OF A VINYL LACTAM AND A POLYMERIZABLE CARBOXYLIC ACID AND SOLUBILIZATION THEREOF

Copolymers of vinyl lactams, e.g. vinyl pyrrolidone or vinyl caprolactam and polymerizable carboxylic acids, e.g. acrylic acid or methacrylic acid, have found application in the photographic industry, as coatings, as biological membranes, in drug release systems, as preservatives, in oil recovery processes, in immunochemicals, and in cosmetic materials.

A solution polymerization process is used for making the copolymers when both reactant monomers and the copolymer product are soluble in a reaction solvent. However, this method suffers from the following disadvantages:

(1) Desirable compositions of the copolymer may not be soluble in a selected solvent; (2) the yields of such copolymer may be low; (3) the copolymer may be colored; (4) the solvent may be a high boiling liquid which is difficult to separate from the copolymer; and (5) the solvent may be a protic liquid, e.g. water or mixtures thereof, which causes considerable hydrolysis of the vinyl lacta under acidic reaction conditions.

The precipitation polymerization method is useful when the monomers are soluble in the reaction solvent and the copolymers are insoluble in the solvent. Benzene, tetrahydrofuran, heptane, acetone and methyl ethyl ketone are known precipitation polymerization solvents. Unfortunately, these solvents have one or more of the following disadvantages:

(1) Useful copolymer compositions may not be insoluble in a selected reaction solvent, which restricts the process to a narrow copolymer compositional range; (2) the copolymer may precipitate only as a gelatinous mass which is difficult to filter; (3) low yields of polymer may be obtained; (4) the solvent may be toxic; and (5) low molecular weight copolymers may be difficult to make by available processes.

For these and other reasons, present solution polymerization and precipitation polymerization processes for making low molecular weight copolymers of a vinyl lacta and an acrylic acid have not been very successful on a commercial scale.

What is described herein is a commercial precipitation polymerization process and copolymer products produced thereby. The polymerization is carried out on a reaction mixture of a vinyl lactam, e.g. vinyl pyrrolidone or vinyl caprolactam, and a polymeriz ^' able carboxylic acid, e.g. acrylic acid or ethacrylic acid, in the presence of a polymerization initiator, e.g. a free radical initiator, in a cosolvent mixture of an aliphatic hydrocarbon solvent, preferably, a C ₃ -C ₁₀ saturated, branched or unbranched, cyclic or acyclic, and, preferably is heptane or cyclohexane, and a secondary alcohol, preferably isopropanol, and, preferably, about 30% or less, most preferably 5-15%, by weight of the latter.

The process herein provides copolymers having a weight ratio of vinyl lactam to polymerizable carboxylic acid of 1:99 to 99:1, and as a white powder, which powder precipitates readily from the aliphatic hydrocarbon solvent and is easily filtered and dried.

The copolymers obtained have a low weight average molecular weight, Mw suitably less than about 20,000, as compared to the medium to high molecular weight copolymers made using the aliphatic hydrocarbon solvent alone.

Suitable vinyl lactams for use herein include vinyl pyrrolidone, vinyl caprolactam and alkylated vinyl derivatives thereof. Suitable poly erizable carboxylic acids include e.g. acrylic acid, methacrylic acid, itaconic acid, maleic acid, and crotonic acid. Acrylic acid itself, or methacrylic acid, is a preferred coreactant monomer in the polymerization. These monomers may be employed in weight ratios over the entire compositional range of the copolymers, i.e. from 1-99 weight percent vinyl lactam and 99:1 weight percent of acrylic acid. Accordingly, weight ratios of VP:AA in the copolymer of 99:1, 75:25, 50:50, 25:75 and 1:99, for example, may be conveniently prepared in this invention in substantially quantitative yields.

Other copolymerizable monomers may be included as additional coreactants, in an amount of 0-30% by weight of the copolymer. Suitable optional comonomers include acrylic acid esters, vinyl esters, acrylamide, N-alkylacrylamides, and the like.

The reaction solvent of the invention includes a ^c ₃ ~ ^c 1 ₀ saturated hydrocarbon which is branched or unbranched, cyclic or acyclic. Preferably the solvent includes a C ₅ -c ₈ aliphatic hydrocarbon or mixtures thereof.

A preferred aliphatic hydrocarbon solvent over other known precipitation polymerization solvents is selected from heptane and cyclohexane. Heptane, the most preferred solvent, provides high yields of a precipitate of the desired copolymer composition as a fine white powder which is easy to filter and dry. This advantageous result is surprising since vinyl pyrrolidone itself readily homopolymerizes in heptane to produce gummy products.

Suitable secondary alcohols for use as a cosolvent herein have the formula R ₁ R ₂ CHOH, where .^ and R ₂ are independently lower alkyl, preferably ^-Cg, and, most preferably R^_ ^anc ^ ^R 2 ^{are metn} yl/ i«e« isopropanol.

The amount of solvent used in the process of the invention should be sufficient to dissolve an appreciable amount of the reactants and to maintain the copolymer precipitate in a stirrable state at the end of the polymerization. Generally, up to about 40% solids, preferably 15-20% solids, is maintained in the reaction mixture.

The precipitation polymerization process of the invention is carried out in the presence of a polymerization initiator, preferably a free radical initiator, and most suitably, a peroxy ester, e.g. t-butylperoxy pivalate, although other free radical initiators such as acylperoxides, alkyl peroxides and azo-nitriles, known in the art or described in the aforementioned references, may be used as well.

The amount of such initiator may vary widely; generally about 0.2-5.0% is used, based on the weight of total monomers charged. Suitably, the initiator may be precharged or added during the polymerization run.

The reaction temperature may vary widely; generally the reactants are maintained at about 50°-150°C, preferably 60°-70°C. , during the polymerization. Pressure usually is kept at atmospheric pressure, although higher and lower pressures may be used as well.

The reaction mixture should be stirred vigorously under an inert atmosphere, e.g. nitrogen, during the polymerization. A stirring rate of about 400-600 rpm in a 1-liter lab reactor is quite adequate to effect the desired polymerization and to keep the precipitate in a stirrable state during the polymerization.

The precipitation polymerization process of the invention may be carried out by first precharging a suitable reactor with a predetermined amount of a vinyl lactam in the solvent mixture of aliphatic hydrocarbon solvent and secondary alcohol, and heating the solution to a desired reaction temperature while stirring vigorously under an inert gas atmosphere. The initiator is then charged into the reactor. Then a selected amount of the polymerizable carboxylic acid, e.g. acrylic acid, optionally with another comonomer, is admitted into the reactor over a period of time, generally about an hour or more. Then the reaction mixture is held for an additional period of time for polymerization to occur. Finally, the mixture is cooled to room temperature. Filtering, washing with solvent, and drying provides the copolymer in yields approaching quantitative, and, substantially, in a composition predetermined by the weight ratio of monomers introduced into the reactor.

Alternatively, the aliphatic hydrocarbon solvent and secondary alcohol can be precharged into the reactor, purged with nitrogen, heated to reaction temperature, the initiator added, and then separate streams of the vinyl lactam monomer, the acrylic acid monomer and optional comonomer are introduced over a period of time into the precharged reactor.

The polymers of the invention are generally characterized by their having high average molecular weights, low hygroscopicity, high glass transition temperatures, and exhibiting polyelectrolyte behavior in water.

EXAMPLE

In a two-liter, four-necked reaction kettle equipped with a condenser, a mechanical stirrer, two dip tubes connected to two metering pumps, a nitrogen purge adaptor, and a thermocouple connected to the temperature controller, and a cosolvent mixture of 1000 grams of heptane and suitable amounts of isopropanol were charged and the reactor was heated to 80°C. in 30 minutes with nitrogen purge throughout the whole process. The reactor temperature was held at 80°C. for 30 minutes. Then t-butylperoxypivalate (Lupersol 11) in the amounts given in Tables l, 2 and 3 were added into the reactor and N-vinylpyrrolidone and acrylic acid (in amounts given in Tables 1, 2 & 3) were fed into the reactor over a period of 4 hours. After completion of feeding of N-vinyl pyrrolidone and acrylic acid monomers, the reaction temperature was raised to 85°C. during 30 minutes and held for another 30 minutes. Thereafter 200 microliters of t-butylperoxy pivalate was charged 4 times every 2 hours and the reaction was kept stirring at 85°C. The reactor was cooled to room temperature and its contents were collected. The polymer was dried in a hood overnight, and, thereafter, further dried under vacuum oven at 90°C. overnight.

TABLE 1 - CONTINUED

VP/AA 75/25 HEPTANE/ISOPROPANOL

Mn** K-Value*** Yield %

4 16,300 4 12,300 4 16,500 4 11,200 4 17,200 4 13,300

* weight average molecular weight ** number average molecular weight *** Fikentscher K-value

•μ J -> ->

TABLE 2 - CONTINUED

VP/AA 50/50 HEPTANE/ISOPROPANOL

Feed Ti e(hr) MW Mn

4 10,000 4 9,600 4 11,000 4 13,500 4 20,000 4 9,700 4 9,100 4 14,500 4 8,800 4 13,500 4 10,700

TABLE 3 CONTINUED

VP/AA 25/75 HEPTANE/ISOPROPANOL

Water insoluble vinyl pyrrolidone/acrylic acid (VP/AA) copolymers, such as made herein, are important coating and textile sizing agents which are compatible with onovalent inorganic salts, plasticizers and many soluble gums and resins and which possess excellent moisture resistance. These copolymers are also useful in adhesive formulations and display good anti-deposition properties for reduced soiling of textiles. Additionally, hard, transparent films of these copolymers. can be cast for use in photographies or as water proof coatings on paper, metal, textile and plastic surfaces.

Notwithstanding these valuable properties, the utility of VP/AA copolymers has been somewhat limited due to complications encountered in coating operations which derive from their insolubility both in water and in alcohol. Previously, in order to form VP/AA copolymer solutions, it has been the practice to neutralize the copolymer, however, thus destroying its anionic property, which is responsible for hydrogen bonding and many of the above advantages, or to employ a high boiling solvent, such as dimethyl formamide, N-methyl pyrrolidone, dimethyl acetamide and others. However, such high boiling compounds are impractical for applications involving paper, textiles and many plastics which cannot tolerate the high temperature needed for solvent removal.

In accordance with another feature of this invention, there is provided a solubilizing composition for normally water insoluble copolymers of vinyl pyrrolidone and acrylic acid which comprises a mixture of water and an oxygen-containing solubilizing agent in a critical weight ratio of between about 5:1 and about 1:3, preferably between about 2:1 and about 1:1.5. The organic solubilizing agents which are contemplated for this

invention are oxygen-containing compounds having an oxygen to carbon ratio of from about 1:1 to about 0.1:1, preferably from about 1:1 to about 1:4. Suitable solvents include methanol, ethanol, isopropanol, acetone, ethylene glycol, methyl cellosolve and tetrahydrofuran and mixtures thereof. Of this group, the organic solvents or their mixtures which boil below 125°C. are more desireable and ethanol and isopropanol are most preferred.

The weight ratio of solvent to polymer can vary between about 20:1 and about 1:1, however a ratio between about 10:1 and about 4:1 is most desirable.

Solutions of the present copolymers are formed by agitating the polymeric product in the water/solvent mixture for a period of from about 0.5 to about 8 hours at a temperature of from about 20° to about 50°C. , preferably under ambient conditions until a desired amount of polymer is dissolved. In accordance with this invention stable polymer solutions containing up to 40 wt. % of dissolved polymer can be obtained.

The prepared solutions can be coated on a surface and cured at low temperature by techniques, without damage to the substrate. Further, the resulting anionic VP/AA polymeric coatings, which retain hydrogen bonding properties, have excellent adherence to the substrate and provide valuable protective coatings.

EXAMPLE

To 10 grams of VP/AA (75 parts/25 parts) copolymer having a weight average molecular weight of about 110,000, was added 90 grams of a 50/50 water-isopropanol mixture. The resulting mixture was shaken in a glass jar and stirred overnight at room temperature. A liquid solution was obtained with no detectable precipitate.

Other Solvent Mixtures VP/AA Copolymer Result

H ₂ 0/ethanol (60/40) 75/25 no precipitate

H ₂ 0/ethanol (40/60) 25/75 no precipitate

H ₂ 0/glycol (60/40) 75/25 no precipitate

H ₂ 0/methyl cellulose (50/50) 50/50 no precipitate