Monovinylidene aromatic polymers with improved toughness and rigidity and a process for their preparation

A process for preparing a rubber-modified monovinylidene aromatic polymer comprising the steps of:

• (a) continuously supplying a reaction mixture comprising monovinylidene aromatic monomer and a dissolved diene rubber to a reactor means, (b) continuously polymerizing the monovinylidene aromatic monomer in the presence of the dissolved diene rubber in the reactor means under conditions whereby phase inversion subsequently occurs, (c) continuously removing from the reactor means a diene rubber-reinforced monovinylidene aromatic polymer, which process is characterized in that:
• (d) the diene rubber which is dissolved in the reaction mixture has I) from 20 to 80 percent, based on the total weight of said diene rubber, of a high solution viscosity component having a viscosity value ranging from 120 to 500 centipoise and II) from 80 to 20 percent, based on the total weight of said diene rubber, of a low solution viscosity component having a viscosity value ranging from 1 to 100 centipoise, further characterized in that the ratio of solution viscosity of high solution viscosity component to the solution viscosity of low solution viscosity ranges from 1.2 to 500, wherein both components I and II have a 1,4 cis content of greater than 30 and, optionally, (e) the process conditions prior to phase inversion are adjusted to produce different groups of diene rubber particles from the high and low solution viscosity components of the diene rubber, each group having a different average rubber particle size.