This is an extended commentary to the question.
In the beginning of Matter and Memory Bergson clearly declares his dualism:
This book affirms the reality of spirit and the reality of matter...
It is, then, frankly dualistic. (p.XI)
In principle, this should prohibit any possibility of physicalist development. However, a literal understanding of his writings may lead to confusion because one can find in the book many statements like these:
...we compress within its narrowest limits the problem of the union of
soul and body... (p.325)
...we can conceive an infinite number of
degrees between matter and fully developed spirit... (p.296)
They depart apparently from his declared dualist position and evoke some questions about the mutual independency of spirit and matter.
To resolve this difficulty V. Delbos had proposed an interpretation based on the analysis of this book and of another of Bergson's works, Time and Free Will (Essay):
Commenting on this discussion, Victor Delbos noted in his 1897 review
that Matter and Memory allows us 'to surmount the dualism with which
the Essay had been content and which here [in Matter and Memory] is
conceived only as a critical procedure resulting in a provisional
conclusion'. The real conclusion is memory, or more precisely,
duration, understood as a sort of monistic substance where substance
itself is not understood as something stable but rather as unstable
differentiations of spirit into matter. (Lawlor L. The Challenge of
Bergsonism - Phenomenology, Ontology, Ethics. London: Continuum, 2003,
p.XIII)
One can say that the dualism of Bergson is not a Cartesian clear-cut dualism. It is something more nuanced. According to his views, the whole world is "made from the same stuff". This, probably, could open a way to a physicalism.
However, Bergson was strongly opposed to any attempt to reduce mental phenomena to material movements. That opposition he had justified in various ways. Perhaps, it can be demonstrated based on the very special and decisive role that time plays in his philosophy. First, he states
Questions relating to subject and object, to their distinction and
their union, should be put in terms of time rather than of space.
(p.77)
This means, that all the existence of a spirit or consciousness with all the phenomena is within time, and if we try to concentrate or imagine this existence exclusively in the present moment, it dissolves completely. Matter, in its turn, is completely immersed in the present moment, and time does not bring anything to its being:
If matter does not remember the past, it is because it repeats the
past unceasingly, because, subject to necessity, it unfolds a series
of moments of which each is the equivalent of the preceding moment and
may be deduced from it: thus, its past is truly given in its present.
(p.297);
One can say, that they are merely extremities, limiting cases, abstractions, and, indeed, Bergson prefers to speak about an infinite number of degrees between them (for example, living beings), that fill the world. He had even speculated that
...the material universe itself is a kind of consciousness, a
consciousness in which everything compensates and neutralizes
everything else, a consciousness of which all the potential parts,
balancing each other by a reaction which is always equal to the
action, reciprocally hinder each other from standing out.(p.313)
Anyway, from his point of view there is a principal obstacle to any possibility to reconstruct a conscious life from physical ingredients of inert matter because the latter is alien to progression of time. Therefore, it is hard to imagine that Bergson was a physicalist.
