Discussion
Norrish-Smith effect or Trommsdorff effect during free radical bulk polymerization at high conversion is characterized by ___.
Answer: A
The effect is an increase in rate of polymerization with time due to increase in viscosity.
At high conversion in bulk free radical polymerization the medium becomes very viscous. Termination needs two large growing chains to meet, and their diffusion slows dramatically, so the termination rate constant falls. Small monomer molecules can still reach the radical ends, so propagation continues.
With fewer terminations the radical concentration builds up, and the rate of polymerization and the molecular weight rise sharply: autoacceleration, the gel effect. This can cause a dangerous temperature runaway.
The options with a decreasing rate describe the opposite behaviour, and those attributing the effect to falling viscosity have the cause backwards.
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