  Noncatastrophic encoders are an important class of polynomial generator
matrices of convolutional codes. When these polynomials have coefficients in a
finite field, these encoders have been characterized are being polynomial left
prime matrices. In this paper we study the notion of noncatastrophicity in the
context of convolutional codes when the polynomial matrices have entries in a
finite ring. In particular, we need to introduce two different notion of
primeness in order to fully characterize noncatastrophic encoders over the
finite ring Z_{p^r}. The second part of the paper is devoted to investigate the
notion of free and column distance in this context when the convolutional code
is a free finitely generated Z_{p^r}-module. We introduce the notion of
b-degree and provide new bounds on the free distances and column distance. We
show that this class of convolutional codes is optimal with respect to the
column distance and to the free distance if and only if its projection on Z_p
is.
