Chain regularity

Catalysts control the position of the monomer unit added, relative to the end of the growing chain. The monomer side group can be on the same side as the last one - a meso (m) placement - or on the opposite side - a racemic (r) placement. The stereoregularity of polymer chains is often idealised as being of one of three types. 

Isotactic the side groups X are all on the same side of the chain 

Syndiotactic the side groups alternate from side to side Atactic the side groups have random positions 

These words are based on the Greek roots iso- = same, a- = not, tactos = form. Isotactic chains contain exclusively mm monomer placements syndiotactic chains contain exclusively mr or rm placements, while atactic chains contain 25% rr, 25% mm and 50% mr placements. Special catalysts are necessary to produce isotactic polypropylene, which is highly crystalline, whereas atactic polystyrene is produced without the use of special catalysts. It is possible to produce isotactic polystyrene and atactic polypropylene, but these have very limited markets. 

Stereoregularity is more accurately described by the statistical distribution of sequences of neighbouring side group placements. In the simple (BemoulHan) form of chain growth statistics, the probability a of an m 

The stereochemical microstructure (tacticity) of polymers can greatly affect their thermal and mechanical properties. Therefore, the ability to investigate the tacticity is of great importance for understanding structure-property relationships. Also, studying the chain regularity may yield information on the monomer-addition process. NMR is the prime technique in this context (Pichot etal, 1981 Marciniec Malecka, 2003 Phillipsen, 2004). NMR has been applied to study the chain structure of many different homopolymers and even of some complex copolymers (Koinuma et al, 1982 van der Velden, 1983). 

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In spite of possessing a flexible backbone and low interchain attraction polyethylene is not a rubber. This is because its chain regularity enables a measure of crystallinity which does not disappear until temperatures of the order of 100°C are reached. It therefore follows that if crystallinity can be substantially reduced it should be possible to obtain an ethylene-based polymer which is rubbery. The means by which this objective has been achieved on a commercial scale may be classified into three categories ... 

The disruption of chain regularity by the introduction of lateral substituents or kinks on repeating units is a supplementary means to decrease the melting temperature of aromatic polyesters.72 This is illustrated in Table 2.9, where the melting temperatures of unsubstituted and methyl-substituted aromatic-aliphatic and aliphatic acids are reported. Regularity disruptions often cause significant... 

The diamine part has six methylene units and the diacid part four methylene units and two acid carbon atoms. Thus, it has an even number of methylene units in both die diamine and the diacid part, which gives the polymer chain regularity so it is able to crystallize easily. 

Since the chain regularity is an attribute of a development of higher levels of structural hierarchy, namely crystalline domains, stereoisomerism affects greatly the morphology and, thus, the mechanical properties of semicrystalline thermoplastics. 

Natural rubber and guttapercha consist essentially of polyisoprene in a s-1,4 and trans-1,4 isomers, respectively. Commercially produced synthetic polyisoprenes have more or less identical structures but reduced chain regularity, although... 

Natural rubber (NR) and guttapercha consist essentially of polyisoprene in cis-l, 4 and trans-1,4 isomers, respectively. Commercially produced synthetic polyisoprenes have more or less identical structure but reduced chain regularity, although some may contain certain proportions of 1,2- and 3,4-isomers. Microstructure differences not only cause the polymers to have different physical properties but also affect their response to radiation. The most apparent change in microstructure on irradiation is the decrease in unsaturation. It is further promoted by the addition of thiols and other compounds.130 On the other hand, antioxidants and sulfur were found to reduce the rate of decay of unsaturation.131 A significant loss in unsaturation was found, particularly in polyisoprenes composed primarily of 1,2- and 3,4-isomers.132,133... 

Aryl-aliphatic copolyamides (Ar-Al-PA) are industrial technical polymers whose uses as transparent plastics require performances in terms of temperature behaviour, or resistance to solvents and stress cracking. The transparency of materials necessitates a completely amorphous structure, which is obtained by using not only para-substituted phenyl rings, but also meta-substituted phenyl rings that decrease the chain regularity. 

Within each of these three categories a large number of main types exists, each characterized by a a specific structure of the macromolecule. Within each main type several variations occur, e.g. concerning chain length, chain regularity, copolymerisation (presence of more than one monomer in the chain), etc. 

With conventional polymerization processes, atactic chains are predominantly formed for the formation of isotactic and syndiotactic chains a special catalyst system is required, e.g. Ziegler-Natta catalysts. Such a process is called stereospecific polymerization. It enables the manufacture of, i.a., technically usable PP and also unbranched PE (see 4.1). The newest development is the metallocene katalyst it enables the building-up of chains-to-measure with very high degrees of chain regularity also the manufacture of syndiotactic polystyrene is technically possible in this way (see Qu. 2.47). 

Branching disturbs the chain regularity if the branches are situated at random positions along the chain. In particular with PE a number of branching types... 

The extent of chain regularity in poly(a-olefin)s is determined primarily by high-resolution nuclear magnetic resonance, 1H NMR and especially 13C NMR, spectroscopy. Both the H and 13C NMR signals of the macromolecule are most conveniently related to its microstructure [421]. 

We have chosen to focus on the molecular variables that influence fatigue resistance in a stress cracking environment molecular weight, chain regularity, and molecular parameters of the medium. In most cases, we will differentiate between amorphous, crystalline, and cross-linked polymers. In a subsequent section we will examine the impact of sample preparation on the fatigue resistance sterilization, cross Unking, orientation. Another section will focus on the different strategies to improve the ESCR. 

Maccone P, Brinati G, Arcella V (2000) Environmental stress cracking of poly(vi-nylidene fluoride) in sodium hydroxide effect of chain regularity. Polym Eng Sci 40(3) 761-767... 

It must be amorphous in its unstressed state. (Polyethylene is not an elastomer, but copolymerization of ethylene with suflicient propylene reduces chain regularity sufficiently to eliminate crystallinity and produce a useful elastomer.)... 

For stock dyeing no more than 0,1-0,5 weight % is introduced, as such amount of dye caimot influence intermolecular interaction, chain regularity and polyester crystallinity [288],... 

Polymer properties are directly dependent on both the Inherent shape of the polymer and on its treatment. Contributions of polymer shape to polymer properties are often complex and interrelated, but can be broadly divided into terms dealing with chain flexibility, chain regularity, interchain forces, and steric effects. 

Fig. 7.14. Models for the strueture of the chain layer CoO in which L- and R-chains regularly alternate. The unit mesh is indicated (subscript o basic cell, s supereell). Small circles represent oxygen atoms, large circles Co atoms, squares oxygen vaeaneies and blaek dots are the perovskite Cu positions.

In recent years, significant improvement in the synthesis of electroactive polymers has been observed leading to the preparation of soluble conjugated polymers with high chain regularity. These two features... 

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