No, he didn't.
In Metaphysical Foundations of Natural Science, he assumes as an empirical premise a notion of matter, which he analyzes to derive certain, in his view important, insights about Newtonian physics.
This work isn't really a work of 'transcendental philosophy', as you might think of it, but rather a rational reconstruction of a branch of physics (like what Carnap executes in his Aufbau, but without Carnap's phenomenalism, and written in a way which makes it unreadable).
The way he describes his enterprise (as a "construction") is confusing, perhaps because Kant was himself confused about what he was doing in the work (I myself think this is likely, and I believe he might have realised this problem later in the Opus Postumum, because it is known he became dissatisfied with his Metaphysical Foundations, recognizing a 'gap' in his critical enterprise). Hegel brilliantly, albeit too briefly, analyzes Kant's work in his Science of Logic (in the first part, i.e. The Doctrine of Being) and in his lectures concerning the history of philosophy.
Two great quotes from Hegel's Science of Logic:
Kant's method in the deduction of matter from these forces, which he
calls a construction, when looked at more closely does not deserve
this name, unless any exercise of reflection, even analytical
reflection, is to be called a construction; and later philosophers of
nature have in fact given the name of construction to the shallowest
reasoning and the most baseless concoction of unbridled imagination
and thoughtless reflection — and it is especially for the so-called
factors of attraction and repulsion that such philosophers have shown
a predilection.
For Kant's method is basically analytical, not constructive. He
presupposes the idea of matter and then asks what forces are required
to maintain the determinations he has presupposed. Thus, on the one
hand, he demands the force of attraction because, properly speaking,
through repulsion alone and without attraction matter could not exist;
and on the other hand he derives repulsion, too, from matter and gives
as the reason that we think of matter as impenetrable, since it
presents itself under this category to the sense of touch by which it
manifests itself to us. Consequently, he proceeds, repulsion is at
once thought in the concept of matter because it is immediately given
therein, whereas attraction is added to the concept syllogistically.
But these syllogisms, too, are based on what has just been said,
namely, that matter which possessed repulsive force alone, would not
exhaust our conception of matter.
And one quote from Lectures on the History of Philosophy:
For Kant assumes all such conceptions as that matter has motion and
also a power of attraction and repulsion, instead of demonstrating
their necessity. The Principles of Natural Philosophy have
nevertheless been of great service, inasmuch as at the commencement of
a philosophy of nature, attention was called to the fact that physical
science employs thought-determinations without further investigation;
and these determinations constitute the real foundations of its
objects.
Michael Friedman, who wrote a whole book on Kant's Metaphysical Foundations (Kant's Construction of Nature) writes:
This emphatically does not mean, however, that he attempts to demonstrate by
pure reason, independently of experience, what Newton has discovered by observation
and experiment. The point is rather, on my reading, that Kant attempts
to isolate just those features of the concept of matter in virtue of
which Newton has successfully mathematized its quantity, and he
analyzes or explicates this concept (in terms of motion, the filling
of space, inertia, and so on) precisely to reflect these features.
As Friedman explains elsewhere, for Kant, Newton's laws play a certain a priori necessary functional role within the system of physics (roughly, they correspond to the three analogies of the Critique of Pure Reason), and only in this sense they're a priori.
But Kant doesn't attempt to demonstrate that Newton has the last word in the game of physics. In this sense his analysis is contingent, a posteriori, because other laws might play the equivalent role in some other theory.
