CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a continuation-in-part of pending U.S. patent application, Serial No. 726,415, filed July 5, 1991, and assigned to the same assignee as herein.

In one aspect, the invention relates to a toxin free copolymer of maleic anhydride and alkyl vinyl ether having a predetermined specific viscosity within the range of from as low as 1 up to 5.5.

In another aspect the invention relates to a process capable of producing maleic anhydride/methyl vinyl ether copolymers having number average molecular weights greater than 1,000,000 in the presence of a solvent having a relatively high chain transfer constant. The invention also relates to terpolymers of maleic anhydride and mixtures of lower alkyl vinyl ethers of high viscosity.

DISCUSSION OF THE PRIOR ART

Various commercial processes are available for making copolymers of maleic anhydride and a lower alkyl vinyl ether utilizing different solvents and reaction conditions to achieve copolymers of dissimilar specific viscosities, or molecular weights. It is known that the molecular weight of the copolymer can be controlled in part by adjusting the temperature of the polymerization, the concentration of the polymerization initiator and/or the particular species of solvent utilized in the polymerization reaction. However, it is generally believed that the purpose of the solvent used is to combine with the free radicals produced by the initiator, which effect leads to early termination of the reaction. The degree of

solvent interference in the polymerization can be determined from its chain transfer constant; thus, solvents having a low chain transfer constant, such as benzene and chlorinated aliphatic hydrocarbons, exhibit only a relatively slight tendency to interfere with the chain reaction type of polymerization; whereas solvents with a significantly higher chain transfer constant such as toluene, xylene, ethyl benzene, are believed to exert a notable higher degree of interference and chain termination in such polymerizations. Thus, for a given maleic anhydride vinyl ether polymerization, it has been accepted that, a low chain transfer constant solvent produces higher specific viscosity (higher molecular weight) copolymeric products, e.g. up to 3.5, in contrast to higher chain transfer constant solvents which provide lower viscosity products.

Several processes of the prior art, in order to avoid the use of toxic benzene, have experimented with alternative solvents. For example, toluene having a higher chain transfer constant has been substituted to dissolve the maleic anhydride monomer in the feed charge introduced to the reactor where it contacts comonomer in the liquid state for better distribution. However, this and similar processes have not been able to achieve copolymeric products of number average molecular weights above about 50,000; consequently, the resulting maleic anhydride/vinyl ether copolymers have limited application and are not suitable for use as adhesives, pastes, medicinal thickeners, etc.

Accordingly, it is an object of this invention to overcome the above process deficiencies and to provide unique toxin-free maleic anhydride/lower alkyl vinyl ether polymers having molecular weights up to about 2,000,000 or more.

Another object of this invention is to provide a general process using a solvent having a high chain transfer constant in such a manner as to avoid interference with the polymerization reaction and to permit long polymer chain development.

Still another object is to produce a benzene-free maleic anhydride/methyl vinyl ether copolymer having an individual predetermined specific viscosity falling within the range of between about l and about 5.5.

Another object is to provide a general process using the same high chain transfer solvent to achieve polymers of predetermined specific viscosity within the range of from 1 to 5.5.

Yet another object of this invention is to produce a toxin free maleic anhydride/vinyl ether copolymer which is suitable for adhesives and use in packaging of comestible products and pharmaceutical creams, pastes and the like.

These and other objects of this invention will become apparent from the following description and disclosure.

THE INVENTION

In accordance with this invention there is provided a process for the preparation of a toxin-free maleic anhydride/lower alkyl vinyl ether copolymer having a specific viscosity within the range of from about 1.0 to 5.5 which comprises (a) precharging to a reaction zone a cosolvent mixture of a lower alkyl substituted benzene, a liquid C ₆ to C ₈ alkane, a saturated lower alkyl ether or mixture thereof as the solvent and a controlled amount of a lower alkyl vinyl ether or mixture of said lower alkyl vinyl ethers as the cosolvent; (b) premixing maleic anhydride monomer and an excess of a lower alkyl vinyl ether monomer reactant external of said reaction zone at

below reaction temperature to form a polymerizable complex of said monomers; (c) feeding to said precharged reactor the premixed complexed monomers at a controlled feed rate over an extended period of several hours; (d) gradually introducing into said reaction zone an initiating amount of a free radical initiator dissolved in said lower alkyl substituted benzene or said liquid C _g to C ₈ alkane or mixture thereof; (e) polymerizing the complexed monomeric mixture at an elevated temperature and pressure while thoroughly mixing said complex with the gradually introduced initiator solution; (f) forming a slurry of the resulting polymeric product in said solvent and (g) recovering said polymer from said solvent as the product of the process. This process can be operated in a batch or a continuous manner as will become apparent from the following disclosure.

Suitable cosolvent systems employed in the present process include as a solvent individual compounds or mixtures of a lower alkyl substituted aromatic compound having a relatively high chain transfer constant, such as toluene, xylene, ethyl benzene, etc., a liquid C ₆ to C ₈ alkane such as heptane, cyclohexane, hexane, etc., and a saturated lower alkyl ether such as methyl t-butyl ether, diisopropyl ether, dibutyl ether, methyl butyl ether, etc. in admixture with a controlled amount of a cosolvent which is the same or a different lower alkyl vinyl ether as the comonomer in the polymerization reaction or mixtures thereof. Generally, the weight ratio of the solvent to cosolvent in the precharged solvent mixture can vary between about 4:1 and about 1:12, depending upon the product specific viscosity desired. It has been found that the specific viscosity and molecular weight of the resulting polymer varies directly with the amount of alkyl vinyl ether employed in the solvent system. Hence, solvent mixtures of 4:1-1:1 will produce polymers having

viscosities of from about 1 to about 3; whereas mixtures of 1:1.4-1:12 will produce polymers having viscosities up to 5.5 which indicates a number average molecular weight of 1,500,000-2,000,000 or more, heretofore unachievable.

In the above process, suitable lower alkyl vinyl ether comonomer reactants include ethyl vinyl ether, isopropyl vinyl ether, n-butyl vinyl ether, iso-butyl vinyl ether, tertiary butyl vinyl ether and methyl vinyl ether; the latter being preferred.

At least a slight excess of the lower alkyl vinyl ether comonomer reactant with respect to the maleic anhydride monomer is employed in the premixing zone. Such molar excess can vary between about l.l and about 2, although a molar ratio of from about 1.3:1 to about 1.7:1 is preferred. The maleic anhydride and vinyl ether monomers are thoroughly premixed at below reaction temperature before entering the reactor in order to form a polymerizable monomer-comonomer complex species on which to build polymer chains and to provide a substantially alternating polymeric product. Monomer mixing is beneficially effected at between about 30° and about 40°C. at atmospheric pressure.

The complexed monomers are gradually introduced into the reaction zone, containing the precharged cosolvent mixture, over a period of from about 2 to about 5 hours, preferably from about 2.5 to about 3.5 hours or until the ensuing polymerization reaction has reached a desired level. Generally, monomer/comonomer complex to cosolvent mixture feed rates of from about 1:1.8 to about 1:3 part/parts is recommended for optimum mixing and temperature control, although somewhat higher or lower feed rates can be employed, if desired. The polymerizable complexed mixture is then thoroughly agitated and brought to reaction temperature at from about 45° to about 90°C.

under a pressure of from about 25 to about 100 psig while simultaneously contacting a free radical initiator which is continuously fed to the reactor throughout the polymerization reaction. Since the free radical initiator is usually a solid, it is dissolved in a lower alkyl substituted benzene or in the C^ to C ₈ alkane, preferably in the same aromatic or alkane compound or mixture thereof as employed in the cosolvent mixture, prior to being introduced into the reactor. The recommended reaction temperature and pressure for the preferred monomeric mixture, i.e. maleic anhydride/methyl vinyl ether, is from about 55°C. to about 70°C. under from about 35 to about 70 psig. Ethyl, propyl and butyl vinyl ether comonomers may require somewhat more severe reaction conditions within the above temperature and pressure ranges.

Suitable free radical initiators are those known in the art and include but are not limited to peroxides such as benzoyl peroxide, lauryl peroxide, decanyl peroxide, tertbutyl peroxy pivalate, terta yl peroxy pivalate, etc. The initiator solution is continuously and intimately mixed with the monomeric mixture throughout the exothermic polymerization reaction. The concentration of the initiator with respect to maleic anhydride in the feed mixture is generally between about 0.00005 and about 0.05:1 parts by weight, preferably between about 0.0001 and about 0.005 parts by weight.

In the reactor, the monomers are reacted under relatively mild reaction conditions and excess unreacted vinyl ether comonomer together with a portion of the solvent of the cosolvent mixture are removed as a vaporous overhead during the exothermic reaction. This overhead mixture can be directly recycled and combined with the cosolvent feed mixture to the reactor or the components can be separated and individually recycled in the process.

After the addition of complexed monomers, initiator solution and cosolvent mixture is complete, the reactor contents may be held at reaction temperature and pressure for an additional period of up to about 2 hours to insure quantitative conversion of maleic anhydride to the desired polymeric product. At completion of the polymerization reaction, additional solvent can be added as needed to provide a crude product mixture containing from about 15 to about 30 wt. % solids in the resulting slurry. The slurry is then treated to recover the polymeric product from diluent in a substantially pure state.

Product recovery can be accomplished by any known means. One method involves vacuum drying the crude product mixture at a temperature above the boiling point of the solvent. The solvent vaporized at this stage can be condensed and can be recycled to one of the appropriate feeds to the reactor.

An important advantage of the present process stems from the surprising discovery that by preforming the copolymerizable complex before contact with the high chain transfer solvent and initiator, the expected tendency of the aromatic solvent to limit polymeric chain development is overcome. Also, the continuous feed of initiator during reaction minimizes the influence of the aromatic solvent in the copolymerization reaction. Thus, the only factor controlling molecular weight of the product resides in the amount of the vinyl ether solvent fed to the system which is easily monitored. It is now discovered that certain aromatic solvents, i.e. those having relatively high chain transfer constants, do not cause termination of the reaction as in the case of benzene, and instead, can be usefully employed in a cosolvent mixture to extend chain propagation and to produce high number average molecular weight products, e.g. from about 1,000,000 to about 2,000,000 or more, which heretofore were unachievable.

Such products have extended the field of application for maleic anhydride/vinyl ether copolymers to include their use as adhesives, particularly denture adhesives since they can be prepared to contain no traces of toxic components. Other new uses suited to the present high molecular weight copolymers include applications in the packaging of foods or use in ostomy applications on human skin, toothpaste additives and the like. When the cosolvent of the solvent mixture contains a lower alkyl vinyl ether dissimilar to the lower alkyl vinyl ether in the comonomer complex, the dissimilar cosolvent alkyl vinyl ether may also react to form a high viscosity terpolymeric product also suitable for the above uses.

A more detailed description and best mode embodiment of the present process is provided by the following description of accompanying Figure 1. In accordance with the illustrated flow sheet describing a batch operation, 4381 pounds of the alkyl vinyl ether (methyl vinyl ether) is fed into valved line 2, 1367 pounds of which are introduced into high shear mixer 6 at 25°C. wherein it is intimately mixed with 1495 pounds of maleic anhydride introduced from valved line 4 to form a complexed comonomer. The remaining 3014 pounds portion of the methyl vinyl ether feed is passed to valved line 8 and combined with 3014 pounds of toluene entering the system through valved line 9 to provide the cosolvent mixture which is precharged directly into nitrogen purged evacuated reactor 10 equipped with an efficient mixing device 11. After precharging the toluene/methyl vinyl ether cosolvent mixture, the monomer-comonomer complex which is formed in high shear mixer 6 is fed to reactor 10 through valve line 12 at a feed rate effective to provide a weight ratio of complexed monomer to cosolvent mixture of about 1:2 upon initial introduction. Simultaneous with the introduction of the complexed monomer species, the reactor is charged

with a solution of about 2.2 pounds lauryl peroxide initiator in 3010 pounds of toluene from valved lines 14 and 18. The complexed monomer/comonomer species and initiator solution are gradually introduced into sealed reactor 10 over a period of about 2.5 hours during which the exothermic solution polymerization is effected at about 65°C. under about 80 psig. During reaction, the pressure can be maintained constant by pressure regulating valve 44 which can be adapted to release a desired portion of vaporous unreacted methyl vinyl ether/toluene as an overhead mixture in line 20 and thence into condenser 22 wherein, at a temperature of 32°C. , vaporous methyl vinyl ether is removed by line 50 and subsequently compressed for recovery. The remaining condensed methyl vinyl ether and uncondensed toluene are recycled to reactor 10 via line 24.

Upon completion of the polymerization reaction, the crude copolymeric solids in toluene forms a slurry of between about 18-25 wt. % solids. This slurry is then diluted with toluene recovered within the system from valved line 28 which may or may not be supplemented by feed toluene from valved line 30. The post dilution with toluene reduces the solids content in the crude product to about 15 wt. %. Pressure valve 44 is then fully opened and remaining unreacted methyl vinyl ether is vaporized and removed via line 20. For most efficient operation, a holding tank (not shown) may be employed to receive the crude product slurry and to clear reactor 10 for the next feed batch. In the drawing, the crude product is withdrawn from reactor 10 by means of line 36 and then passed to vacuum dryer 38 wherein at a temperature of about 230°C. under 28" Hg vacuum, solid methyl vinyl ether/maleic anhydride copolymer product in quantitative yield and at least 99% purity is withdrawn and recovered from line 40. Toluene vaporized in vacuum dryer 38 is removed by line 42, passed to cooler-condenser 48 and recycled to the process in line 28 to supply the aforesaid post dilution of the crude product slurry in reactor 10.

It will be appreciated from the above description of Figure l that the process can be readily adapted to continuous operation, e.g. by employing alternating pressure reactors. Additionally, it will be understood that other alkyl vinyl ether comonomers can be substituted for methyl vinyl ether and that other aromatic or C _g to C ₈ alkane solvents as well as other peroxide initiators can replace those described in Figure 1 without departing from the scope of this invention and that many modifications, substitutions or alterations of the described procedure are also included herein.