After reviewing your problem and process description, several observations can be made.  While an anchor style agitator is never a great design, it provides almost no top to bottom mixing, is may be adequate to help remove carbon buildup along the walls of your reactor.  As long as the mixing requirements for the polymer are minimal, long processing time and slow reaction, the anchor agitator should work
Assuming you stay with your anchor style agitator, with two arms, each 200 mm wide, the maximum viscosity for a 25 hp motor is about 60,000 cp at 20 rpm.  To handle 100,000 cp, the mixer speed should be reduced to 16 rpm.  If you want to run at 20 rpm, with 100,000 cp viscosity, you would need a 40 hp motor, a larger gear reducer and a larger shaft diameter.  The current 4" diameter agitator shaft is the minimum diameter for the motor power and speed.  A 4.5" or 5" diameter shaft would be better.  Blade thickness, reinforcement, and cross-arms (number and location) are all interrelated for strength.  The blade design and reinforcement must be adequate for a 11,000 N (2,500 lbf) distributed load.  Whether you use stainless or carbon steel is a choice.  The carbon steel should be a little stronger, but might not be enough stronger to reduce the thickness of the agitator blades.  The carbon steel will be less expensive, although most of the blade cost will be in the fabrication, not the material.
Your bent blades may have been a result of striking a lump in the polymer and not a design flaw.

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