Method of preventing the polymerization of liquid vinyl monomer
United States Patent Sato Apr. s, 1975 [75] Inventor: Ryozi Sato, Yokohama. Japan [73] Assignee: The Japanese Geon Company, Ltd., Tokyo, Japan [22] Filed: Mar. 30, 197i [2]] Appl. No.: 129,573  
 [30] Foreign Application Priority Data Apr. 1 1970 Japan 45-26970 [52] US. Cl 260/486 R [51] Int. Cl. C07c 69/54 [58] Field of Search 260/486 R [56] References Cited UNITED STATES PATENTS 1446984 8/1948 Rogers et al 260/486 FOREIGN PATENTS OR APPLICATIONS 567.130 l/l945 United Kingdom 260/486 R Primary Examiner-Lorraine A. Weinberger Assistant E.\&#39;aminer-Paul J. Killos Attorney. Agent, or Firm-Sherman &amp; Shalloway [57] ABSTRACT A method of preventing the polymerization of a liquid vinyl monomer selected from the group consisting of acrylic acid esters, methacrylic acid esters or mixtures thereof. which comprises adding to the monomer as a polymerization inhibitor, hexamethylenetetramine, in an amount sufficient to inhibit polymerization.  
 8 Claims, No Drawings METHOD OF PREVENTING THE POLYMERIZATION OF LIQUID VINYL MONOMER The present invention is concerned with the stabilization of acrylic acid esters, methacrylic acid esters or mixtures thereof, that is, a method of preventing the polymerization.  
  Acrylic acid esters and methacrylic acid esters (hereinafter referred to simply as monomer or monomers) are well known as very useful compounds. On the other hand, however, it is also known that these monomers are unstable compounds which are liable to cause the polymerization with ease by virtue of heat, light, peroxide, etc. Consequently, in the carrying out of the purification, storage and transportation of these monomers, it goes without saving that one should take measures to inhibit polymerization in order to remove possible troubles, such as clogging and contamination of apparatus caused by the precipitation of polymerized products followed by interruption of operation or loss of the compounds concerned.  
  Conventionally, polymerization inhibitors used in such cases include, for instance, hydroquinone, hydroquinone derivatives or phenol derivatives, etc. However there are drawbacks with these stabilizers in that they do not exhibit stabilization effects sufficiently when the aforesaid monomers are exposed particularly to elevated temperatures. Further, if these stabilizers are present in the polymerization system in the production of useful polymers by the polymerization of monomers, it gives rise to unfavorable effects such that the polymerization time is remarkably delayed, the catalyst is wasted in excess amounts as compared with the case under the usual polymerization conditions and reproducibility of the polymerization is lowered, causing unfavorable influences upon the polymerization yields, molecular weight distribution, etc., or polymers are colored.  
  Consequently, when polymerizing acrylic acid esters or methacrylic acid esters stabilized by the aforesaid conventional polymerization inhibitors, it was necessary to provide in advance a process for the removal of inhibitors in accordance with a method such as distillation, extraction and adsorption and disadvantages caused therefrom, e.g., increased cost of expenditure for the installation of equipment therefor and loss of monomers were not of negligible nature.  
  The object of the present invention is to provide an effective method for preventing the polymerization of acrylic acid esters or methacrylic acid esters, or mixtures thereof by the discovery of a stabilizer free from the aforesaid drawbacks.  
  Other objects of the present invention will be obvious from the statement given below.  
  In accordance with the present invention, in the case of adding hexamethylenetetramine to acrylic acid esters or methacrylic acid esters or mixtures thereof, monomers are sufficiently stabilized not only at low temperatures but also under such conditions that the monomers are exposed to elevated temperatures (for instance, about 50C., particularly 70 150C.) in the distillation and purification processes, etc, and further the polymerization of monomers is inhibited in the portion where monomers condensed from the gaseous phase are present, for instance, column top within the distillation tower, periphery of the cooler or even in the portion where the monomer mist stays. Moreover, despite such strong polymerization inhibiting effects, hexamethylenetetramine substantially does not exhibit the polymerization retarding operation such as is shown by hydroquinone even if it is present in the polymerization system in the case of polymerizing a monomer in the presence of a polymerization initiator. This is one of unexpected, surprising effects in the present invention. Further, hexamethylenetetramine very seldom colors monomers and polymers. Not only that, even when a monomer is present in the organic solvent solution, hexamethylenetetramine sufficiently prevents the polymerization of monomer. Examples of such organic solvents are aliphatic hydrocarbons with 5 17 carbon atoms; aromatic hydrocarbons such as benzene, toluene, xylene, ethylbenzene; ethers such as methylethyl ether, ethylbutylether, di-n-propyl ether; ketones such as methylisobutylketone, methylphenylketone, diethylketone, cyclohexanone; aliphatic alcohols with l-18 carbon atoms; esters such as the ethyl ester of propionic acid; or mixtures thereof.  
  Furthermore, depending upon the amount of hexamethylenetetramine added, environment of the monomer containing hexamethylenetetramine, is placed such as temperature conditions, time elapsed after addition thereof and type of the solvent present in conjunction with monomer, there are some cases where precipitation of the added hexamethylenetetramine is partially caused from the stabilization monomer solution and it is present in the form of an insoluble portion. Even in such cases, no troubles are caused in the polymerization inhibiting effects of hexamethylenetetramine and it has been observed that the stabilization of monomers can be rather promoted. Accordingly, in accordance with preferable embodiments of the present invention, by causing hexamethylenetetramine to be partially present in the form of the insoluble portion, monomer can be stored stably to the full extent. In the case of polymerizing such monomer, however, in advance of this, it is desirable to separate hexamethylenetetramine present as the insoluble portion. This separation can be accomplished without any difficulty following the usual separation operation of liquids and solids.  
  As the acrylic acid esters or methacrylic acid esters useful in the present invention, acrylic acid or methacrylic acid alkylesters having 1 18 carbon atoms in the alkyl group are suitable. Specific examples of such esters are methyl-, ethyl-, propyl-, butyl, n-octyl-, 2- ethylhexyl, lauryl-, cetyl-, stearyl-ester, etc. I  
  The amount of hexamethylenetetramine added can be suitably selected in conformity with the conditions under which acrylic acid esters or methacrylic acid esters are exposed. In general, it is 0.001 3.0 percent by weight, preferably 0.005 2.0 percent by weight based on the weight of the monomer. However, as aforesaid, in view of the fact that the presence of the insoluble portion does not cause disadvantages in the stabilization of monomer and brings about rather favorable results, hexamethylenetetramine may be added to the monomer in amounts exceeding 3.0 percent by weight.  
  Further, in order to enhance the stabilization effects, it is also possible to use the known inhibitors for use with unsaturated monomers in conjunction with hexamethylenetetramine appropriately in conformity with purposes.  
  The embodiments of the present invention will be explained in accordance with the following non-limitative Examples.  
 allowed to stand over a period of 24 hours at 100C. After standing the sample was colorless, transparent and no viscosity rise was observed.  
  Example 1 Example 4 Hexamethylenetetramine in the amounts indicated in 0.5 percent by weight of hexamethylenetetramine Table l aS add t g of Purlfied methyl methawas added to 10.0 g each of purified 2-ethylhexyl methcrylate and Sea ed n a glaSS mp and allowed to acrylate and lauryl methacrylate and allowed to stand stand in the constant temperature oil tank maintained over a i d f 24 hours at 100C i h h result h at 100C Table 1 Indicates the Condition of changes 10 each sample was colorless, transparent and no viscosity with the lapse of time in viscosity rise of methyl methai was b d. crylate. in the table, by solidification it is meant that Example 5 the entire sample has been solidified by the polymerization f monomer 0.2 percent by weight of hexamethylenetetramine The results of Table 1 clearly show that hexamethywas added to 8 each of Punfied methyl acrylate, lenetetramine sufficiently prevents the polymerization z&#39;ethylhexyl acrl&#39;late and lauryl acrylate and allowed to of monomers stand over a period of 24 hours at 100C. Each sample T bl l was colorless, transparent and no viscosity rise was oba 6 served.  
  Added amounts Example 6 of hexame- Exp thylenetetmmine 0.05 percent by weight (based on the solution) of Na (weighh ppm) viscosity rise hexamethylenetetramine was added to a xylene solution and a light oil solution, each containing 30 percent 1 &#39;g s g ifig gzf fg by weight of purified methyl methacrylate, and allowed observed in 1.5 hrs. to stand for 24 hours at a temperature of 100C. Each 3 Vismsily slighlly 6 hrs of samples was colorless transparent and no viscosit and solidification was observed y in 2 hm rise was observed. 3 50 No viscosity rise was observed 7 hrs later and solidification Example 7 was observed after 22 hrs. 4 m0 viscosity rise was observed after 200 g of purified methyl methacrylate and 0.1 per- 20 hours. cent by weight of hexamethylenetetramine were charged into a three neck distillation flask provided XAMPLE 2 with a glass made spherical type cooler and present in conjunction with a cylindrical carbon steel material (in Compounds i he amount i di ated in T bl 2 w view of practical environmental conditions in industrial added to 10.0 g of purified methyl methacrylate and appara u and heated to a mp r r of about the sample thus obtained was allowed to stand for 24 0C 0 polymer Was Obser ed hen the Surface of hours under the same conditions as in Example l. Then the internal walls of the cooler was examined 8 hours after observing the appearance of the sample and vislater. Further, monomer in the flask was also colorless, cosity rise, a part of the sample was put into 10 times transparent and no viscosity rise was observed. On the the volume of the sample of normal hexane to check other hand, when the sample experiment was repeated whether polymer was formed or not. In the table Experby adding 0.1 percent by weight of hydroquinone, a iment Nos. 3 10 are Comparative Examples. thin film-like polymer was observed on the internal wall The results in Table 2 clearly show that of the many surface at the lower part of the cooler and it was also compounds hexamethylenetetramine exhibits outobserved that viscosity of monomer in the flask had standing effects as a stabilizer for the monomer. risen.  
 Table 2 Experiment Added amounts Condition of samples after No. (/2 by weight) 24 hours later Polymer in normal hexane l hexamethylenetetramine 0.] colorless. transparent. no Very small amounts of polymer were formed.  
  viscosity rise 2 hexamethylenctetramine. 2.0 colorless. transparent. no None of polymer were formed.  
  (with the insoluble portion) viscosity rise 3 no addition solidified in 2 hours 4 hydroquinone. 1.0 light brown. transparent. rise Large amounts of polymer formed.  
  was observed 5 chloranil. 0.] became viscous in l8 hours* Large amounts of polymer formed. 6 phenol. (Ll same as above Large amounts of polymer formed. 7 cresol. 0.1 solidification 8 bisphenol. l).l viscosity rise was observed Large amounts of polymer formed. 9 phenothiazine. 0.| viscosity rise was observed Large amounts of&#39;polymer formed.  
 diethylhydroxylaminc. 0.]  
 Viscosity rise was observed Large amounts of polymer formed.  
 solidification had not been finally reached. but the sample had become very viscous.  
 Example 3 Hexamethylenetetramine (1.0 percent by weight) was added to 10.0 g of purified butyl methacrylate and Example 8 ture of percent by weight to methyl methacrylate and 90 percent by weight of normal hexane, sample (B) prepared by adding thereto 0.01 percent by weight of hydroquinone and sample (C) with no addition was polymerized in the presence of benzoyl peroxide as a catalyst at a temperature of 60C the polymerization induction periods of(A) and (C) were almost equivalent and the polymerization yields after a lapse of hours were 42 percent in (A), almost 0 percent in (B) and 46 percent in (C). From this result it can be observed that hexamethylenetetramine shows no substantial polymerization retarding effects with regard to the polymerization of monomers use of a polymerization initiator.  
 Example 9 EXAMPLE 10 Each of solidified product of sample (A) in Example 9, solidified product obtained by allowing sample (B) to stand further and solidified product of sample (C) was dissolved in the ratio of 5.0 g per 100 cc of dimethylformamide, and absorbance was measured at 430 mp. with the result that it was 2 X 10 with regard to the solidified product of (A), 9 X 10 with the solidified product of(B) and O with the solidified product of (C).  
 This result indicates that in case of hexamethylenetetramine polymerized products are less colored than in the case with hydroquinone.  
 I claim:  
  1. A method of preventing the polymerization of a liquid vinyl monomer selected from the group consisting of acrylic acid esters, methacrylic acid esters or mixtures thereof in the absence of a polymerization catalyst while not substantially retarding catalyzed polymerization, which comprises adding to said monomer, as a polymerization inhibitor, hexamethylenetetramine in an amount sufficient to inhibit premature polymerization.  
  2. The method of claim 1, wherein said vinyl monomer is present in an organic solvent solution. I  
  3. The method of claim 1, wherein said vinyl monomer is exposed to elevated temperatures.  
  4. The method of claim 1, wherein hexamethylenetetramine is added in an amount of at least 0.001 percent by weight based on the weight of said vinyl monomer.  
  5. The method of claim 1, wherein hexamethylenetetramine is added in an amount of at least 0.005 percent by weight based on the weight of said vinyl monomer.  
  6. The method of claim I, wherein hexamethylenetetramine is added in an amount of 0.001 3.0 percent by weight based on the weight of said vinyl monomer.  
  7. The method of claim 2, wherein said organic solvent is selected from the group consisting of aliphatic hydrocarbons, aromatic hydrocarbons, ketones, esters, ethers, alcohols and mixtures thereof.  
  8. The method of claim 1, wherein said acid ester is an alkyl acid ester having 1 8 carbon atoms in the alkyl group.