This lecture summarizes some of our recent results about reactions with synthetic polymers. By cationic ring-opening polymerization o f 4 H , 7H-1,3-dioxepin a linear polyacetal containing one C-C-double bond per repeating unit is obtained. Copolymerization o f this monomer with trioxane o r dioxolane is possible. Various addition reactions can be carried o u t with these unsaturated polyacetals, polymeric epoxides being o f special interest. Some other unsaturated cycloacetals a r e briefly mentioned. Amide moieties in linear polyamides can be modified by different reactions. Preparation and properties o f poly-N-chloroamides and poly-N-trifluoroacetylamides a r e described. By reaction o f metalated polyamides with alkyl halides poly-N-alkylamides are available. Anionic grafting reactions can also be carried out. Reduction o f the amide groups leads t o the formation o f polyiminoalkylenes. By topochemical polymerization o f esters o f 10,12-docosadiyne-l,22diol soluble polydiacetylenes are obtained. These polymers react in homogeneous solution with SbC15 o r FeC13. Furthermore the hydrolysis o f the ester groups and the reacylation is described.