Does Mars contain the materials required to make plastic? If not, can they relatively easily be made, or an adequate substitute found?
Yes - for silicone plastics.
Mars does have plenty of silica (aka sand) so there are the basic materials to synthesize silicone - which can be used in place of most organic plastics. (Often performing better than conventional carbon-based plastics.)
Methane, chlorine, water, and carbon-dioxide all exist on the surface as well. It should be stated that this is speculative - the exact industrial processes to create plastics in any quantity given the Martian resources is far beyond me at this point. However, the raw elements seem to be available.
The key material to produce to make plastics is the production of ethylene, which is $C_2H_4$. According to The Case for Mars, this can be produced by the reaction $2CO+4H_2 \rightarrow C_2H_4+2H_2O$, with the presence of an Iron catalyst.. And the carbon monoxide comes from $6H_2+2CO_2 \rightarrow 2H_2O+2CO+4H_2$. Thus, the key to making plastics on Mars is Carbon Dioxide and Water. We know both materials exist on Mars, thus, it should be easy enough to make plastics, given the right catalysts.
Scientists at Waseda University discovered a new OCM reaction mechanism occurring at a temperature as low as 150 degrees C. The novel catalytic reaction, which demonstrated both high yield and catalytic activity, was done in an electric field, and could provide a more cost-effective method of synthesizing ethylene in the future. The findings were published in the Journal of Physical Chemistry C on January 22, 2018.
"Performing OCM in an electric field dramatically lowered the reaction temperature, and we succeeded in efficiently synthesizing C2 hydrocarbon, including ethylene, from oxygen in the atmosphere with methane," says Shuhei Ogo, assistant professor of catalytic chemistry at Waseda.
The article links to Shuhei Ogo et al, Electron-Hopping Brings Lattice Strain and High Catalytic Activity in the Low-Temperature Oxidative Coupling of Methane in an Electric Field, The Journal of Physical Chemistry C (2018). DOI: 10.1021/acs.jpcc.7b08994
above: Reaction mechanisms for the oxidative coupling of methane (OCM) over Ce2(WO4)3 catalysts at low temperatures in an electric field. From here. Credit: Waseda University
The earlier, Phys.org article A radical solution comes from mixing tools says:
The molten surface of a sodium-based material could assist the direct conversion of methane to useful building blocks.
Free radicals, molecules with an unpaired valence electron, such as the hydroxyl radical, play a crucial role in the industrially important conversion of natural gas, primarily methane, to ethylene: a vital organic compound that forms the building blocks of many commodities and polymers. To enhance this process, known as oxidative coupling, it is vital to develop selective catalysts.
The KAUST team—led by Kazuhiro Takanabe and his student Abdulaziz Khan—used in situ tools to measure the state of a catalyst under reaction conditions. They discovered that the active species in the catalytic reaction that exits on the molten surface of the sodium tungstate, a chemical necessary for the reaction, is sodium peroxide. This catalyst is unique in that instead of activating methane directly, it firsts activates water and then generates gaseous hydroxyl radicals.
The article links to Kazuhiro Takanabe et al. Integrated In Situ Characterization of a Molten Salt Catalyst Surface: Evidence of Sodium Peroxide and Hydroxyl Radical Formation, Angewandte Chemie International Edition (2017). DOI: 10.1002/anie.201704758