Patents form a major part of the chemical literature. There are many search engines and portals that provide free access to patent documents from agencies around the world. They are mainly useful for locating a patent document you already know about. They are not intended for patentability or prior-art searching, which should be done by professional patent searchers. Most patent office public search sites have poorly designed interfaces, so patience and determination are essential. Pay special attention to the entry format of patent numbers: most databases won't tolerate any variation in the parsing of numbers and country codes.
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