Polyethylene may be considered either as -[CH-CH-]- with a repeat unit of -[CH-CH]-, or as [-CH-]-, with a repeat unit of -[CH]-. Chemists tend to consider the repeat unit as -[CH-CH]- since this polymer is made from the monomer ethylene (CH=CH).
More complex repeat units can occur in vinyl polymers -[CH-CHR]-, if one hydrogen in the ethylene repeat unit is substituted by a larger fragment R. Polypropylene -[CH-CH(CH)]- has the repeat unit -[CH-CH(CH)]. Polystyrene has a chain where the substituent R is a phenyl group (CH), corresponding to a benzene ring minus one hydrogen: -[CH-CH(CH)]-, so the repeat unit is -[CH-CH(CH)]-.