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As far as I understand, Polymers are long molecules made of smaller monomers which are Alkenes. But polymers don't have double bonds, so they are saturated. What is the difference between Alkanes and Polymers structures.

The first one is an Alkane, but if the chain was much longer, it would be structurally the same as the Polymer (Second image).

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    $\begingroup$ Polymers aren't necessarily made from alkenes ! What is more, you can have all kinds of functions on polymers. $\endgroup$ – Hippalectryon Mar 19 '16 at 17:17
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    $\begingroup$ Some polymers are by definition alkanes. The difference is basically size. We wouldn't normally consider something a polymer without a molecular weight of at least 1000 g/mol. However this is an arbitrary distinction. $\endgroup$ – Lighthart Mar 19 '16 at 19:34
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Some points to note: You said polymers are saturated. This generalization is not true in many cases, e.g. polystyrene, nylon, and much more. Alkanes are always saturated.

Alkanes can have branchings. The ones you have in mind are called normal alkanes, in which no carbon has higher order than two. An alkane can have any kind of branching, as long as it is saturated.

Now, maybe the most important one: polymers are made of so-called monomers. All polymers can be constructed from exactly one monomer (even co-polymers). An alkane which is sort-of tree-like randomly branched, will not have such an unit.

A normal alkane has some similarities with polymers, tho. As others mentioned it, the size is the deciding factor there. A normal alkane with a carbon count of 10000 is for sure a polymer.

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There are two types of common polymers that are basically just alkanes with very long chains: polyethylene and polypropylene. But there are a large number of other polymers that are made of much more complex parts.

The essential differences between a polymer like polypropylene or polyethylene and an alkane are the size of the molecule and the purity of the molecules. Polymers are big, really big, often containing from thousands to hundreds of thousands of carbons. And, they are often made of mixtures rather than a pure alkane of a fixed number of carbons. These properties arise from the mechanisms of their creation which usually involve long chains of reaction where small molecules (monomers such as ethylene or propylene) are joined together often via a radical reaction to create long chains. These reactions can't be controlled precisely enough to give a single product so a mix of products with a distribution of chain lengths and "branchiness" usually results. To some extent the branchiness can be controlled and this yields different grades of polymer (some clever catalysts can give much more control of chain length).

The properties of polymers differ from those of alkenes mainly because of the size of the molecules. Though you can get some idea of the types of property to expect by following the properties of progressively larger alkanes. Small alkanes are gases, middle-sized alkanes are volatile liquids, larger alkanes (dozens of carbons) are often waxy solids. Polyethylene is just like an extreme version of the waxy solid (depending a little on how it is processed).

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