What is the science behind multi viscosity oil?

There are additives added into the base oil to increase viscosity at higher temperatures (when oil tends to thin out until it is like water) and to decrease viscosity at low temperatures (when oil tends to thicken, sometimes to the consistency of mud!) Most of these are used specifically for their action in a specific temerature range. Most comercial motor oils have an additive package containing 6 to 8 diferent viscosity enhancers.What is the science behind multi viscosity oil?
In brief, these oils contain various polymers which increase the viscosity as the temperature increase. Most of the polymers are like rubber in chemical structure.