How to vertically align letter before brackets in chemfig?

how can I vertically center this little letter n before the left bracket? I want it to be not at top but instead at middle. Thanks in advance.

\documentclass{article}
\usepackage{chemfig}

\begin{document}

\schemestart
n
\chemleft[
\chemfig{M([6]-R^{+;-})}
\chemright]
\arrow{->[Polymerisation][]}[0,2]
\chemfig{-[@{upleft,0.5},1]M([6]-R^{+;-})-[@{upright,0.5},1]}
\polymerdelim[delimiters ={[]}, height = 10pt, depth = 40pt, indice = n]{upleft}{upright}
\schemestop
\end{document}


Using an invisible arrow (\arrow{0}[,0]) will vertically center the n with respect to the arrow:

\documentclass{article}
\usepackage{chemfig}

\begin{document}
\schemestart
n  \arrow{0}[,0]
\chemleft[
\chemfig{M([6]-R^{+;-})}
\chemright]
\arrow{->[Polymerisation][]}[0,2]
\chemfig{-[@{upleft,0.5},1]M([6]-R^{+;-})-[@{upright,0.5},1]}
\polymerdelim[delimiters ={[]}, height = 10pt, depth = 40pt, indice = n]{upleft}{upright}
\schemestop
\end{document}