When A is the methyl radical —CH3 and B is the radical —COOCH3, then the polymer is polymethyl methacrylate, which was mentioned in Chapters 9 and 12. It generally has a molecular weight between 10s and 106 Daltons (Da, g/mol), and since the molecular weight of the monomer CH2=C(CH3)C02CH3 is 100 Da, there are between 1000 and 10,000 monomers in this polymer. Polymers are also formed from a number of other radicals, such as allyl (CH2=CHCH2—) and vinyl (CH2=CH—). Natural rubber is a polymer based on the isoprene molecule:

which has a pair of double bonds. The second double bond is used to form crosslinkages between the linear polymer chains. The methyl group —CH3 at die top of the molecule is an example of a side chain on an otherwise linear molecule.

11.2.2. Sizes of Polymers

Polymers are generally classified by their molecular weight, and to discuss them from the nanoparticle aspect, we need a convenient way to convert molecular weight to a measure of the polymer size d. The volume V in the units cubic nanometers (nm3) of a substance of molecular weight Mw and density p is given by where Mv has the unit dalton or g/mol (grams per mole) and p has the conventional units g/cm3. If the shape of the nanoparticle is fairly uniform, with very little stretching or flattening in any direction, then a rough measure of its size is the cube root of the volume (11.10), which we call die size parameter d:

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