Role of substituents in radical initialized polymerization
The compound IV is a great compound for cationic polymerization , the explanation is the oxygen which is giving electron with resonance : The form where the oxygen has a plus charge and the end carbon a minus is the reason why this compound will be able to start a radical polymerization but with a really low constant of polymerization (kinetic constant), and also the reason why the cationic polymerization will have high constant.

Compounds with strong (electron donor and tiny) resonance will not be good for radical mechanism, but if you put a phenyl group behind the ether function it will be better because the resonance effect will be "diluted".