Is it permitted to answer to kaddish if one is saying brachos of shema?
This would depend at what point you are holding. In between Brachos, and in between the paragraphs of Shema is known as “bein haprakim”. At these points one may answer Amen to any bracha he hears. when in the middle of a bracha or paragraph of Shema one may only answer Amen to the bracha of ה-ל הקדוש and of שומע תפילה.
However with regards to answering Amen to kaddish which is not a bracha, we do not differentiate between the two, and in both instances one may only answer Amen Yehei Shmei Rabba, and Amen after דאמירן בעלמא ואמרו [see Mishna Brura 66:17].