It may be desirable to change the particle diameter of your latex to give improved performance or physical properties for a given application. However, emulsion polymers are complex products, and more often than not changes in one property also result in changes in other properties. This interdependency applies to particle size and molecular weight. The average molecular weight of a latex can be influenced by a number of factors, but it is often not realized that the number of particles in the latex (which is directly related to particle size) is important in determining molecular weight.
The reason is not obvious; in each latex particle, the growth of a new chain starts when a radical generated in the aqueous phase enters a particle and starts adding monomer. Chain growth stops when another radical enters the particle and terminates the growing chain. The time span between two radicals entering a particle represents how long the chain has to grow; a longer time interval gives longer chains (higher molecular weight).