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Is there a specific rule on how and when to use に after a 的 adjective to make it function adverbially?
For example, 具体的 seems to always be followed by に before verbs, i.e. 具体的に話してほしい "Please speak concretely."
However, I was told that the phrase 比較的に読みやすい "Relatively easy to read." is unnatural, and one should write 比較的読みやすい instead.
比較的 is used as an adverb by itself and not normally used as an adjective. (That’s except, of course, when 比較的な is used in the sense of 比較のような or 比較みたいな in very colloquial speech.) In fact, the word is listed as an adverb in dictionaries, unlike 具体的 and most other words that end with 的, which are all listed as な-adjectives, or 形容動詞.
可及的 is the only other adverb as far as I can find in the dictionaries I have access to on my computer or online. But I don’t think I have ever seen or heard this word used without being followed by 速やかに. It is almost like 可及的速やかに is one adverb.
Some of the dictionaries have an entry for [快快的]{カイカイデー}. It seems this Chinese expression was used in Japan in the Taisho era. Nobody uses it any longer. (If it is an adverb, I would think it should be written as 快快地, though...)
So, practically speaking, 比較的 seems to be the only exception.
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