Problem 4. What is the metal catalyst that is used in the Grignard metathesis (GRIM) polymerization mechanism? a) Nickel b) Magnesium c) Palladium d) Iron e) Tin Next, draw the chemical structures of two repeat units that would depict the head-to-head, tail-to-tail, and head-to-tail stereoisomers of a poly(3-alkylthiophene) derivative