For me, dialectic is expressed in two different senses, and each sense can be subdivided into homological pairs of opposing dispositions, based on either dialogic or monologic orientations.
One sense involves successive interlocution with a hypothetical interlocutor. This is the “constructive” sense of dialectic which merely mimics, in impure form, the form of expression peculiar to dialogue, wherein the speaker advances an agenda through the useful foils provided by another character. This constructive sense can be subdivided into a more dialogic disposition wherein the hypothetical interlocutor actually maintains a semblance of self-identity (think of Socrates in Plato’s dialogues) and a more monologic orientation wherein the interlocutors are really momentarily alienated instances of the protagonist himself (think Hegel’s Phenomenology of Geist).
The other sense involves the systematic auto-critique of all possible forms of stable truth for a given reality. This is the “destructive” sense of dialectic which fully embodies, in pure form, the form of expression peculiar to dialectic, wherein the incoherence of a given reality or proposition is made clear by appealing to the fact that the consequences of those propositions contradict the premises assumed by that very proposition, making that proposition fundamentally self-undermining. This destructive sense can be subdivided into a more dialogic disposition where a dialectic is explicated for the reception of a wider audience (Marx’s Das Kapital and Nāgārjuna’s Mūlamadhyamakakārikā are good examples) and a more monologic disposition where the subject who proceeds by means of dialectic and the object which is being immanently critiqued are one and the same thing (monologic dialectic is one of the exalted techniques of the Prāsaṅgika-Madhyamaka school of Buddhist orthopraxy, which is used to induce a meditative state of awareness that is free of the hypostatizing tendency of the deluded mind to reify abstract percepts as if they were concrete realities).
I think it is fair to say that monologic dialectic is the “purest” form of dialectic in that it does not pretend to be what it’s not (has not one iota of dialogic), tolerates no mediation between itself and its object through another, and fulfills itself at every momentary instance of its operation; for these reasons, it is monologic dialectic that best bears the title of “organon of extinction,” since it brings a complete end to all hubristic thought. In this way, we can understand that monologic dialectic fulfills its role as that which makes space for genuinely communicative dialogue to emerge freely without obstruction, because it would have obliterated all obstructions to the clarity of mind.