There have been claims that maleic anhydride was homopolymerized but it is noteworthy that poly(maleic anhydride) is not available commercially. On the other hand, there are many examples of copolymers of maleic anhydride & other monomers such as ethylene or styrene...etc. and you will find companies selling the copolymers.
Also, there are copolymers which contain cyclic carbonate (e.g. 5-membered ring) and one of our Research Gate colleagues has added his publication to RG site as shown below.
Article Cyclic carbonate functional polymers and their applications
Polyolefin grafted with maleic anhydrate is commercially available such as PP-g-MA (polypropylene) and PE-g-MA (polyethylene). Quite easy to find as they are known as compatibilizer in polymer blends.
There are many companies that sell PP-g-MA or PE-g-MA. I had used PP-g-MA from Sigma. For the processing conditions and properties you can also check the following pub, Regards, Apostolos.
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