A new carbon allotrope based sp hybridization containing alternating triple and single bonds (an acetylenic or linear carbon allotrope) has been prepared. Studied of small (8 to 28 carbon atoms) acetylenic carbon model compounds show that such species are quite stable (130 - 140°C) provided that nonreactive terminal groups or end caps, such as t-butyl, triethylsilyl, phenyl, trifluoromethyl, or nitrile groups, are present to stabilize these molecules against further reactions. Using laser based synthetic techniques in the presence of end capping groups, similar to the conditions normally used to generate fullerenes, thermally stable acetylenic carbon species capped with trifluormethyl or nitrile groups have been prepared with chain lengths in excess of 300 carbon atoms. Under these conditions, the production of fullerenes is suppressed and only a negligible quantity of fullerenes is produced. Acetylenic carbon compounds are not particularly moisture or oxygen sensitive but are moderately light sensitive. Carbon ( 1 3 C) nuclear magnetic resonance (NMR), infrared (IR) spectroscopy, and laser desorption mass spectroscopy are used to characterize these carbon chain species and confirm that capped acetylenic chain polymers, (-C C-C -) n , join graphite, diamond and fullerenes-nanotubes as the fourth allotrope of the element carbon.