  The S=1/2 Heisenberg chain with bond alternation and randomness of
antiferromagnetic (AFM) and ferromagnetic (FM) interactions is investigated by
quantum Monte Carlo simulations of loop/cluster algorithm. Our results have
shown interesting finite temperature magnetic properties of this model. The
relevance of our study to the observed results of the material
(CH$_3$)$_2$CHNH$_3$Cu(Cl$_x$Br$_{1-x}$)$_3$ is discussed.
