# Why must polymers have a repeating unit?

In organic chemistry, we learned that small molecules can form a polymer via a process called polymerization. For example, $\ce{CH2=CH-Br}$ molecules can form the polymer

\begin{align} \ce{nCH2=CH-Br-> -[-CH2 -}&\ce{CH -]_n -}.\\ &\;\ce{|}\\ &\ce{Br} \end{align}

So the polymer is a periodic chain $\ce{-CH2-CHBr-CH2-CHBr-\cdots}$. But since every monomer can have two orientations ($\ce{-CH2-CHBr -}$ or $\ce{-CHBr-CH2 -}\!$), there is no requirement that all monomers must be in the same orientation, does the polymer have to be a peroidic chain? Can it be a random chain with a structure that looks like

$$\ce{-CH2-CHBr-CHBr-CH2-CHBr-CH2-CH2-CHBr-CH2-\cdots}\,?$$

Most textbooks emphasize that $n$ is random, but still assume that the unit repeats.

• I recommend you look at the roots of ‘polymer’ poly means many, mer means segment, mono means one. Monomer - one segment. Polymer - many segments. – JSCoder says Reinstate Monica Jan 16 '18 at 3:36
• @JavaScriptCoder, the discussion is periodic vs random. I'm not sure if a random chain is actually possible, or it is simply energetically not favorable on the scale of thermal fluctuations $k_BT$. – Zhuoran He Jan 16 '18 at 4:14
• honestly I can’t see why it couldn’t be random, I mean DNA could be random and still considered a polymer – JSCoder says Reinstate Monica Jan 16 '18 at 4:18
• Random chains are very much possible. Then again, this particular one is probably repeating. – Ivan Neretin Jan 16 '18 at 4:20
• Well, that's what happens in practice, too. Related reading: en.wikipedia.org/wiki/Tacticity. – Ivan Neretin Jan 16 '18 at 5:45

• I know the chain is not straight. In a simplified model that only cares whether adjacent monomers have the same or opposite orientations, the rotational degrees of freedom of the sp3 bonds would be uncoupled to the ordering of the orientations. To make the mean defect-free length long, we need to reduce $\exp(-\Delta E/k_BT)$ by increasing $\Delta E$, the energy cost of having a defect. If $\Delta E=10k_BT$, then the defect-free length could be $\sim 10^4$. – Zhuoran He Jan 16 '18 at 21:01
• I think they still can reach equilibrium during the reaction. You see the number $n$ can randomly change. What this means is that the bonds between monomers are forming and breaking all the time. To preserve the defectless chains sifted from the reaction system, the temperature would then have to be lowered. – Zhuoran He Jan 17 '18 at 16:11