CROSS-REFERENCE TO RELATED PATENT APPLICATION

This application is a continuation-in-part of copending U.S. patent application, Serial No. 735,230, filed July 24, 1991, and assigned to the same assignee as herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to cationic polymerization of vinyl ethers without formation of highly colored materials, and, more particularly, to the use of defined polymerizable additive comonomers in predetermined amounts during such polymerization to effect the desired result without affecting the properties of the polymer.

2. Description of the Prior Art

Vinyl ether monomers can be cured by ultraviolet light, electron beam radiation, or thermal energy-induced cationic polymerization to produce useful coatings and films. However, the polymerized mixtures, and the polymers, coatings and films produced therefrom, typically contain undesired, highly colored materials which appear during the polymerization process. These colored products are believed to result from the formation of fully conjugated polyenes.

For example, the cationic polymerization of isobutyl vinyl ether (R = isobutyl) can produce the conjugated polyene IX by the following mechanism*:

* See Oshi a et al., Polymer Journal I i, No. 3, pp. 249-258 (1984)

In the first step of the polymerization, the propagating species III rearranges itself into oxonium ion V via the direct migration of the beta-proton to the penultimate ether oxygen or via beta-proton abstraction by the counter-anion (to give olefin IV) followed by protonation of the penultimate ether oxygen in IV. Intermediate V then can release an alcohol, ROH, resulting in formation of conjugated carbocation VI. The driving force of this dealcoholation is the presence in V of a good leaving alcohol, coupled with generation of a more delocalized (allylic) carbocation VI. Cation VI then

remains in eguilibrium with butadiene-molecule VII and, thereafter, intermediates can release additional alcohol molecules to produce a cation having extended conjugation. Successive dealcoholations from the propaging species III will lead to a fully conjugated polyene cation VIII which, via deprotonization, will produce polyene IX. Both VIII and IX are colored species.

This chain-transfer mechanism is unique for vinyl ethers which have a good leaving group, such as an alkoxy or aryloxy group, which can be eliminated readily in the acidic environment present during cationic polymerization.

The color produced as a result of such cationic polymerization of certain vinyl ethers can be reduced if the path by which the fully conjugated polyene termini in the polymers, and their stable protonated forms, can be inhibited, interrupted, or eliminated.

Accordingly, it is an object of the present invention to provide a composition and method for cationic polymerization of vinyl ethers which uses a polymerizable additive therein to inhibit, interrupt or eliminate the formation of undesired colored materials ordinarily formed during such polymerization.

SUMMARY OF THE INVENTION

What is provided herein is a method and composition for cationic polymerization of vinyl ethers without color formation. The reaction composition includes an additive comonomer which inhibits the formation of undesired colored materials during such polymerization. The additive used herein is an unsaturated comonomer which will copolymerize with the vinyl ether to provide a copolymer having spacer molecules of the additive therein.

The presence of the spacer molecules in the copolymer will preclude the formation of long chain conjugation in the copolymer which ordinarily would appear as undesired, colored compounds. To carry out this inhibiting function, the unsaturated comonomer additive includes a poorer leaving group therein than the leaving group of the vinyl ether. Suitable inhibitor additive comonomers include cyclic aliphatic epoxides cycloalkene oxides, unsaturated carboxylic acids and esters, styrene, styrene ethers and styrene oxide. In the presence of the additive, the resulting copolymer is observed to be clear, or only slightly colored.

The color-inhibiting additive used herein is effective in small amounts, for example, when present in an amount of less than 5% by weight of the composition. In these amounts, the desired colorless product has the same properties as the polymer obtained without the additive.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the present invention, there is provided herein a composition for polymerizing vinyl ethers cationically without forming highly colored materials in the resulting polymer, and, subsequently, in films and coatings thereof. Vinyl ethers which are amenable to the treatment of the present invention ordinarily are observed to form, such colored materials during polymerization. Typical vinyl ether in this category include alkyl vinyl ethers, such as methyl, ethyl, isobutyl, hexadecyl and dodecyl vinyl ethers; cyclic vinyl ethers; such as cyclohexyl vinyl ethers; vinyl ether prepolymers, such as vinyl ether esters and urethane vinyl ethers, optionally including reactive diluents, e.g. divinyl ethers such as triethylene glycol divinyl ether; cyclohexanedimethanol divinyl ether; hexanediol divinyl ether; tetrahydrofurfural vinyl ether; and propenyl ether of propylene carbonate; and the like.

The color-inhibiting additive in the composition of the present invention will copolymerize with the vinyl ether to provide a stable spacer molecular unit in the copolymer to preclude the formation of any fully conjugated polymeric species from polymerization of the vinyl ether. The stability of the spacer molecular unit in the copolymer obtained is enhanced by the presence in the additive of a poorer leaving group therein the corresponding leaving group in the vinyl ether. For example, a typical copolymer product of the cationic polymerization of an alkyl vinyl ether, e.g. cyclohexyl vinyl ether, and an epoxide additive, e.g. cyclohexene oxide, has the following formula:

!additive J j spacer , vinyl ■molecular* vinyl ether ι unity J ether

• !

-CH _P -CHJ-CH,-CH-O→CH -CH-

0 i R ' 0 I i ' I

^R ιi : ^R ι

where -OR- _j^ is the leaving group present in the alkyl vinyl ether, and -R is the corresponding poorer leaving group of the additive. The presence of the R group inhibits further dealcoholoations of the copolymer which would lead to full conjugation and, thereby, colored products.

Suitable cationic initiators for use in the cationic polymerization of the invention are described Crivello in U.S. Patent 4,161,405 such as aromatic onium salts of Group Via elements. Conventional acidic initiators such as BF ₃ OC ₂ H ₅ ; CF3SO3H;

CH3SO3H; p-toluene sulfonic acid and the like, also may be used, particularly when polymerization is effected by a solution polymerization process. Amine blocked acids also may be used where the cationic initiator is generated by heating.

EXAMPLES A-G

The vinyl ether monomer, predetermined amounts of a additive comonomer, and the cationic initiator components of the polymerization composition of the invention were mixed in a solvent until homogeneous and placed in a quartz tube. The composition was then exposed to UV light of sufficient intensity to form a polymer. The resulting appearance of the polymer solutions produced is summarized in Table 1 below.

TABLE 1

SAMPLES

CONTROL

COMPONENT B D E

Vinyl Ether Monomer

Cyclohexyl Vinyl Ether 0.5000 Additive Comonomer

Maleic Anhydride

Cyclohexene Oxide

Styrene

Styrene Oxide

% Additive 0

Cationic Photinitiator

(4-octadecyloxypheny1) phenyliodonium hexa- fluoroantimonate 0.0025 0.0031 0.0031 0.0031 0.0031 0.0030 0.0030 Solvent

Toluene 2.0000 2.4560 2.4452 2.4620 2.4408 2.4052 2.4248

Resulting Color Dark Red Clear SI. Yell Clear Clear SI. Yell. Clear

The results show that control Sample A, which contained no comonomer additive, turned dark red upon polymerization. The test samples B-G, however, containing an effective amount of about 1-3.8% of the inhibiting comonomer additive, were clear upon polymerization, or showed only slight coloration.

EXAMPLES H-J

The polymerization components given in Table 2 were blended at 50°C. until homogeneous. The resulting formulations were then coated onto a glass slide using a # 40 Mayer bar to give a coating thickness of approximately 3.5 mils. The coated slides exposed to UV light to effect the desired polymerization. The results are shown in Table 2 below.

TABLE 2

Control Samples

COMPONENT H I J K

Vinyl Ether

Urethane Vinyl Ether* 25.40 25.40 25.40 25.40

Reactive Diluent

*** FX-512 (3M) 0.90 0.90 0.90 0.90

Resulting Color Dark Brown Clear Brown SI.Brown

* Rx product of hydroxybutyl vinyl ether and Mondure B701 (isocyanate of trimethylol propane - Mobay) CHVE - 1,4-cyclohexane dimethanol divinyl ether PEPC - 4-propenyloxymethyl-l,3-2-dioxolane

** 3,4-epoxycyclohexylmethyl-3• ,4 •-epoxycyclohexane carboxylate

*** Triaryl sulfonium hexafluorophosphate salts in gamma-butyrolactone

As shown in Table 2, Control Sample H, which did not contain any inhibiting comonomer additive, produced a polymer film which was dark brown. In contrast, test Samples I-K, which included an inhibiting amount of the comonomer additive of 2-3.8% of the composition, showed a significant reduction in color formation. Sample I, which contain a diepoxide additive, produced a clear polymer film.

EXAMPLES L-N

The procedure of the above examples is followed using similarly effective amounts of the following styrene ethers: vinyl anisole (L) , isoeugenol (M) and anethol (N) as color inhibiting comonomers. Polymer solutions having substantially no coloration in the product are obtained.

While the invention has been described with particular reference to certain embodiments thereof, it will be understood that changes and modifications may be made which are within the skill of the art. Accordingly, it is intended to be bound only by the following claims, in which: