**Bruce S. Hudson**

Department of Chemistry, Syracuse University, Syracuse, NY 13244-4100, USA; bshudson@syr.edu; Tel.: +1-315-443-5805

Received: 31 December 2017; Accepted: 31 January 2018; Published: 6 February 2018

**Abstract:** Polyacetylene, the simplest and oldest of potentially conducting polymers, has never been made in a form that permits rigorous determination of its structure. *Trans* polyacetylene in its fully extended form will have a potential energy surface with two equivalent minima. It has been assumed that this results in bond length alternation. It is, rather, very likely that the zero-point energy is above the Peierls barrier. The experimental studies that purport to show bond alternation are reviewed and shown to be compromised by serious experimental inconsistencies or by the presence, for which there is considerable evidence, of finite chain polyenes. In this view, addition of dopants results in conductivity by facilitation of charge transport between finite polyenes. The double minimum potential that necessarily occurs for polyacetylene, if viewed as the result of elongation of finite chains, originates from admixture of the 11Ag ground electronic state with the 21Ag excited electronic singlet state. This excitation is diradical (two electron) in character. The polyacetylene limit is an equal admixture of these two 1Ag states making theory intractable for long chains. A method is outlined for preparation of high molecular weight polyacetylene with fully extended chains that are prevented from reacting with neighboring chains.

**Keywords:** polyacetylene; double-minimum potential; Peierls barrier; zero-point level; cross-linking
