The present invention is a rubber modified monovinylidene aromatic polymer composition where the rubber is a sulfanylsilane chain end modified elastomeric polymer. In the present invention the functionalized rubber provides sufficient grafting levels in standard graft polymerization process technologies to prepare desired rubber particle size distributions and morphologies. Across a range of rubber modified monovinylidene aromatic polymers, it has been found that the use of the modified elastomeric polymers according to the present invention can improve the combinations of resin properties, particularly clarity, surface and physical properties, including both rubber modified monovinylidene aromatic homopolymers (e.g., high impact polystyrene and copolymers (e.g., ABS). The functionalized rubbers have sulfanyl functional groups that provide an optimized reactive grafting site. This ensures that under graft polymerization conditions a monovinylidene aromatic polymer block is formed grafted to the butadiene polymer block. This is particularly desirable to enable the use of polybutadiene homopolymer rubbers which are more cost effectively used in rubber-modified monovinylidene aromatic polymers.