630 GIACOBBE, YOCUM,NYQUIST
Macromolecules
Syntheses of Monomers from Hydroxyethyl Acrylate and Methylolmelamine Which Polymerize by Low Doses of Radiation T. J. Giacobbe,* R. H. Yocum, and E. B. Nyquist The Dow Chemical Company, Midland, Michigan 48640. Received January I , 1971
ABSTRACT: A process has been developed for the syntheses of monomers from 2-hydroxyethyl acrylate (HEA), hexamethylolmelamine, and another alcohol (optional). Two of the monomers, tris(acryloyloxyethoxymethyl)tris(methoxymethy1)melamine and hexakis(acryloyloxyethoxymethyl)melamine, were found to polymerize to dry, hard films when initiated by very small doses (0.1 and 0.04 Mrad, respectively) of high-energy electrons (2 Mev).
T
he growing interest in systems which can be cured by high-energy radiation prompted us t o investigate monomers derived from melamine, formaldehyde, and hydroxyethyl acrylate (HEA). Radiation-initiated polymerizations have been the subject of numerous papers’s2 a n d several patents. 3-6 Interest in radiation-initiated polymerizations partially stems from the speed of the process, the fact that solvents (a source of air pollution) are often not required, and that it is a facile method for coating solids.
Preparation of the Monomers T h e preparation of new monomers which contained varying amounts of 2-hydroxyethyl acrylate residues attached t o a melamine nucleus was approached by two routes. T h e first was a n acid-catalyzed transetherification between hexakis(methoxymethy1)melamine and 2-hydroxyethyl acrylate. This transetherification procedure’ was successfully employed for preparation of the compound which contained one H E A residue, pentakis(methoxymethyl)acryloyloxyethoxymethylmelamine. However, the preparation of compounds by this procedure which contained three o r more H E A residues was unsuccessful. The reaction was very slow a t r o o m temperature a n d polymeric material was formed a t elevated temperatures. Both concentrated hydrochloric acid a n d p-toluenesulfonic acid were employed as acid catalysts. T h e second route to the desired monomers was a n acidcatalyzed etherification of hexamethylolmelamine. This route provided a n easy entry into a series of monomers which contained multiple H E A residues per melamine. The reactions were rapid (2-11 hr) a t r o o m temperature and the monomers were obtained in yields of 43-73 %. A comparison of the probable meclianisms of the two routes for monomer formation suggested that the acid-catalyzed etherification of hexamethylolmelanline (A) should be superior, since it was expected that water should be superior t o methanol as a leaving group (see Scheme I). The superiority of the second route was proven experimentally. Several acid catalysts (concentrated hydrochloric acid, picric acid, p-toluenesulfonic acid, and concentrated sulfuric acid) were used t o catalyze the etherification of hexamethyl(1) W. H. T. Davison, J . Oi! Colour Chem. Ass., 52,946 (1969). (2) W. Vanderbie, P. Spencer, and C. Swanholm, Paint Vurn. Prod., 59 (3), 39 (1969).
(3) W. J. Burlant, U. S. Patent 3,437,514 (1969); to Ford Motor Company. (4) P. Timmerman, U. S . Patent 3,467,488 (1969); t o Radiation Processing, Inc. (5) J. J. Brophy and R. R. Perron, U. S. Patent 2,668,133 (1954); to United Shoe Machinery Corp. (6) W. H. T. Davison, British Patent 949,191 (1964); to T. I. (Group Services), Ltd. (7) J. K. Magraiie and R. E. Layman, U. S . Patent 3,020,255 (1962); to American Cyanamid Co.
olmelamine with 2-hydroxyethyl acrylate. Concentrated sulfuric acid was found t o be a superior catalyst for the etherification. T h e mixed ethers containing both alkoxy and acryloyloxyethoxy groups could be synthesized i n a single-step procedure. This was accomplished by premixing the appropriate alcohol and H E A prior t o reactioh with hexamethylolmelamine. Table I lists the compounds prepared i n this manner. Hammond has reported the reaction of hydroxylalkyl acrylates a n d hexamethylolmelamine t o form monomers similar t o those discussed in this report.8 T h e relative nucleophilicities of methanol a n d H E A were determined by competitive reactions between hexamethylolmelamine, methanol, and H E A . It was then possible t o prepare a monomer coiitaining a predetermined number of the methoxy and acryloyloxyethoxy groups. T h e ratio of methanol t o H E A moieties on the methylmelamine nucleus could be approximated accofding t o the equation H E A residues/melamine
-
CHIOH residues/melamine mol of H E A charged X 0.6 mol of C H I O H charged
(1)
T h e substitution equation (1) was altered if the commercial grade H E A was not purified to remove acrylic acid. T h e relative reactivities of methyl, ethyl, n-propyl, and n-butyl alcohol i n the etherification of hexamethylolmelamine have been accurately determined, a n d similar results were obtained by Ahderson, Netzel, aiid T e s ~ a r i . ~ The monomers were Casily purified by dissolving in benzene, extracting with water, and concentrating the benzene solution. T h e excess alcohols (HEA and methanol o r butanol) were the major impurities a n d were extracted by water. T h e monomers prepared by the reaction of H E A , hexamethylolmelamine, and another alcohol (optional) were TABLEI MONOMERS PREPARED FROM HEXAMETHYLOLMELAM~NE (A) ~~~
_________ R_____ --.--~~
No. of alkoxy residues
No. of HEA residues
Methyl 0 3 3.4
2.6
4.2 3.3
1.8 2.7
Butyl
6 3
% yield 7j 61 59 43
(8) R . Hammond, Bkitish Patent 628,150(1949); toI.C.I., Lta. (9) D. G . Anderson, D. A. Netzel, and D. J. Tessari, J . Appl. Polym. Sci., 14,3021 (1970).
HYDROXYETHYL ACRYLATE A N D METHYLOLMELAMINE 631
Vol. 4 , No. 5 , September-October 1971 TABLE I1
RATIOOF NMRPEAK AREAS Compound
Ratio
Theory
Found
0.75
0.79
1.5
1.46
(3)’BIC
1
1
(4) AIC (511 AIB
0.75 1.5 0.5
0.76 1.65 0.46
(1) A / C
1
+ B)/B
(2) ( A
2
(6jBIC
SCHEME I
A
PROPOSED MECHANISM FOR THE FORMATION OF THE M ~ J O M E R ~
N(CHzOI;b)z NAN (ROCHA8
H+
-----t
ANAN(CH,OR),
A R = H or CH3 ‘R’ = CHzCHzOOCCH:=CH,
[-N-CHzOR I I @ d - N - C HI z @
]-
d - N = C 1H z
ROH
0
I
-N-cHzoR’
-k RoH
characterized by nuclear magnetic resonance (nmr) spectroscopy. Anderson, et d . , 9 also characterized their aliphatic melamine ethers by nmr spectroscopy. T h e nmr spectra of tris(acryloyloxyt:thoxymethyl)tris(methoxymethyl)melamine (1) and hexakis(acryloyloxyethoxymethy1)melamine (2) were selected as representative examples (see Figures 1 a n d 2). These nmr spectra bear some additional comments. Various ratios of the peak areas could be obtained which provided C;H,0CH,.N,CH,0CH2CH20CCH
li
CH30CH,
If O . 75, then diacrylate present If > 1.5, then contamination by HEA and/or methanol and/or diacrylate If < 1.5, then some polymer formation and/or incomplete etherification If > 1, then incomplete etherification If < 1, then diacrylate and/or HEA present Same as (1) Same as ( 2 ) If > O . 5 , then etherification not complete If
