How is Polymethylene polyphenyl polyisocyanate (CAS: 9016-87-9) typically synthesized?

Polymethylene polyphenyl polyisocyanate (CAS: 9016-87-9) is synthesized through a two-step process. First, phenylene isocyanate is polymerized using a polyphenyl carbonate as a chain extender. This step involves the use of organometallic catalysts such as tin(II) octoate. The second step is the polymerization of the intermediate product, which typically requires high vacuum and elevated temperatures. The yield and selectivity of this process can be improved by optimizing reaction conditions and catalysts.