A series of poly[B-(methylamino)borazine] were synthesized by thermolysis of a monomeric B-tri-(methylamino)borazine at various temperatures between 150 and 200 °C and then characterized for suitability as a fiber precursor. Polymerization mechanisms and polymer architectures are discussed. It was shown that poly-[B-(methylamino)borazine] represents a network combining a majority of -N(CH3)- bridges with a small proportion of B-N bonds, both connecting borazine rings, and -N(H)CH3 groups. Both the ratio between flexible-N(CH3)- bridges and rigid B-N bonds and the relative amounts of plasticizing -N(H)CH3 groups cause different responses to thermal properties and spinnability (glass transition, spinning temperatures, melt throughput, and fiber drawing). Based on fiber shape visualization using CCD camera during extrusion, appreciable meltspinnable compounds are prepared between 160 and 185 °C. Such polymers display a chemical formula of [B3.0N4.4(0.1C2.0(0.1H9.3(0.2]n (n ∼ 7.5), a glass transition between 64 and 83 °C, tailored flexibility, and sufficient plasticity to successfully produce fine-diameter green fibers.