kkim123,
You need to find some very detailed, Plating On Plastic (POP) instructions for your particular substrates.
The plastic is first alkaline cleaned & rinsed.
Conditioning: Some plastics are then slightly softened with chlorinated organics to make more susceptible to the acid.
Etching: The acid (often, hot chromic+sulfuric) attacks the plastic surface, breaking molecular bonds, converting it into a microscopically porous, wettable surface.
Neutralizing (sensitizing): A rinse containing complexing or reducing agents (for oxidizing acids such as chromic & nitric) follows.
Catalyzing (sensitizing): A stannous chloride or dilute palladium chloride solution is used to catalyze (ativate) the surface for metal deposition.
Acceleration: Sometimes, a step using fluorides is used.
Silver can then be deposited using the Brashear Formula solution.
Caution: The procedure above follows ASTM B727, 'Standard Practice for Preparation of Plastics Materials for Electroplating,' which doesn't mention PMMA or PU.