I was working at Monsanto up until just before the acquisitions, mergers, and spin-offs hit. The cause was not that investors wanted to focus their investment, the ultimate cause was that the management team wanted to be in the higher profit margin biotech/pharmaceutical business. (not a surprise; the Monsanto CEO at the time used to be the Searle CEO, IIRC.)
Monsanto acquired Searle to get into that business, then spun off Solutia as their commodity chemical business – and saddled Solutia with most of the debt from the Searle (and other) acquisition. Solutia also inherited a lot of liabilities in the form of old plants with potential environmental issues to deal with. Monsanto kept the (highly profitable) agricultural chemicals (e.g., Roundup, Lasso) and Searle’s biotech/pharm stuff.
Monsanto then merged with Pharmacia, and the ag chemicals (and seed) business became a wholly-owned subsidiary – and the portion to keep the “Monsanto” name. The parent organization was Pharmacia.
Monsanto was later spun off on its own. But the core of the original management team that was still around was in Pharmacia – they had shifted out of the chemicals market, into biotech/pharm, and got rid of the debt for doing so by spin-offs. Pharmacia and Monsanto did still share a few facilities here in St. Louis.
Pharmacia later merged with Pfizer. As I understand it from my friends there, this was largely done to get access to drugs in the developmental pipeline, which is a common occurance in the pharm industry (small company develops drugs, then merges/is acquired by big company who funds the expensive commercialization); I’m not clear who wanted what, though. Most of the Pharmacia facilities in St. Louis have been sold back to Monsanto, other than some critical stuff like the pilot plant.