A new synthetic pathway to various mono-alkyl-substituted phosphinoboranes HRP-BH2-NMe3 has been developed. The new synthetic route starting from alkyl halides and NaPH2 followed by metalation and salt metathesis is performed in a one-pot procedure and leads to higher yields and purity of the resulting phosphinoboranes, as compared to previously reported routes. Additionally, the scope of accessible compounds could be expanded from short-chained linear alkyl substituents to longer-chained linear alkyl substituents as well as secondary or functionalized alkyl substituents. The reported examples include primary alkyl-substituted phosphinoboranes RHP-BH2-NMe3 (R = n-butyl, n-pentyl, n-hexyl; 1a-c), the secondary alkyl-substituted derivatives iPrPH-BH2-NMe3 (2), and the functionalized alkyl-substituted 4-bromo-butyl-phosphinoborane (BrC4H8)PH-BH2-NMe3 (3). Compounds 1a, 1c, and 2 were additionally used for preliminary polymerization reactions via a thermal and a transition metal-catalyzed pathway, revealing the formation of high-molecular-weight polymers under certain conditions. Detailed investigations on the influence of temperature, concentration, substituents and reaction time on the respective polymerization reactions were performed.