Sharp Over Range
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Two questions of Polymerisation of Alkenes?
I'm doing some reseach, and I'm having trouble trying to find the answers to the following questions. These are A-Level Standard, so those of you under 16 won't know the answers - please only answer if you are an expert in this area!
What is the empirical formula, with reason, of polypropene (I think it is C3H6!)
and
"Suggest why polypropene does not have a sharp melting temperature, but softens over a range of temperature"
Thanks,
Tom
Sharp melting temperatures are a characteristics of those substances which have a definite fixed structure, with always the same number of atoms and bonds throughout. This description would includes almost all substances we know because all have a fixed molecular mass; but this doesn't apply to polymers, because the polymers chains produced in the industry are of varying lengths - they would have the same characteristic repeating unit throughout the chain; but their chain lenghts can vary from a few hundred single units (joined together) to tens of thousands of them. A polymer with a short chain length will have less surface area - therefore less contact - and consequently less number of weak vanderwaal forces among them. A less quantity of these forces in a polymer would require less energy to be broken and hence have a less melting temperature (and vice versa). So polypropene would "soften over a range of temperatures" until all the long and short- chained polymer structures melt out.
As for the empirical formula, yes I guess you're right - though it's better to write it as (C3H6)n, where 'n' denotes many repeating units. I thought it would be CH2 if we simplify C3H6 further, but then CH2 would be the empirical formula for propene. (as in any homologous series)
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