Polymer structure initiator-derived

NMR methods can be applied to give quantitative determination of initiator-derived and other end groups and provide a wealth of information on the polymerization process. They provide a chemical probe of the detailed initiation mechanism and a greater understanding of polymer properties. The main advantage of NMR methods over alternative techniques for initiator residue detection is that NMR signals (in particular nC NMR) are extremely sensitive to the structural environment of the initiator residue. This means that functionality formed by tail addition, head addition, transfer to initiator or primary radical termination, and various initiator-derived byproducts can be distinguished. 

Living radical polymerizations have received considerable attention because they provide a convenient alternative for synthesizing block copolymers, polymers of narrow polydispersity and complex polymer structures (1-5). Because of their ability to initiate living free radical polymerizations, iniferters have been examined extensively after Otsu et al. (6) introduced them in 1982. In particular, dithiocarbamate derivatives have been studied more closely by several researchers. Lambrinos et al (7) have examined the molecular weight evolution during the polymerization of n-butyl acrylate using p-x ylylene bis(A,A-diethyl... 

The copolymerization of furan and 2-methylfuran with dienophiles such as maleic anhydride leads to polymer structures with furan pendent functionality. Furan, 2-methylfuran, and 2,5-dimethylfuran have been copolymerized with acrylic monomers (51,52) and acrylonitrile (52,53). The furan ring of furan, 2-methylfuran, and 2,5-dimethylfuran participates as a diene in a free radical copolymerization with acrylonitrile. The initial step for furan and for 2,5-dimethylfuran is the attachment of an acrylonitrile radical at the 2-position, but for 2-methylfuran, the attack is at the-5-position. Propagation proceeds by the attack of the furan radical on an acrylonitrile molecule, to leave one olefinic bond in the structure derived from the furan ring. If this bond is in the 4,5- or 2,3-position, it may be involved in a second additional reaction by the return of the propagating chain. 

The nomenclature of step-reaction polymers is even more complicated than that of vinyl polymers and can be quite confusing. These polymers are usually named according to the source or initial monomer(s) and the type of reaction involved in the synthesis. For example, nylon 6,6 (4) is usually designated poly(hexamethylene adipamide), indicating an amidation reaction between hexamethylene-diamine and adipic acid. Nylon 6 is called either poly(6-hexanoamide) or poly(e-caprolactam). The former name indicates the structural and derivative method while the latter, which is more commonly used, is based on the source of the monomer. 

It should be quite apparent that, although all major architectural polymer classes are derived from common or related repeat units, the covalent connectivity is truly discrete and different. Furthermore, mathematical analysis of the respective propagation strategies clearly illustrates the dramatic differences in structure development as a function of covalent bond formation. It should be noted that linear, branched, and dendritic topologies differ substantially both in their covalent connectivity, as well as their terminal group to initiator site ratios. In spite of these differences, these open, unlooped macromolecular assemblies clearly manifest thermoplastic polymer type behavior in contrast to the looped, bridged connectivity associated with cross-linked, thermoset systems. In fact, it is now apparent that these three open assembly-topologies (i.e., (I) linear, (III) branched. 

A new class of polymers with repeating cycloketonic units along their chain can be prepared by reaction of polydiene containing adjacent structural units derived from 1,4-d.s polymerization of conjugated dienes with carbon monoxide. This is best achieved in the presence of a free radical initiator and, preferably, a compound capable of acting as a hydrogen donor [36]. Infrared studies clearly indicate that the carbonyl moiety is incorporated in the forms of cyclopentanones and cyclohexanones. The content of carbonyl units as well as the ratio of cyclopentanones/cyclohexanones depends strongly on the experimental conditions. The simple mechanistic Scheme 5 for the chain modification of the polymer is not adequate to explain such a behavior. 

Dense polymer structures with well-controlled compositions and dimensions are preferentially grown from a wide range of surfaces by atom transfer radical polymerization (ATRP). Different bromo-terminated molecules ranging from silanes, thiols, or disulfides to aromatic moieties derived from diazonium ions " have been proposed to initiate the polymer growth. 

---