\documentclass[12pt,border=1pt,crop,multi=false,tikz,class=scrartcl]{standalone}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{chemfig}
\usepackage{lmodern}

\chemsetup[chemformula]{font-shape=sf,format=\sffamily}
\renewcommand*{\familydefault}{\sfdefault}
\renewcommand*\printatom[1]{\ensuremath{\mathsf{#1}}}
\setatomsep{2em}
\setdoublesep{.6ex}
\setarrowdefault{,1,semithick}
\setbondstyle{thick}

\newcommand\setpolymerdelim[2]{
	\def\delimleft{#1}
    \def\delimright{#2}
}
\def\makebraces[#1,#2]#3#4#5{%
	\edef\delimhalfdim{\the\dimexpr(#1+#2)/2}%
	\edef\delimvshift{\the\dimexpr(#1-#2)/2}%
	\chemmove{%
		\node[at=(#4),yshift=(\delimvshift)] {%
        	$\left\delimleft\vrule height\delimhalfdim depth\delimhalfdim
			width0pt\right.$
        };%
		\node[at=(#5),yshift=(\delimvshift)]{%
        	$\left.\vrule height\delimhalfdim depth\delimhalfdim
			width0pt\right\delimright_{\rlap{$\scriptstyle#3$}}$
        };
    }
}
 
\makeatother
\begin{document}
\schemestart
\setpolymerdelim[]
\chemfig{
	>:[@{op,.5}0] *6(-(=O)-[,,,,draw=none](=O)-(<:O-[@{cl,.25}])-O-(<-[::-60]OH)-)
}
\makebraces[2.1cm,1.6cm]{\!\!n}{op}{cl} 
\schemestop
\end{document}