Is it appropriate to use "so much so" at the beginning of a sentence?

She is terrified of flying…so much so that often she has to fight an inner turmoil even to step on the plane.

So much so that Lenin commissioned a leading party organizer and Georgian, Josef Stalin, to write a counter-attack (Both are from BNC).

I favored "so much so" in the first sentence more than that in the second. I think it is not good to start a sentence with "so much so".

Solution 1:

So much so that Lenin commissioned a leading party organizer and Georgian, Josef Stalin, to write a counter-attack (Both are from BNC).

This particular sentence would be considered incorrect. It would be classified as a sentence fragment and needs something before the "so much so".

Your first example was the correct usage:

She is terrified of flying…so much so that often she has to fight an inner turmoil even to step on the plane.

Do note that in an informal context you can split these two parts into different sentences:

She is terrified of flying. So much so, that often she has to fight an inner turmoil even to step on the plane.

This deliberately breaks the rule about sentence fragments as a conscious stylistic choice. This is common in fictional literature and would not be recommended in a professional or academic context.

Solution 2:

The first rule of style is not to let the so-called hard-and-fast rules of grammar kill your prose. As long as the context is laid out well before using so much so, either in continuation or as the first words of a new sentence, I do not think you have a problem to fret over. That goes for academic writing as well, so much of it is written often in tedious and boring form.