Condoms are made from latex, which comes from the tropical rubber tree Hevea brasiliensis. Liquid latex, a dispersion of rubber particles (30%) in water (70%), lies between the bark and wood and is collected by slashing through the bark and allowing the latex to flow out. Ammonia is added to counter the acid production of waste products from the bacteria that naturally feed on the latex and cause it to curdle.
The liquid latex is centrifuged to concentrate the rubber particles so that it is 60% solid rubber. Other chemicals including an antioxidant, a sulfur-based vulcanizing agent, and a vulcanizing accelerator, are added to increase both the rate and extent of cross-linking in the latex compound during vulcanization - a process in which polymer molecules are cross-linked by sulfur atoms or carbon-carbon bonds under high heat. This makes the latex harder and more durable.
This is where there is some ethical debate. The manufacture of condoms usually involves casein, a phosphoprotein that occurs in milk and cheese and so is from cows and not vegan friendly. Casein is added at this stage to toughen up the latex. Condomi have replaced casein with cocoa powder. It is thought that these condoms might be a tiny bit weaker.
I'm not sure which option is the best for the planet as the latex produced is still largely not biodegradable, but it's good to see that the best brains on the planet have made life a bit easier for a couple of vegans. I always figured there was some connection between sex and chocolate...