Polyesters containing double bonds are often crosslinked by reacting the polymer with styrene. a. Draw the structure of the copolymer of \(\mathrm{HO}-\mathrm{CH}_{2} \mathrm{CH}_{2}-\mathrm{OH} \quad\) and \(\quad \mathrm{HO}_{2} \mathrm{C}-\mathrm{CH}=\mathrm{CH}-\mathrm{CO}_{2} \mathrm{H}\) b. Draw the structure of the crosslinked polymer (after the polyester has been reacted with styrene).

First, we draw the structures of the two monomers. 1. Monomer 1: \(\mathrm{HO}-\mathrm{CH}_{2} \mathrm{CH}_{2}-\mathrm{OH}\) (this monomer consists of a diol with hydroxyl groups on each end) 2. Monomer 2: \(\mathrm{HO}_{2} \mathrm{C}-\mathrm{CH}=\mathrm{CH}-\mathrm{CO}_{2} \mathrm{H}\) (this monomer is a dicarboxylic acid with a double bond)

To create the copolymer structure, we can combine the two monomers using esterification reaction mechanism between hydroxyl groups (from monomer 1) and carboxylic acid groups (from monomer 2). The ester bond is formed via the elimination of a water molecule from the reaction. The copolymer structure: \(\mathrm{OOC}-\mathrm{C}(\mathrm{HO})=\mathrm{CH}-\mathrm{COO}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{O}\)

In order to draw the structure of the crosslinked polymer, we need to understand the structure of styrene. Styrene: \(\mathrm{C}_{6}\mathrm{H}_{5}-\mathrm{CH}=\mathrm{CH}_{2}\)

For crosslinking, the double bond present in the copolymer will react with the double bond present in the styrene via free radical polymerization. Crosslinking occurs when the styrene molecules react with multiple double bonds along the copolymer chain. To create the crosslinked polymer structure, we can combine the copolymer (from step 2) and styrene (from step 3). In this reaction, the radical from styrene will bind to the double bond present in the copolymer, joining two copolymer chains to form a 3D structure. Crosslinked points will occur wherever a styrene molecule reacts with the double bonds along the copolymer chains. The crosslinked polymer will look like this: \(\mathrm{OOC}-\mathrm{C}(\mathrm{HO})=(\mathrm{C}_{6}\mathrm{H}_{5}-\mathrm{CH}_{2}-\mathrm{C})(\mathrm{OOC}-\mathrm{C}(\mathrm{HO})=\mathrm{CH}-\mathrm{COO}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{O})(\mathrm{CO}_{2}\mathrm{H})\) Here, the styrene (\(\mathrm{C}_{6}\mathrm{H}_{5}-\mathrm{CH}_{2}-\mathrm{C}\)) is the crosslinked point between the copolymer chains.