1. Field of the Invention
The invention relates to a process for the synthesis of mono and polyfunctional aliphatic propenyl ethers by an improved synthesis and rearrangement of allyl ethers. The invention also relates to novel mono and polyfunctional propenyl ethers that are available by the process of the invention and to compositions for preparing polymeric coatings containing the propenyl ethers.
2. Information Disclosure
Polymers of vinyl ethers are well known as coating materials, sealants, varnishes, adhesives and inks. A common method for the production of such polymers involves the cationic polymerization of vinyl ether monomers (I), according to equation 1.: ##STR1##
The commercial synthesis of vinyl ethers involves the base catalyzed condensation of alcohols with acetylene at high temperatures and under pressure in an autoclave as shown in equation 2.: ##STR2##
Although the starting materials--acetylene and alcohols--are economically attractive, the usual synthesis of the monomer is inconvenient and expensive to carry out. Since equation 2 describes an equilibrium process, it is difficult to achieve high conversions, particularly of di and multi-functional vinyl ethers. Furthermore, many side reactions, such as cyclizations, also occur with these starting materials substantially limiting the utility of this synthetic method. As a result, only a few difunctional vinyl ether monomers are currently available from commercial sources. Monomers that have the advantage of the high reactivity of vinyl ethers, but which are more easily prepared would be most desirable.
Propenyl ethers have been reported to undergo facile cationic polymerization. These monomers differ structurally from vinyl ethers only by the presence of a methyl group on the .beta.-carbon. The methyl group further activates the double bond towards cationic polymerization by increasing the electron density of the double bond but at the same time, slightly retards its reactivity by steric hindrance. Comparisons between the rates of cationic polymerization of vinyl and propenyl ethers have shown that, when the catalyst was boron trifluoride etherate, the propenyl ethers were more reactive than the vinyl ethers. Further investigation showed that the cis isomer was more reactive than the trans isomer in these cationic polymerizations. ##STR3## It thus appears that, if a good synthesis of the propenyl ether monomers (III) were available, an attractive alternative to vinyl ether polymerization would be provided.
Mizote et al. [J. Polym. Sci., Polym. Chem. Ed. 5, 1727 (1967)] disclosed the preparation of various monofunctional alkyl-substituted propenyl ethers by the sequence of reactions given in Scheme 1. ##STR4## In this scheme, propionaldehyde is condensed with an excess of the desired alcohol under acidic conditions to form the corresponding acetal. The acetal is then heated in the presence of p-toluenesulfonic acid to generate the desired propenyl ether with the elimination of one mole of the alcohol. Generally, the overall yields are in the range of 60-80% depending on the structure of the starting alcohol. When n-butanol is used, a mixture of 75% trans and 25% cis n-butylpropenyl ether is produced. If desired, the isomers can be separated and purified by fractional distillation.
While this method works well for low molecular weight propenyl ethers, complications due to side reactions make it less effective for the preparation of high molecular weight members of this series of monomers and particularly for multifunctional propenyl ethers. In fact, the literature is almost devoid of examples of multifunctional propenyl ethers. Only one reference can be found to the preparation of such compounds, and it discloses only polypropenyl ethers centered on an aromatic nucleus (U.S. Pat. No. 4,864,054). Compositions consisting of cationically photopolymerizable mono and multifunctional aliphatic propenyl ethers do not appear to have been described in the literature.
There is thus a need for a general synthesis of aliphatic polypropenyl ethers from readily available starting materials.