Here are two more versions of your table. In both, I have corrected the chemical formulae of the polymer molecules using \polymerdelim from the chemfig package. With this command, the square brackets are correctly drawn through the outermost bonds, instead of around them. I have also corrected some of the typos in the compound names and in the application examples.
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\documentclass[a4paper, 12pt]{scrartcl}\usepackage[ngerman]{babel}\usepackage{tabularx}\usepackage{chemfig}\usepackage{adjustbox}\usepackage{pdflscape}\usepackage[column=0]{cellspace}\usepackage{makecell}\setlength\cellspacetoplimit{8pt}\setlength\cellspacebottomlimit{8pt}\begin{document}\begin{landscape}\noindent\begin{tabularx}{\linewidth}{|0l|0c|0c|>{\raggedright\arraybackslash}X|} Name & Monomer & Polymermolekül & Einsatzbeispiel\\ \hline& & & \\ \makecell[cl]{Polyethen\\ (PE)} & \adjustbox{valign=c}{\chemfig{ H -[1]C (-[3]H) =C ( -[1]H) -[7]H }} & \schemestart \chemfig{ -[@{upleft,0.5},1]C ( -[2]H) ( -[-2]H) -C ( -[2]H) ( -[-2]H) -[@{upright,0.5},1] } \polymerdelim[delimiters ={[]}, height = 45pt, depth = 40pt, indice = n]{upleft}{upright} \schemestop& Plastiktüten \\ \makecell[cl]{Polypropen\\ (PP)} & \adjustbox{valign=c}{\chemfig{ H -C ( -[2]H) ( -[6]H) -C ( -[2]H) =C ( -[1]H) ( -[7]H) }} & \schemestart \chemfig{ -[@{upleft,0.5},1]C ( -[2]H) ( -[-2]H) -C ( -[2]{CH_3}) -[@{right,0.5},1] } \polymerdelim[delimiters ={[]}, height = 45pt, depth = 40pt, indice = n]{upleft}{upright} \schemestop& Flaschendeckel, Brotdosen \\ \makecell[cl]{Polyvinylchlorid\\ (PVC)}& \adjustbox{valign=c}{\chemfig{ H -[1]C ( -[3]H) =C ( -[1]Cl) -[7]H }} & \schemestart \chemfig{ -[@{upleft,0.5},1]C ( -[2]H) ( -[-2]H) -C ( -[6]H) ( -[2]{Cl}) -[@{right,0.5},1] } \polymerdelim[delimiters ={[]}, height = 45pt, depth = 40pt, indice = n]{upleft}{upright} \schemestop& Rohrleitungen, Vinylböden, Schallplatten \\ \makecell[cl]{Polytetrafluorethen\\ (PTFE)} & \adjustbox{valign=c}{\chemfig{ F -[1]C ( -[3]F) =C ( -[1]F) -[7]F }}& \schemestart \chemfig{ -[@{upleft,0.5},1]C ( -[2]F) ( -[6]F) -C ( -[2]F) ( -[6]F) -[@{right,0.5},1] } \polymerdelim[delimiters ={[]}, height = 45pt, depth = 40pt, indice = n]{upleft}{upright} \schemestop& Pfannenbeschichtung (Teflon), Funktionskleidung (Goretex) \end{tabularx}\end{landscape}\setlength\cellspacetoplimit{15pt}\setlength\cellspacebottomlimit{15pt}\renewcommand{\tabularxcolumn}[1]{m{#1}}\noindent\begin{tabularx}{\linewidth}{|0l|0c|0c|>{\raggedright\arraybackslash\hspace{0pt}}X|} Name & Monomer & Polymermolekül & Einsatzbeispiel\\ \hline \makecell[cl]{Polyethen\\ (PE)} & \adjustbox{valign=c}{\chemfig{ H -[1]C (-[3]H) =C ( -[1]H) -[7]H }} & \schemestart \chemfig{ -[@{upleft,0.5},1]C ( -[2]H) ( -[-2]H) -C ( -[2]H) ( -[-2]H) -[@{upright,0.5},1] } \polymerdelim[delimiters ={[]}, height = 45pt, depth = 40pt, indice = n]{upleft}{upright} \schemestop& Plastiktüten \\ \makecell[cl]{Polypropen\\ (PP)} & \adjustbox{valign=c}{\chemfig{ H -C ( -[2]H) ( -[6]H) -C ( -[2]H) =C ( -[1]H) ( -[7]H) }} & \schemestart \chemfig{ -[@{upleft,0.5},1]C ( -[2]H) ( -[-2]H) -C ( -[2]{CH_3}) -[@{right,0.5},1] } \polymerdelim[delimiters ={[]}, height = 45pt, depth = 40pt, indice = n]{upleft}{upright} \schemestop& Flaschendeckel, Brotdosen \\ \makecell[cl]{Polyvinyl-\\chlorid\\ (PVC)}& \adjustbox{valign=c}{\chemfig{ H -[1]C ( -[3]H) =C ( -[1]Cl) -[7]H }} & \schemestart \chemfig{ -[@{upleft,0.5},1]C ( -[2]H) ( -[-2]H) -C ( -[6]H) ( -[2]{Cl}) -[@{right,0.5},1] } \polymerdelim[delimiters ={[]}, height = 45pt, depth = 40pt, indice = n]{upleft}{upright} \schemestop& Rohrleitungen, Vinylböden, Schallplatten \\ \makecell[cl]{Polytetra-\\fluorethen\\ (PTFE)} & \adjustbox{valign=c}{\chemfig{ F -[1]C ( -[3]F) =C ( -[1]F) -[7]F }}& \schemestart \chemfig{ -[@{upleft,0.5},1]C ( -[2]F) ( -[6]F) -C ( -[2]F) ( -[6]F) -[@{right,0.5},1] } \polymerdelim[delimiters ={[]}, height = 45pt, depth = 40pt, indice = n]{upleft}{upright} \schemestop& Pfannenbeschichtung (Teflon), Funktionskleidung (Goretex) \end{tabularx}\end{document}