as illustrative of the invention 
and are not to be considered as limiting. The periods of time stated in 
the Examples were measured from the beginning of mixing of the aziridino 
component with the sulfonium initiator. 
EXAMPLE 1 
1.0 Gram of a polyether having terminal aziridino groups and having an 
average molecular weight of about 6500 and the preparation of which is 
described in U.S. Pat. No. 3,453,242 (Example 13) is mixed with 4 mg of 
1-methylimidazole. This mixture is then homogeneously blended with 0.08 g 
of .beta.-(S-lauryl-S-ethylsulfonium)butyronitrile fluoroborate. After 3 
minutes gelling occurs, and after 5.5 minutes a nontacky, rubber-elastic 
composition has formed. 
COMATIVE EXAMPLE 1 
The process of Example 1 is repeated without the addition of the imidazole. 
At a room temperature of about 23.degree. C. gelling occurs after 1 minute 
and 50 seconds; after 4 minutes and 30 seconds, a nontacky, rubber-elastic 
composition has formed. 
EXAMPLE 2 
1.0 Gram of the aziridino polyether used in Example 1 is mixed with 8 mg of 
1-laurylimidazole. Thereafter, 0.16 g of a 1:1 mixture of diethylhexyl 
phthalate and the sulfonium salt mentioned in Example 1 are homogeneously 
incorporated therein. After 3 minutes the composition starts gelling, and 
after 5 minutes and 30 seconds it has hardened to a nontacky, 
rubber-elastic composition. 
EXAMPLE 3 
For the production of an impression composition for dental purposes, 800 g 
of the bifunctional aziridine compound mentioned in Example 1 are 
masticated with 150 g of fine kieselguhr; moreover, 3.2 g of 1-methyl 
imidazole are added thereto. 
30 g of the paste are mixed with 4.8 g of a 1:1 mixture of diethylhexyl 
phthalate and the sulfonium salt described in Example 1. The dentist has 3 
minutes in order to bring the impression material into the patient's mouth 
by means of a suitable tray. After 5 minutes and 30 seconds, the rubber 
formation is completed to such an extent that the impression can be taken 
out of the patient's mouth without any risk of undergoing deformation. 
EXAMPLE 4 
In 1.0 g of the aziridino polyether mentioned in Example 1, 0.5 mg of 
1-methyl imidazole are dissolved. Thereafter, 0.2 g of a 1:1 mixture of 
acetyl tributyl citrate and 2-ethylhexyl oxycarbonylmethyl ethylsulfonium 
propionic acid tetradecyl ester fluoroborate are homogeneously 
incorporated therein. Gelling occurs after 3 minutes; after 9 minutes a 
nontacky, rubber-elastic composition has formed. 
COMATIVE EXAMPLE 2 
The process of Example 4 is repeated without the addition of the imidazole. 
Gelling occurs after 2 minutes and 40 seconds; after 8 minutes a nontacky, 
rubber-elastic composition has formed. 
The invention being thus described, it will be obvious that the same may be 
varied in many ways. Such variations are not to be regarded as a departure 
from the spirit and scope of the invention, and all such modifications as 
would be obvious to one skilled in the art are intended to be included 
within the scope of the following claims.