SOLVENT-FREE COPOLYMERS OF MALEIC ANHYDRIDE AND ALKYL VINYL ETHERS HAVING A SPECIFIC VISCOSITY OF 0.5 TO 5 BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a process for making a copolymer of maleic anhydride and a Ci-C4 alkyl vinyl ether, and, more particularly, to a solvent- free process for making such copolymer product in the form of free-flowing, fine white powders having a predetermined, substantially reduced specific viscosity, SV which is characterized by addition of a small amount of an active chain transfer agent to the reaction mixture, which agent can be easily volatilized without leaving substantially any residue, odor or taste in the product.

2. Description of the Prior Art M. Tazi et al, in U. S. Pat. 5,003,014, described a process for making copolymers of maleic anhydride and a Ci-C4 alkyl vinyl ether having specific viscosities extending over a wide range by addition of large, solvent quantities of toluene as chain transfer agent in the polymerization. However, toluene is a relatively inactive chain transfer agent for this polymerization. Accordingly, such solvent amounts of toluene were required to effect any reduction in the specific viscosity of the copolymer product. Furthermore, toluene was difficult to remove during the process steps of venting of excess alkyl vinyl ether and drying of the polymer, resulting in a toluene residue in the product, and some objectionable odor and taste.

Shimizu, H., in Jap. Kokai pat. Appln. No. Hei 6 [1994]-569; published March 1,1994, described a process of making very low molecular weight copolymers of maleic anhydride and alkyl vinyl ethers by including large amounts of higher alkyl aldehydes as a solvent in the system. Such aldehydes, however, are non-volatile and remain in the product giving it a particularly unpleasant odor.

Accordingly, it is an object of the present invention to provide a solvent- free process for making copolymers of maleic anhydride and a Ci-C4 alkyl vinyl ether in the presence of an active chain transfer agent which can be included in small amounts in the polymerization mixture to effect a substantial reduction in the specific viscosity of the copolymer product, and which can be easily volatilized during removal of excess alkyl vinyl ether without degrading the copolymer and without leaving substantially any residue, odor or taste in the product.

A specific object is to provide copolymers of maleic anhydride and methyl vinyl ether which have a specific viscosity SV of about 0.5 to 5, preferably 1.0 to 4, by including about 0.01 to 1.9%, preferably 0.02 to 1.0, by weight of the reaction mixture, of acetaldehyde therein.

IN THE DRAWING The FIGURE is a plot of SV of copolymer product vs. concentration of acetaldehyde added to the reaction mixture.

SUMMARY OF THE INVENTION What is described herein is a solvent-free, free-flowing, fine white powder copolymer of maleic anhydride and a Ci-C4 alkyl vinyl ether having a predetermined specific viscosity of about 0.5 to about 5, preferably 1.0 to 4, as measured on a 1% wVvol solution of copolymer in methyl ethyl ketone at 25°C, a product substantially without residue, odor or taste.

As another feature of the invention, a solvent-free process is described for making such a copolymer product of maleic anhydride and a Ci-C4 alkyl vinyl ether by copolymerizing maleic anhydride and excess alkyl vinyl ether with a radical initiator, stripping off excess alkyl vinyl ether, followed by drying the polymer at 40-100°C under vacuum to remove any traces of remaining alkyl vinyl ether, wherein acetaldehyde is added to the reaction mixture as an active chain transfer agent in a predetermined amount of about 2% or less by weight, preferably 0.01 to 2%, and, most preferably 0.02 to 1.0%, thereby producing a copolymer product having a predetermined, substantially reduced specific viscosity, whose value is determined directly in relation to the amount of acetaldehyde added. The acetaldehyde is particularly characterized by its easy volatilization during removal of excess alkyl vinyl ether, without degrading the copolymer or leaving substantially any residue, odor or taste in the product.

The solvent-free process here is preferably embodied when the amount of acetaldehyde is about 0.01 to 1.9%, most preferably 0.02 to 1.0, whereupon the specific viscosity of the copolymer is about 0.5 to 5, preferably 1 to 4, as measured on a solution of copolymer of 1 % wt/v in methyl ethyl ketone (MEK) at 25°C.

DETAILED DESCRIPTION OF THE INVENTION The solvent-free polymerization process of the invention is carried out using an excess of alkyl vinyl ether as the reaction medium, characterized by precharging the alkyl vinyl ether into the reactor, adding a predetermined amount of acetaldehyde, adding a radical initiator, heating to a reaction temperature of about 50° to 100°C, and, thereafter, feeding molten maleic anhydride into the reactor over a predetermined period of time, followed by holding the reactants at a defined temperature for a predetermined period of time.

The remaining AVE is stripped off, the polymer is dried at about 40-100°C under vacuum, to remove any remaining traces of AVE. The desired copolymer is obtained as a solvent-free, fine powder, of predetermined SV, which is odorless and tasteless.

In a preferred embodiment of the invention, the amount of acetaldehyde added is less than 2% by weight of the reaction mixture, preferably 0.01% to 2%, and optimally 0.02% to 1%. The MAN reactant preferably is introduced into the precharged MVE over a period of at least 1 hour and the % solids during the polymerization is about 10-40%. The MVE: MAN mole ratio is about 25: 1 to 5: 1, most preferably 12: 1 to 7: 1. The initiator concentration is 0.01 to 0.2%, most preferably 0.05 to 0.1 % by wt., based on the copolymer obtained.

The invention will now be described in more detail with reference to the following examples.

EXAMPLE 1 A 1-liter Buchi pressure reactor was sparged with nitrogen and charged with 400 ml (300 g, 5.16 mol) of methyl vinyl ether (MVE), 0.55 g TrigonoxR 21 (t-butylperoxy-2-ethylhexanoate, an initiator made by Akzo Nobel Chemicals, Inc.), a concentration of 0.08% by weight based on copolymer obtained, and with 0.034 g of acetaldehyde, or 0.01 % by weight based on total weight of the reaction system. The precharged reactor was heated to 75°C, with agitation, over 15 min. Then 41 g (0.418 mol) of molten maleic anhydride (MAN), was fed in over 1 hour, resulting in a MVE/MAN mole ratio of 12.3. The solids content of the resulting mixture was 16%. The temperature of 75°C was maintained for 1.5 hours. Then excess MVE was stripped off, whereupon the pressure dropped to atmospheric. Upon opening the reactor, a fine, free- flowing copolymer powder was observed. The product was dried in a vacuum oven at 65°C for 4 hours. A total of 64.8 g (99.37 % of theoretical yield) of product was obtained. 13C NMR analysis the product indicated a 1: 1 MVE/MAN copoiymer having a specific viscosity (SV) of 4.1 (1 % w/v polymer solution in MEK at 25°C). The product was odorless and tasteless (2.5% aqueous solution, pH 7).

EXAMPLES 2-4 Examples 2-4 were carried out as in Example 1, using different amounts of acetaldehyde.

The results of Examples 1-4 are summarized in the Table below: TABLE Example No. 1 2 3 4 Acetaldehyde 0.034 0.34 3.4 6 added, g Acetaldehyde 0.01 0.1 1.0 1.75 added, % on reaction blend Specific viscosity 4.1 2.1 1.2 0.55 of copolymer (SV, 1 % wt/v in MEK), after drying Appearance of Solvent-free, fine white powders, without odor and copolymer taste The attached Figure shows the effect of acetaldehyde addition to the polymerization mixture in accordance with the data in Examples 1-4. What is plotted is SV of copolymer vs. % acetaldehyde added. The results show a substantial reduction in SV with an increase in acetaldehyde added.

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: