polymerised mass to the mean degree of polymerisation. [@zhang_rev]

The two principal moments expressed in our variables read \[eq4\] P = \_[i=n\_c]{}\^ y\_i, for the polymer concentration and \[eq5\] M = \_[i=n\_c]{}\^ i y\_i, for the polymerised mass. We obtain the dynamical equations for $P$ and $M$ by extracting the first two principal moments from the full polymer length distribution described by Eq. \[eq2\]. In the process of deducing the dynamical equations for the moments, the only approximation we employ is that we neglect all terms arising from the disintegration of nuclei, which for early times are negligible in number anyway, at least in the limit $k_n^+ \rightarrow 0$. See Appendix A for details.

The dynamical equation for the number of polymers $P(t)$ that we obtain reads \[eq6\] &=& - P(t)\^2 + + k\_n\^+ \^[n\_c]{}, and for the time evolution of the the polymerised monomeric mass $M(t)$ we find \[eq7\] &=& 2 + (-M(t)) (M(t)+P(t))\
& &+ (2 P(t)-M(t))+ n\_c k\_n\^+ (-M(t))\^[n\_c]{}, where $\mtot$ is the total concentration of monomers in the system. A detailed derivation of Eqs. \[eq6\] and \[eq7\] from Eqs. \[eq2\] and \[eq3\] is provided in Appendix A. For this general set of equations the initial polymerised 