Side-Group Changes

Of course, reactions of the foliage, the side chains on polymers, can take place without altering the molecular weight or the chain stiffness. Changes in solubility, compatibility, color, and mechanical and electrical properties may result. The main chains of poly(ethyl acrylate) or poly(vinyl acetate), for example, are not affected by hydrolysis. But in either case, hydrolysis changes water-insoluble precursors into water-soluble resins  

CH2CH3 Poly(ethyl acrylate) water-insoluble 

In the case of poly(vinyl alcohol), the reaction is a commercially useful one. But in either case, a drastic change in solubility has been made. It is clearly an undesirable change if the precursors are being used as water-insoluble protective coatings. 

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Provided that an optically active molecular aggregate is photochemically perturbed to change the state of molecular alignment, the effect of a chiral environment on an achiral chromophore incorporated in the molecular aggregate will be also altered. It has been known that polypeptides bearing photochromic side groups change their optically active properties as a result of photochromic reaction(10-12). This phenomenon is likely to be related to non-linear photoresponsiveness. 

The side groups and the repeating structure of the side groups change the chemical and physical properties of the polymer, and this defines the chemical and physical characteristics of the different polypeptide molecules. Not all natural macromolecules, however, are polymers. For example, insulin is a natural macromolecule with a molecular weight of 5733 kg/kg-mol. Insulin has long linear chains that are connected by 21 sulfur crosslinks. When it is decomposed 51 residual molecules result. Insulin is not a polymer because it does not have repeating units of monomers. 

Fig. 15. Dynamic modulus and loss tangent (at 1 Hz) of a series of w-alkyl methacrylates showing how side groups change the a and transitions. After Heijboer (44).

Control of the Glass Transition Temperature (Tg) Through Skeletal and Side Group Changes... 

Specific applications of DIRLD spectroscopy are presented for several polymer systems to demonstrate the types of information obtainable from this powerful rheo-optical characterization technique. A DIRLD study of atactic polystyrene revealed the existence of highly localized motion of various molecular constituents induced by a macroscopic dynamic strain.It was discovered the rate of reorientational motion of the polystyrene backbone differs considerably from that of the phenyl side groups. The reorientation direction of the phenyl side groups changes dramatically as the temperature of the system is raised above the glass transition temperature. This result may be interpreted as the onset of a new submolecular... 

FiaaHy, the use of photoreversible change of the circular dichroism for optical data storage is of iaterest. This technique offers an advantage over photochromic materials ia that the data can be read ia a way that does not damage the stored information. These chirooptic data storage devices have been demonstrated with the example of chiral peptides with azobenzene side groups (155). 

Liquid crystal polymers are also used in electrooptic displays. Side-chain polymers are quite suitable for this purpose, but usually involve much larger elastic and viscous constants, which slow the response of the device (33). The chiral smectic C phase is perhaps best suited for a polymer field effect device. The abiHty to attach dichroic or fluorescent dyes as a proportion of the side groups opens the door to appHcations not easily achieved with low molecular weight Hquid crystals. Polymers with smectic phases have also been used to create laser writable devices (30). The laser can address areas a few micrometers wide, changing a clear state to a strong scattering state or vice versa. Future uses of Hquid crystal polymers may include data storage devices. Polymers with nonlinear optical properties may also become important for device appHcations. 

Benzene rings in both the skeleton structure and on the side groups can be subjected to substitution reactions. Such reactions do not normally cause great changes in the fundamental nature of the polymer, for example they seldom lead to chain scission or cross-linking. 

Microstmcmral changes on irradiation have been observed by IR and UV spectroscopy. Changes in absorption bands due to vinyhdene double bonds [356,357], substituted double bonds, and ethyl and methyl groups give a measure of modifications in the presence of radiation. The ratio of the double bonds (located mainly at the end of a polymer chain) and scission is reported by some investigators [356-358] and found to be independent of temperature and dose. This is beheved to be due to the reaction of the methyl radical side group with hydrogen atoms on the backbone of the parent chain. 

The purpose of this chapter is to introduce a new class of polymers for both types of biomedical uses a polymer system in which the hydrolytic stability or instability is determined not by changes in the backbone structure, but by changes in the side groups attached to an unconventional macromolecular backbone. These polymers are polyphosphazenes, with the general molecular structure shown in structure 1. 

Molecular structural changes in polyphosphazenes are achieved mainly by macromolecular substitution reactions rather than by variations in monomer types or monomer ratios (1-4). The method makes use of a reactive macromolecular intermediate, poly(dichlorophosphazene) structure (3), that allows the facile replacement of chloro side groups by reactions of this macromolecule with a wide range of chemical reagents. The overall pathway is summarized in Scheme I. 

The electron transfer properties of the cytochromes involve cycling of the iron between the +2 and +3 oxidation states (Cytochrome)Fe + e" (Cytochrome)Fe ° = -0.3Vto+ 0.4V Different cytochromes have different side groups attached to the porphyrin ring. These side groups modify the electron density in the delocalized iz system of the porphyrin, which in turn changes the redox potential of the iron cation in the heme. 

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