Everything from the 1 lessons of this topic on a few pages: the key idea of each lesson, the diagram to remember, the points to learn and the traps examiners set. Read it, cover it, test yourself.
In addition polymerisation many alkene molecules (the monomers) join to form one very long saturated chain (the polymer). The weaker π bond of each C=C breaks and the electrons form new C–C σ bonds to neighbouring monomers, so nothing else is produced and the polymer has the same empirical formula as the monomer. The repeat unit is the monomer with its C=C turned into C–C and a bond sticking out from each end of the 2-carbon backbone, written in brackets with a subscript n. To find the monomer from a chain, mark off 2-carbon units along the backbone and put the double bond back. Poly(alkenes) are saturated with strong, non-polar C–C and C–H bonds, so they are unreactive and non-biodegradable: they persist in landfill, and burning them releases CO, CO2 and, from PVC, toxic HCl.
Poly(ethene) is saturated, so never draw a C=C inside a repeat unit, and never write “polyethene contains double bonds”. Only the two C=C carbons form the backbone; side groups such as CH3 or Cl hang off it, so the repeat unit always has a 2-carbon backbone (per monomer) with bonds through the brackets and an n. Addition polymerisation produces no by-product such as water. Poly(alkenes) are non-biodegradable because they are unreactive, not because they are toxic, and it is specifically PVC that gives HCl on burning.