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This type of polymerization takes place via mechanism involving carbocation intermediates. The steps involved are initiation and propagation. A good monomer for cationic polymerization needed to produce a relatively stable carbocation intermediate in the initiation step to form the initiating chain via protonation by acid catalyst. In propagation step, the polymer is formed by successive addition of other monomer molecules to the cationic end of the growing chain.

(a) The acid catalyst boron trifluoride (BF₃) reacts with water ( H₂O) to form an oxonium intermediate.

The oxonium intermediate protonates the vinyl chloride that forms the initiating chain of the polymer.

The carbocation of the initiating chain reacts with another molecule of vinyl chloride which lengthens to form the growing chain of the polymer.

The growing chain reacts continuously until the chain lengthening process of the polymer chain ends when there are no more available vinyl chloride molecules.

Hence, vinyl chloride is the good monomer for cationic polymerization since it produces a stable secondary carbocation intermediate in its initial and growing chains.

(b) The acid catalyst boron trifluoride ( BF₃) reacts with water (H₂O) to form an oxonium intermediate.

The oxonium intermediate protonates the vinyl acetate that forms the initiating chain of the polymer.

The carbocation of the initiating chain reacts with another molecule of vinyl acetate which lengthens to form the growing chain of the polymer.

The growing chain reacts continuously until the chain lengthening process of the polymer chain ends when there are no more available vinyl acetate molecules.

Hence, vinyl acetate is not a good monomer for cationic polymerization since the carbonyl group is strong electron withdrawing group that forms destabilized carbocation which does not polymerize well.

(c) The acid catalyst boron trifluoride (BF₃ ) reacts with water (H₂O ) to form an oxonium intermediate.

The oxonium intermediate protonates the that forms the initiating chain of the polymer.

The carbocation of the initiating chain reacts with another molecule of which lengthens to form the growing chain of the polymer.

The growing chain reacts continuously until the chain lengthening process of the polymer chain ends when there are no more available molecules.

Hence, is not a good monomer for cationic polymerization since the carbonyl group and cyano group are strong electron withdrawing groups that forms destabilized carbocation which does not polymerize well.