  Dimerized quantum spin systems may appear under several circumstances, e.g\
by a modulation of the antiferromagnetic exchange coupling in space, or in
frustrated quantum antiferromagnets. In general, such systems display a quantum
phase transition to a N\'eel state as a function of a suitable coupling
constant. We present here two path-integral formulations appropriate for spin
$S=1/2$ dimerized systems. The first one deals with a description of the dimers
degrees of freedom in an SO(4) manifold, while the second one provides a
path-integral for the bond-operators introduced by Sachdev and Bhatt. The
path-integral quantization is performed using the Faddeev-Jackiw symplectic
formalism for constrained systems, such that the measures and constraints that
result from the algebra of the operators is provided in both cases. As an
example we consider a spin-Peierls chain, and show how to arrive at the
corresponding field-theory, starting with both a SO(4) formulation and
bond-operators.
