Process for the Production of Acrylonitrile-Propylene Copolymers

1. Field of the Invention

This invention relates to acrylonitrile-propylene copolymers. More specifically, this invention relates to a process for producing acrylonitrile-propylene copolymers that contain a major portion of polymerized acrylonitrile. This process is further characterized as employing a free radical generating polymerization initiator.

2. Description of the Prior Art

Free radical polymerization is a means of chain growth polymerization which employs as an initiator or intermediate a molecular fragment having one or more unpaired electrons. These molecular fragments are usually short lived and highly reactive. In free radical polymerization chain growth results when the radicals attack and react with a substrate and the radicals lost by this reaction are regenerated for further attack, reaction and combination with additional substrates.

The instant invention pertains to copolymers of acrylonitrile and propylene. Copolymers of acrylonitrile and various olefins are known in the art. For example, U.S. Pat. No. 3,227,697 teaches the preparation of acrylonitrile-ethylene copolymers and U.S. Pat. No. 3,652,731 describes copolymers of acrylonitrile with isobutylene. Typically, such copolymers are produced by mechanisms other than free radical polymerization. Copolymers produced by free radical techniques, typically do not contain ethylene or propylene because these monomers are much less reactive in free radical polymerization than the larger olefins.

A means for copolymerizing acrylonitrile and ethylene by free radical polymerization techniques has been developed. U.S. Pat. No. 3,791,603 describes the polymerization of ethylene with numerous comonomers including acrylonitrile using a free radical-forming polymerization catalyst. However, until the instant invention, techniques for an acrylonitrile-propylene copolymer by a free radical mechanism remained unknown.

An object of the instant invention is to provide a process for the polymerization of acrylonitrile and propylene using a free radical mechanism.

It is a further object of the instant invention to produce acrylonitrile-propylene polymers containing a major proportion of acrylonitrile by such a process.

A thermoplastic polymeric composition resulting from the polymerization in the presence of a free radical initiator

(A) a major portion of olefinically unsaturated nitrile monomers, and

(B) a minor portion of propylene wherein the molar ratio of the propylene to the olefinically unsaturated nitrile during polymerization is maintained at a ratio greater than the molar ratio of propylene to the olefinically unsaturated nitrile in the final polymeric composition.

Random copolymers of an olefinically unsaturated nitrile and propylene, containing a major proportion by weight of the nitrile are prepared by free radical polymerization at moderate pressures. Preferably these copolymers contain 65 to 95 percent by weight of the nitrile and more preferably these copolymers contain 75 to 85 percent by weight of the nitrile.

The olefinically unsaturated nitriles suitable for the present invention include the alpha,beta-unsaturated mononitriles of the formula: ##STR1## where R is hydrogen, a lower alkyl group having from 1 to 4 carbon atoms or a halogen. Such compounds include acrylonitrile, alpha-chloro acrylonitrile, alpha-flouro acrylonitrile, methacrylonitrile, ethylacrylonitrile and the like. The most preferred olefinically unsaturated nitriles for the present invention are acrylonitrile, methacrylonitrile and mixtures thereof.

The process of the present invention is unique in that it produces copolymers of olefinically unsaturated nitriles and propylene. However, equally suited for polymerization with the olefinically unsaturated nitriles by the process of the instant invention are other alpha-olefins such as ethylene, butylene, isobutylene and the like.

The polymerization of an olefinically-unsaturated nitrile and propylene to produce a random copolymer containing a major portion of the nitrile is achieved through the proper control of the comonomer ratios during the copolymerization reaction. Specifically, the molar ratio of the propylene monomer to the nitrile monomer in the reaction is maintained at a ratio greater than the molar ratio of propylene to the olefinically-unsaturated nitrile in the final copolymer. For example, the preferred level of propylene in the final copolymer is between 5 to 35 mole percent and more preferably 15-25 mole percent; in order achieve these compositions when using a continuous nitrile addition method in emulsion or suspension polymerization, it is preferred to begin the reaction with a propylene:nitrile molar ratio of between 3:1 and 6:1 in the reactor and to add the nitrile during the course of the polymerization such that the final propylene:nitrile molar ratio (unreacted) in the reactor is in the range of about 1:1 to 1.5:1. The actual comonomer ratios employed will vary depending upon the desired copolymer composition, whether batch or continuous monomer addition methods are utilized, and the solubility of the monomers in the polymerization system.

In a typical polymerization, olefinically unsaturated nitrile-propylene copolymers containing a major portion of the nitrile are produced when the propylene to nitrile molar ratio is in excess of 1:1. Preferably, the propylene to nitrile molar ratio at the start of polymerization is in excess of 1.1:1. More preferably this ratio is in excess of 2:1. Most preferably the propylene to nitrile molar ratio at the start of polymerization is between 3:1 and 5:1. However, in all cases by the end of the polymerization reaction, the molar ratio of propylene to the nitrile may be allowed to drop as low as 1:1.

A catalyst, i.e. a free radical initiator, is required for a satisfactory polymerization rate. The catalyst may be any of those commonly employed for the polymerization of nitriles with other monovinyl monomers including the azo initiators such as azobisisobutyronitrile, the various peroxygen compounds such as hydrogen peroxide, benzoyl peroxide, pelargonyl peroxide, cumene hydroperoxide, tertiary butyl hydroperoxide, tertiary butyl diperphthalate, tertiary butyl perbenzoate and others such as those disclosed in U.S. Pat. Nos. 2,471,959 and 2,491,471. The azonitrile compounds are preferred.

Also useful as polymerization initiators are the sodium, potassium and ammonium persulfates, the water-soluble oxidation-reduction or "redox" types of catalysts and the heavy metal activated, water-soluble peroxygen and redox catalysts. Included in this list are the water-soluble persulfates; the combination of one of the water-soluble peroxygen compounds such as potassium persulfate with a reducing substance such as a polyhydroxy phenol, an oxidizable sulphur compound such as sodium bisulfite, sodium sulfite and the like; the combination of a water-soluble peroxygen compound such as potassium persulfate and dimethylaminopropionitrile; the combination of a water-soluble peroxygen compound with a reducing sugar or with a combination of a dimercapto compound and a water-soluble ferricyanide compound and others. Heavy metal ions which greatly activate potassium persulfate and the redox catalyzed polymerizations include those of iron, cobalt, and others. The preferred range of catalyst as above defined, is from about 0.01 to 5 parts by weight per one-hundred parts by weight of monomers.

The copolymerization of olefinically unsaturated nitriles and propylene may be conducted in batch or continuous flow processes in solution, suspension or emulsion systems. Suitable emulsifiers include fatty acid soaps such as sodium laurate; organic sulfates and sulfonates such as sodium lauryl sulfate; the alkali metal salts of sulfonated petroleum or paraffinic oils; the sodium salts of aromatic sulfonic acids. such as the sodium salts of naphthalene sulfonic acids; the sodium salts of dodecane-1-sulfonic acid, octadecane-1-sulfonic acid, etc.; aralkyl sulfonates such as sodium isopropyl benzene sulfonate, sodium isobutyl naphthalene sulfonate, and alkali metal salts of polymerized alkyl naphthalene sulfonic acids; alkali metal and ammonium salts of polymerized alkyl naphthalene sulfonic acids; alkali metal and ammonium salts of sulfonated dicarboxylic acid esters and amides such as sodium dodecyl sulfosuccinate, sodium N-octadecyl sulfosuccinamate; the polyalkyl and polyalkaryl alkoxyalkylene phosphonate acids and salts more fully described in U.S. Pat. No. 2,853,471; the so-called cationic emulsifiers such as the salts of strong inorganic acids and organic bases containing long carbon chains, for instance, lauryl amine hydrochloride, the hydrochloride of diethylaminoethyl decylamine, trimethyl cetyl ammonium bromide, dodecyl trimethyl ammonium bromide, the diethyl cyclohexylamine salt of cetyl sulfonic ester and others may be used.

In addition to the above and other polar or ionic emulsifiers, still other materials which may be used, singly or in combination with one or more of the above types of emulsifiers include the so-called "nonionic" emulsifiers such as the polyether alcohols prepared by condensing ethylene or propylene oxide with higher alcohols, the fatty alkylamine condensates, the diglycol esters of lauric, oleic and stearic acids, and others. It is often desirable to add post-polymerization emulsifiers to the latices embodied herein for improved stability.

Although the polymerization may be carried out in the presence of air, the rate of reaction is ordinarily faster in the absence of oxygen and hence, the polymerization in an evacuated vessel, at reflux, or under an inert atmosphere such as nitrogen is preferred. The polymerization is performed at conventional temperatures of approximately 40.degree. to 85.degree. C., preferably approximately 60.degree. to 70.degree. C. The pressure at the beginning of polymerization depends upon the solubility of the propylene in the polymerization system and the polymerization temperature. For example, in solution polymerization at approximately 60.degree. C., the pressure may range from approximately 1 to approximately 6 atmospheres, while in suspension and emulsion polymerizations, the pressure may range as high as approximately 30 to approximately 50 atmospheres. Although the pH of the polymerization system is not critical, it is preferred that a pH of about 6 be employed during the polymerization reaction. The polymer latex resulting from emulsion systems may be adjusted to any desired pH.

In batch polymerization processes, one or more of the components, i.e., monomers, catalyst, activator, emulsifier, etc. may be added incrementally or continuously during the polymerization reaction. Typically in a batch polymerization process, continuous monomer addition is preferred, with the more active nitrile being added to the reactor to maintain the desired ratio of propylene to the nitrile. Continuous nitrile addition permits the more efficient utilization of the nitrile and the preparation of a more homogeneous product.

In order to better illustrate the instant invention the following examples are provided.

Clear, dry 8 oz. round screw cap bottles were charged with 50-100 ml. of solvent and purged for 10-15 minutes with nitrogen. The initiator, acrylonitrile, and a slight excess of liquid propylene were added. The excess propylene was allowed to evaporate until the desired weight was attained, and the bottles were then tightly capped and placed in a rotating constant temperature water bath for several hours. The unreacted propylene was then vented and the products were recovered by vacuum filtration. The polymers were dried at 50.degree. C. under reduced pressure for 24-48 hours.

Table I illustrates the results of several solution polymerizations for acrylonitrile and propylene in various solvents with selected initiators. The physical properties of several copolymers prepared in this manner are shown in Table II.

Distilled water was boiled to expel oxygen and the cooled to ambient temperature under a nitrogen sparge. While the water was still warm, a suspending aid was dissolved in the water. The cool water/suspending aid solution was then charged to a stainless steel pressure reactor, followed by about 30 wt. percent of the total acrylonitrile polymerized and polymerization modifiers. The reactor was then purged with nitrogen by pressurizing the reactor to approximately 100-200 psig and then exhausting the nitrogen several times. A nominal 10 psig nitrogen pressure was left in the reactor.

Propylene in amounts shown in Table III was transferred to the reactor from a sample bomb. The exact weight of the propylene was determined by the difference in weight of the sample cylinder before and after the transfer.

The initiator was dissolved in a small amount of a solvent, e.g., ethyl acetate, in a steel cylinder. The cylinder was pressurized to slightly above the reactor pressure with nitrogen, connected to the inlet valve of the reactor, and the initiator solution was then forced into the reactor. The reactor was then heated to the desired polymerization temperature.

The remainder of the acrylonitrile, and optional additional polymerization modifiers were pumped into the reactor at a rate of approximately 0.3 to 0.9 mole/hour. At the end of the monomer addition, the reactor was cooled to ambient temperature and the excess propylene vented. The polymer was recovered from the suspension by filtration, washed with methanol, and dried under reduced pressure at 50.degree.-60.degree. C. for at least 16 hours.

The results of several suspension copolymerizations are shown in Table III. Typical physical properties of such copolymers are given in Table IV.

Acrylonitrile/propylene emulsion copolymerizations are performed according to the procedure of Example 2, except that typical emulsifiers are used in place of the high molecular weight polymeric suspending aids. The resulting system is a latex, from which the copolymer product may be recovered by the standard coagulation methods known in the art, e.g., by diluting the latex with acidified alcohol, etc.

The results of several acrylonitrile/propylene emulsion copolymerizations are illustrated in Table V. The physical properties and optical properties of several of these polymers are listed in Tables VI and VII, respectively.

An acrylonitrile/propylene copolymer prepared according to Example 3 (13 mole percent propylene), 50 g., and 0.5 g. of dioctylphthalate were added over a period of 2 min. to the mixing chamber of Brabender Plasti-Corder. The components were mixed for 7 minutes at 230.degree. C. and 35 rpm. A 3.8 mil. film was compression molded from this compound and tested for gas permeation, with the following result:

Oxygen Transmission Rate: 0.087-0.099 cc./mil 100 in.sup.2 24 hr. atmosphere.

Water Vapor Transmission Rate: 2.1-1.9 g./mil. 100 in.sup.2 24 hr. atmosphere

Although only a few embodiments of this invention have been described above, it is to be appreciated that many additions or modifications can be made without departing from the spirit and scope of the invention. These and all other modifications are intended to be included within the scope of this invention, which is to be limited only be the following claims.