transplant big plants
Is there any chemical substances I can use to maintain the roots while transplanting a big tree?
Transplanting large trees is done successfully. Some expertise may be required for very large trees.
In general, make sure the tree has been well-watered during the period prior to digging it up. It's important to keep the root ball intact as well. This can be difficult for a do-it-yourselfer, especially if the tree is large. Soil weighs A LOT. Timing is also important. Fall is a good time as once transplanted, the tree can use all its energy in making roots and getting re-established.
I also advocate for root pruning prior to the move. There's a pretty good article on this and moving large trees here: https://www.washingtonpost.com/lifestyle/home/how-to-move-a-prized-tree/2013/10/15/3c87faf4-2b91-11e3-8ade-a1f23cda135e_story.html
As far as things to use to help in the process, the use of anti-dessicants can sometimes help, especially on broadleaf evergreens or evergreens. These substances will not make a bad transplanting job (see all the above items) successful. They just help.
Hope this is useful info for you.