transforming a list into a string

[jblazi]

Let us assume I have a list like

['1','2','7','8','12','13]

and would like to transoform it into the string

'{1,2},{7,8},{12,13}'

Which is the simplest way of achiebing this? (The list is in fact much
longer and I may have to cut the resulting strings into chunks of 100 or
so.)

TIA,

jb