The high citation rate is probably due to the multidisciplinary nature of our work and the hot topics our paper deals with. C₆₀ is a wonder molecule with an array of exotic properties but it suffers from poor tractability. Improving its processibility has thus been a subject of extensive research efforts. Creation of conjugated macromolecules is a frontier of contemporary polymer science. Researchers all over the world are in enthusiastic pursuit of utilizing functional polymers as active components for the construction of organic electronic and optical devices. Our paper describes a simple way for molecularly melding fullerene and polyacetylene, and reports unexpected light-emitting behaviors and improved optical-limiting performances of the resultant molecular conjugates. The interesting chemistries, the aesthetically appealing molecular structures, and the unique, useful materials properties of the buckyball-polyacetylene hybrids may have helped our paper attract a wide spectrum of readership.

Yes, it does. The paper reports a number of exciting discoveries. The fullerene has been found to perform dual roles in the acetylene polymerization. It serves as an active comonomer, readily copolymerizing with the phenylalkyne monomers to give fullerene-acetylene copolymers with high molecular weights (more than a half million) in high yields (up to 100%). It meanwhile functions as a cocatalyst, without which, the transition metal-catalyzed acetylene polymerization practically does not proceed. This is the first example of a polymerization reaction, in which the buckyball exhibits useful catalytic activity. Our work establishes a versatile method for incorporating fullerene buckyballs into polyacetylene chains. The buckyball is found to be a luminescence enhancer, when it hooks up several polyacetylene chains together. This is in contrast to the then general belief that fullerene molecules were emission quenchers. The buckyball-polyacetylene conjugates are linear optically more transparent but nonlinear optically more opaque and are hence better optical limiters than their C₆₀ parent.

Prior to our work, many research groups worked on integrating fullerenes with polymers. All the polymers were, however, virtually nonconjugated macromolecules, whose major roles were thus merely to improve the processibility of the buckyballs. For the first time, we molecularly incorporated the fullerene buckyballs into processible and stable conjugated polymers through a one-step polymerization reaction with a one-pot experimental procedure, in anticipation that synergistic molecular interactions between fullerene buckyballs and polyacetylene chains would endow the fullerene-polyacetylene hybrids with novel electronic and optical properties.

1-Phenyl-1-alkynes are disubstituted acetylene monomers, which can be polymerized into high molecular weight polymers by TaCl₅-based catalysts at high temperatures. The tantalum catalysts are, however, functionality intolerant—that is, functional groups can easily "poison" or deactivate the catalysts. While tungsten catalysts such as WCl₆-Ph₄Sn are more functionality tolerant, the polymerizations of phenylalkynes initiated by the tungsten catalyst are sluggish. The addition of a small amount of C₆₀ into the WCl₆-Ph₄Sn mixture dramatically boosts its catalytic activity at room temperature, turning it into an outstanding catalyst for the acetylene polymerization. This brings to light a new useful property of C₆₀: it can magically empower a polymerization catalyst.

Fullerene molecules have been generally regarded as emission quenchers. We prove that this is only partially true. Whilst the fullerenes with no or few substituents are poor emitters, those decorated with multiple pendants can be efficient luminophors. Thus a buckyball attached with one poly (phenylpropyne) chain is faintly fluorescent, whereas that with multiple poly (phenylbutyne) chains emits intense blue light. The strong luminescence is probably due to the nonlinear additive effect of the conjugated species: the multiply substituted fullerene buckyballs, the disubstituted polyacetylene chains, and the buckyball-polyacetylene hybrids all emit in the similar spectral region. Clearly, the buckyballs are working here as a luminescence amplifier, instead of an emission damper.

A fullerene solution allows low-power optical pulses to travel through but blocks the high-power ones from going through. Such optical power limiting property is useful: it can be utilized, for example, to shield battlefield soldiers from the attack of fatal laser weapons, prevent human eyes from blinding by harsh optical pulses, and protect the optical instruments used in space exploration. The buckyball itself is, however, difficult to process into usable objects, yet its linear transmittance is low because of its strong absorption in the visible spectral region. The molecular hybrids of buckyball and polyacetylene are readily processable by simple techniques (e.g., spin coating), exhibit high linear transmittance or optical transparency, and efficiently attenuate the power of intense laser pulses. Our work is thus "two birds, one stone": it improves the tractability of the buckyball and meanwhile boosts its optical limiting performance.

We have worked on fullerene chemistry since I established my research group in Hong Kong in 1994. We initially worked on developing simple ways for incorporating C₆₀ into nonconjugated polymers and sol-gel glasses. One interesting work we did at that time was that we could use the fullerene materials as optical filters to cut off almost every wavelength in the UV and visible region by simply adjusting their C₆₀ contents. From 1997, we shifted our focus to the hybridization of conjugated polymers with buckyballs and nanotubes. In October 1997, we succeeded in wrapping carbon nanotubes with poly (phenylacetylene) chains. The polyacetylene-nanotube hybrids are soluble in common solvents and the nanotube solutions efficiently attenuate intense optical pulses. We were excited by our discovery, because the dissolution of nanotubes would make their wet chemical reactions possible, thus opening up a new area of nanotube chemistry. The publication of our work had, however, encountered some unexpected difficulty but we eventually managed to publish it in Macromolecules in March 1999. This work was later highlighted by the Chemical & Engineering News of the American Chemical Society in its June 7, 1999 issue and has been highly cited by the scientists working in the research area of nanotube materials.

Our study on the fine structures of the polyacetylene-nanotube hybrids suggested that the poly (phenylacetylene) chains were covalently bound to the fullerene hemispheres at the ends of the carbon nanotubes. This prompted us to copolymerize fullerene with phenylacetylene. Because poly (phenylacetylene) itself is not so stable, we carried out the copolymerizations of fullerene with phenylalkynes, the disubstituted derivatives of phenylacetylene. The reactions were very simple and we got all the copolymers in the same month (October) of 1997.

Two observations, however, intrigued us. When we conducted the control experiments to polymerize phenylalkynes by the tungsten catalyst in the absence of fullerene, the polymerizations almost did not occur. When photoexcited, the fullerene-phenylbutyne copolymers emitted strong blue light, but the fullerene-phenylpropyne congeners were nearly nonluminescent. I asked different students to repeat the experiments several times and confirmed that the observations were readily reproducible. We worked hard to understand why and eventually reached the conclusion that fullerene was acting as a cocatalyst in the acetylene polymerization and serving as a luminescence enhancer when it was decorated by multiple polyacetylene chains. After working out all the necessary details, we reported our work in August 1999 at the American Chemical Society national meeting. Dr. Ron Dagani was in the audience and he highlighted our work in the Chemical & Engineering News in its Oct. 18, 1999 issue. The publication of our work was, however, again delayed because it was rejected two times. I am really happy to learn that our paper is enjoying a high citation rate after its publication in Chemistry of Materials in May 2000.