is thought to occur via the anisotropic alignment of graphene layers; these graphite layers lead to structures that have different shapes (e.g., straight, helical, branched) developed by the growth from the different crystal faces of the catalyst particle. [ 1 , 2 , 13 , 14 ] This diversity of morphologies makes CNFs an attractive material for studies that investigate the relationship between structure and function.
To date all fi bers grown have been observed to consist of a single morphology. While their shape may change from helical to straight, the underlying structure remains unchanged, i.e., either plate-like, tubular, or herringbone. [1][2][3][4] It is well established that the morphology of 1D carbon chains can be readily modifi ed by varying the carbon source to yield carbon chains with variable morphologies. Indeed this phenomenon of forming chains of carbon with sections of distinct structure from different monomers is a well-developed area in polymer chemistry and generates block co-polymers. [ 15 , 16 ] Block copolymers are based on the phenomenon of a living polymer, whereby a carbon chain is capable of growing 'indefi nitely'. [ 16 ] Since CNFs can be viewed as a carbon "polymer", it should be possible to synthesize CNFs with a co-block structure analogous to block co-polymers. This should be possible by simply varying the carbon reagent, thereby creating a "living polymer" or more accurately a "living CNF" analogous to a living polymer. Based upon this concept we have been able to produce in a controlled manner novel co-block CNFs, which to the authors' knowledge have not been reported previously.
We begin our discussion by fi rst considering the growth of CNFs from the acetylenes, methylpropiolate, 2-pentyne, and 1-heptyne. It was observed from transmission electron microscopy (TEM) images that the acetylene reactant used ( Figure 1 ) infl uenced the structure of the CNF produced. Methylpropiolate tends to form CNFs that grow simultaneously in a bidirectional manner from a platelet-like catalyst particle (Figure 1 a), to give a regularly striated CNF. The diameter of the CNF is determined by the diameter of the catalyst particle (200-400 nm). This structure stands in stark contrast to CNFs grown from 2-pentyne, in which the CNF grows from the tip of smaller, irregularly shaped catalyst particle (Figure 1 b). The CNF structure is a solid "amorphouslike" strand (diameter, d = 20-150 nm) with no striations (Figure 1 b). Additionally 1-heptyne produced CNFs that grow both bidirectional as well as monodirectional from semiregular particles. These CNFs are solid strands ( d = 50-200 nm) with highly roughened ends that give a stack-like appearance to the CNF. These observations reveal that the different acetylenes preferentially grow from differently sized and