It’s fair to assume that because the abstract and introduction are the first chapters to be read by someone reading your dissertation, it means they must be written first also. But in reality, this isn’t the case. You’ll actually be far better off writing your dissertation introduction, conclusion and abstract after you have written all the other parts of the dissertation.
Firstly, writing retrospectively means that your write a dissertation introduction and conclusion will ‘match’ and your ideas will all be tied up nicely.
Secondly, it’s time-saving. If you write your introduction before anything else, it’s likely your ideas will evolve and morph as your dissertation develops. And then you’ll just have to go back and edit or totally re-write your introduction again.
Thirdly, it will ensure that the abstract accurately contains all the information it needs for the reader to get a good overall picture about what you have actually done.