My supervisor told me that it is not required to publish my papers before submitting my thesis. We are required (in German universities), however, to published our monograph thesis later as a book. One of my chapters is already published as a book chapter in Routledge (it's too good an opportunity to miss). The published chapter is not entirely the same as my thesis chapter, as the Routledge editor edited it too (asking me to add this and that, which I did). Now, another chapter of my thesis can be published in a conference proceeding too, if I so wish. I understand that this is common practice for a cumulative thesis, but can it also be done with a monograph thesis?
From what I can think of there are two potential issues.
First is the copyright issue. In my previous university, for example, when each of the chapters could be either a conference or a journal paper, the university still could publish it online, but it was a grey area. But if you wish to publish your whole thesis to a new publisher as a monograph but excerpts of it, in the form of chapters, are already published with another publisher, then I see a conflict, if you have signed the copyright form. So you need to read on what you have signed, particularly if you are dealing with two different publishers.
The second problem, which could arise even if you are publishing with the same publisher will arise in the form of self-plagiarism. The plagiarism detecting software will obviously flag already published chapters as "copied" and your thesis might not be able to cross the threshold to be accepted for publication. Again, this might require you to convince the editor.
This completely depends on the copyright documents of the publishers which usually vary. So it is necessary to go through that.
Personally, as an aside, I sometimes feel dismayed at the current state of academia when researchers are compelled to look for avenues to regurgitate their work like this. Marketing has taken more prominence over doing original research.
You need to ask the relevant people and sources for the information you need.
- Ask your advisor what he will accept for a PhD thesis. I suppose having the chapters accepted in peer-reviewed journals will be welcomed as it proves a high quality standard.
- Read the copyright agreements of the journals. Usually you transfer the copyright to the journal, but most often they grant you the right to use the paper in your thesis. But you only know it for sure when you read the agreement that you accept(ed) when publishing it there.
In most places, PhD theses whose chapters are simply reproductions of work published elsewhere is acceptable. At the very least that's the case when you just publish the thesis yourself -- the situation might be different if you publish with a company that will likely insist on transferring your copyright to them.
Regardless of this, as in most of the other questions on this forum, the right answer is almost always "Talk to your adviser".