Unbranched Macromolecules

The selectivity of the bis(ylide)nickel catalysts frequently favors the formation of linear macromolecules with unsaturated end groups. In this case the FTIR spectrum of a PE film of defined thickness shows almost exclusively methyl and vinyl end groups. Their presence in equal quantities proves linearity. An example for the catalytically controlled formation of linear, unbranched macromolecules is given in Eq. (12). 

Strongly short-chain-branched polymers can often be spectroscopically or chemically determined. However, with long-chain branching, the number of branch points is very small compared to the number of chain-links. Consequently, the existence and extent of long chain branching is mostly estimated from the dimensions of the macromolecule in solution. A long-chain-branched macromolecule, namely, exhibits smaller molecular dimensions than the unbranched macromolecule with the same molar mass, as can be clearly seen in Figure 2-4. 

Such small and intercatenary cross-linked macromolecules are just like branched macromolecules in generally being soluble in some solvent or other. Thus, they characteristically differ from actual cross-linked networks, which possess many intercatenary cross-links. Such cross-linked molecules, or networks, are infinitely large in comparison with the usual branched or unbranched macromolecules. They are not soluble in any solvent. On the other hand, not all insoluble polymers are cross-linked, of course. 

Branched macromolecules have a higher average segment density than unbranched macromolecules of the same molar mass, and have a lower coil volume. This is easily seen by comparing a star-shaped branched molecule with a linear one. The influence of the branching on the dimensions can be expressed by a g factor... 

The mean functionality of the monomer, however, can also be smaller than the sum of the functionalities of the groups contained in the monomer molecule. For example, in the Diels-Alder polymerization of 2-vinyl butadiene with p-quinone, the total functionality of the 2-vinyl butadiene in chain formation is two per unbranched macromolecule ... 

The macromolecular nature of cellulose was proposed by Staudinger in 1920. Today it is generally accepted that cellulose occurs in nature as an unbranched macromolecule with structural formula (1). The values of x (number of cellobiose units) range as high as 2000-2500. The molecular... 

Shear thinning is often observed in solution containing thread-like macromolecules such as celluloses or unbranched polymers. Here the stress applied causes the macromolecules to align in the direction of the... 

Polymers are macromolecules that consist of repeating molecular units known as monomers covalently bonded during polymerization. While various architectures are possible, linear polymers with unbranched molecular chains are the most common both naturally occurring and synthetic. Homopolymers contain only a single type of monomer, while block copolymers are made up from a sequence of chemically distinct homopolymers (Fig. 1.1). 

Macromolecules differ not only in their constitutional composition and bonding or sequence of monomeric units, but also in terms of their molecular architecture. The molecular architecture relates to the way individual macromolecular chains are bonded to each other. Unbranched chains, branched chains, ordered networks, and disordered or irregular networks can be distinguished. Interpenetrating networks, (IPNs), consist of two interpenetrating networks that are not bonded to each other. 

Branching reduces the hydrodynamic volume relative to the mass of the coil. The intrinsic viscosity of branched macromolecules is thus lower than that of their unbranched (linear) counterparts. The effect is particularly marked in the case of long-chain branching. If the number of branch points in a polymer homologous series increases with the molar mass, then the [rj] values also decrease relative to those of linear molecules. Thus, the slope of the log f (log 2) curve continuously decreases with increasing molar mass,... 

The polymers which give drag reduction and drag increase have all large unbranched or moderately branched macromolecules with extremely high molecular weights. 

On the other hand, modern nomenclature is based on the chemical structure of the macromolecules. The name of a polymer of unspecified degree of polymerization consists of the prefix poly and the name of the smallest repeating unit. With unbranched polymers, the smallest repeating unit is a diradical. The name of this diradical is the same as that met with in the nomenclature for low-molecular-weight organic diradicals. Thus, the group —CH2— is called methylene and the corresponding polymer is called poly(methylene) (example 1 in Table 1-4). The diradicals —CH2—CH2— and —CH2—CH(CH3)— do not have simple definitive names the trivial names ethylene and propylene are retained. Examples of names for other diradicals are as follows ... 

Nucleic acid nu- kle-ik- [fr. their occurrence in cell nuclei] (1892) n. A family of macromolecules, of molecular masses ranging upward from 25,000, found in the chromosomes, nucleoli, mitochondria, and cytoplasm of all cells, and in viruses in complexes with proteins, they are called nu-cleoproteins. On hydrolysis they yield purines, pyrimidines, phosphoric acid, and a pentose, either D-ribose or D-deoxyribose from the last, the nucleic acid derive their more specific names, ribronucleic acid and deoxyribonucleic acid. Nuclear acids are liner (i.e., unbranched) chains of nucleotides in which the 5 -phosphoric group of each one is esterified with the 3 -hydroxyl of the adjoining nucleotide. Black JG (2002) Microbiology, 5th edn. John Wiley and Sons Inc., New York. 

A macromolecule of unknown structure but believed to be of uniform molecular weight and unbranched has an intrinsic viscosity of 2500 ml/g in water at 25 C and the molecular weight determined by light scattering is 1.0 X 10 . What is the terminal relaxation time in this solvent ... 

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