AS Chemistry 9701: Poly(propene), Poly(tetrafluoroethene) and Classifying Polymerisation Type

(a) Propene, $\text{CH}_2=\text{CHCH}_3$ , is polymerised by addition polymerisation to form poly(propene). Deduce the repeat unit of poly(propene). [2]

(b) A section of a different addition polymer chain is represented by the repeat unit $-\text{CF}_2-\text{CF}_2-$ . Identify, by name and structural formula, the monomer used to form this polymer. [2]

(c) For each of the following, state whether it describes a monomer, or pair of monomers, suited to addition polymerisation or to condensation polymerisation, giving a reason based on the functional group(s) present:

(i) but-1-ene, $\text{CH}_2=\text{CHCH}_2\text{CH}_3$ , on its own. [1]

(ii) hexanedioic acid, $\text{HOOC}(\text{CH}_2)_4\text{COOH}$ , together with hexane-1,6-diamine, $\text{H}_2\text{N}(\text{CH}_2)_6\text{NH}_2$ . [2]

Part (a): Repeat unit of poly(propene)

Propene, $\text{CH}_2=\text{CHCH}_3$ , has a terminal double bond between a $\text{CH}_2$ carbon and a $\text{CH}$ carbon that also carries a methyl group. Opening the $\pi$ -bond and connecting the two backbone carbons to their neighbours gives:

$\text{CH}_2=\text{CHCH}_3 \;\longrightarrow\; -\text{CH}_2-\text{CH}(\text{CH}_3)-$

The repeat unit of poly(propene) is:

$\left(-\text{CH}_2-\text{CH}(\text{CH}_3)-\right)_n$

Part (b): Identifying the monomer from a repeat unit

The given repeat unit $-\text{CF}_2-\text{CF}_2-$ has two fluorine atoms on each backbone carbon. Reversing the addition process by turning the two backbone single bonds back into a double bond regenerates all four fluorines on the original alkene:

$-\text{CF}_2-\text{CF}_2- \;\longrightarrow\; \text{CF}_2=\text{CF}_2$

The monomer is tetrafluoroethene, $\text{CF}_2=\text{CF}_2$ (this addition polymer is poly(tetrafluoroethene), known commercially as PTFE).

Part (c): Classifying monomers by polymerisation type

(i) But-1-ene alone. This molecule contains a $\text{C=C}$ double bond and no other functional group capable of reacting further. With only one type of reactive site and no small molecule available to eliminate, but-1-ene can only undergo addition polymerisation.

(ii) Hexanedioic acid with hexane-1,6-diamine. Neither monomer contains a $\text{C=C}$ bond, so addition polymerisation is not possible. However, the dicarboxylic acid’s two $-\text{COOH}$ groups can each react with an $-\text{NH}_2$ group of the diamine, forming an amide bond and eliminating a molecule of water at each junction:

$-\text{COOH} + \text{H}_2\text{N}- \;\longrightarrow\; -\text{CO-NH}- + \text{H}_2\text{O}$

Because both monomers are difunctional, this repeats along the chain. This pair of monomers is suited to condensation polymerisation, forming a polyamide.

Final answers

(a) Repeat unit of poly(propene): $\left(-\text{CH}_2-\text{CH}(\text{CH}_3)-\right)_n$
(b) Monomer: tetrafluoroethene, $\text{CF}_2=\text{CF}_2$
(c)(i) Addition polymerisation (single $\text{C=C}$ bond, no second functional group)
(c)(ii) Condensation polymerisation (amide bonds form between $-\text{COOH}$ and $-\text{NH}_2$ groups, eliminating water)