The definite article is one of the things that make English so fascinating for me, just as it’s the partitive article du/de la/des that does it for French. So here’s a para that I was about to misinterpret for a second, and then after rereading, got the meaning of:

A few words need to be said about the reason for the macro-phase separation. On the onset of postcrosslinking, the entire polymer solution is transformed into a gel. Still, full conversion of the bifunctional crosslinking agent requires time in the two-step Friedel–Crafts reaction. After attaching to one polymeric chain, most bifunctional reagent molecules have to spend some time, until through fluctuations, an aromatic ring of another chain approaches the reagent’s pending second functional group to a distance suitable for reaction.

I placed a query to the author for “have to spend some time,” that does he mean “have to spend some time idle,” which I think is what he most likely meant. As I didn’t have the opportunity to proofread this one, don’t know what the author replied. I of course didn’t want to make changes myself without confirmation from the author. Now the interesting thing is that last bit of phrase highlighted. First, I thought the author means to refer to the Friedel–Crafts reaction once again, and I was about to change to “to a distance suitable for the reaction.” But, an instinct stopped me, thankfully! I grasped that oh, the author simply means that as the ring approaches the functional group nearer, reaction can now happen: the probability of a reaction happening (to what kind of reaction does it belong to doesn’t matter, just that a reaction, an interaction, could).