AS Chemistry 9701: Addition Polymers, Repeat Units and Disposal Hazards

Methyl 2-methylpropenoate, $\text{CH}_2=\text{C}(\text{CH}_3)\text{COOCH}_3$ , is the monomer used to manufacture a transparent addition polymer marketed as acrylic sheeting, widely used for aquarium windows and aircraft canopies.

(a) State the type of polymerisation involved, and identify the structural feature of the monomer that makes this type of polymerisation possible. [2]

(b) Deduce the repeat unit of this addition polymer, showing all atoms and bonds clearly. [2]

(c) A section of a different addition polymer chain is represented by the repeat unit $-\text{CH}_2-\text{CHCl}-$ . Name the monomer used to form this polymer, and give its structural formula. [2]

(d) Both of these poly(alkene)s resist natural degradation. Explain why they are non-biodegradable, and state one additional hazard associated with disposing of the polymer in (c) by incineration, that does not apply to the polymer in (b). [2]

Part (a): Type of polymerisation

Methyl 2-methylpropenoate contains a carbon–carbon double bond, $\text{C=C}$ . In addition polymerisation, this double bond opens up (the $\pi$ -bond breaks), and each monomer forms new single bonds directly to its neighbours on either side, with no atoms or small molecules lost in the process.

Part (b): Deducing the repeat unit

Redraw the monomer with the double bond as two single bonds, ready to connect to the next unit on each side:

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

Every atom from the monomer is kept. The methyl group and the ester side-group, $-\text{COOCH}_3$ , both remain attached to the same backbone carbon they started on. The repeat unit is:

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

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

Given the repeat unit $-\text{CH}_2-\text{CHCl}-$ , reverse the process by turning the backbone single bonds back into a double bond:

$-\text{CH}_2-\text{CHCl}- \;\longrightarrow\; \text{CH}_2=\text{CHCl}$

The monomer is chloroethene, structural formula $\text{CH}_2=\text{CHCl}$ (this addition polymer is poly(chloroethene), commonly known as PVC).

Part (d): Biodegradability and disposal hazards

Both polymers share a carbon–carbon backbone made of strong, non-polar covalent bonds. Water molecules and the enzymes produced by microorganisms cannot attack these bonds, so neither polymer breaks down naturally. Both are non-biodegradable.

However, incinerating poly(chloroethene) is more hazardous than incinerating the acrylic polymer from (b): because it contains chlorine, burning it releases hydrogen chloride gas, which is toxic and corrosive (and incomplete/uncontrolled combustion can also generate trace toxic chlorinated by-products). The acrylic polymer, containing only carbon, hydrogen and oxygen, does not release this additional hazardous gas on combustion.

Final answers

(a) Addition polymerisation; possible because of the monomer’s $\text{C=C}$ double bond, which opens with no atoms lost.
(b) Repeat unit: $\left(-\text{CH}_2-\text{C}(\text{CH}_3)(\text{COOCH}_3)-\right)_n$
(c) Monomer: chloroethene, $\text{CH}_2=\text{CHCl}$
(d) Non-biodegradable due to the inert, non-polar $\text{C-C}$ backbone; poly(chloroethene) additionally releases hydrogen chloride gas on incineration.