responsible for their self-diffusion in the liquid (Einstein 1905 ). The epoch-marking
ideas above, along with the flourishing of quantum mechanics, created a solid
foundation for atomic and molecular views of chemical substances. The atomic
view has been reinforced by modern techniques, for example, scanning tunneling
microscopy, which is capable of visualizing and even manipulating individual
atoms (Binnig and Rohrer 1986 ). Nowadays, we define the molecules, including
ions and mono-atomic molecules, as the smallest units that maintain the chemical
properties of pure substances, and define the atoms as the smallest units that
represent the properties of elements in molecules and in chemical reactions.
Molecules, as the minimal units maintaining the chemical properties, imply that
their molar mass could not be too large. Therefore, when Staudinger proposed the
concept of “Macromolecules” in 1920 (Staudinger 1920 ), he met a strong objection
from the whole academic community. However, he unflinchingly fought for his
argument, and collected various concrete evidences to prove that the chemical
compounds in his hand contained more than 1,000 atoms, and their molar masses
reached more than 10 kilograms per mole. He eventually persuaded his colleagues
in the community and won the Nobel Prize in Chemistry in 1953 for his work on
macromolecules. Nowadays, it has been well known that the molar mass of
polymers could be so large that, removing several repeating units would not
significantly affect their chemical or physical properties. The concept of
“Macromolecules” has indeed challenged our common sense that molecules are
the smallest structural units maintaining the properties of pure substances.
In 1996, the International Union of Pure and Applied Chemistry (IPUAC)
published the recommendation of polymer terms (Jenkins et al. 1996 ). It provided
the definition below:
Macromolecule; polymer molecule
A molecule of high relative molecular mass, the structure of which essentially
comprises the multiple repetition of units derived, actually or conceptually, from molecules
of low relative molecular mass.
Notes:
(1) In many cases, especially for synthetic polymers, a molecule can be regarded as having
a high relative molecular mass if the addition or removal of one or a few of the units has
a negligible effect on the molecular properties. This statement fails in the case of
certain macromolecules for which the properties may be critically dependent on fine
details of the molecular structure.
(2) If a part or the whole of the molecule has a high relative molecular mass and essentially
comprises the multiple repetition of units derived, actually or conceptually, from
molecules of low relative molecular mass, it may be described as eithermacromolecu-
larorpolymeric,orbypolymerused adjectivally.
The definition above is flexible enough to accommodate the diverse macromo-
lecular compounds encountered by chemists. But such a definition is not satisfac-
tory to physicists, because it does not reflect the basic molecular structure that
determines most of the unique physical behaviors of polymers.
2 1 Introduction