The values given below have been taken from my lecture hand out, and they weren't cited so I don't know where my instructor got them from
The $k_\mathrm{p}$ (rate constant for propagation in radical polymerisation ) step for the following monomers is given below:
$$\begin{array}{cc} \hline \text{Monomer} & k_\mathrm{p}~/~\mathrm{mol^{-1}~s^{-1}} \\ \hline \text{Styrene} & 106 \\ \text{Vinyl ester} & 3000\mathrm{-}4000 \\ \text{Ethylene} & 16 \\ \hline \end{array}$$
And the instructor explained the difference between the first two as follows:
the benzyl radical is stabilised by conjugation, however the radical produced in a vinyl ester lacks stabilisation by conjugation and thus reacts more rapidly.
Seems like a fair explanation, however in the same table the $k_\mathrm{p}$ for ethylene is given as $16~\mathrm{mol^{-1} s^{-1}}$, the slowest propagation step of the three. Thus his explanation is not consistent with this observation I believe, because similar to the vinyl ester radical this one too is not stabilised by conjugation and should be similarly eager to react.
My instructor did not offer a satisfactory explanation and I was wondering if people could weigh in on this here.