Sequential Repeat unit polymerization

Extension of a polymeric chain through sequential addition of oligosaccharide repeating units (stepwise synthesis). 

Wu et al. reported on a rod—coil diblock copolymers based on mesogen-jacketed liquid crystalline polymer as the rod block and polystyrene as the coil block (Scheme 6).82 Styrene was polymerized by TEMPO mediated radical polymerization, followed by sequential polymerization of 2,5-bis[4-methoxyphenyl]oxy-carbonylstyrene (MPCS) to produce the rod—coil diblock copolymer (20) containing 520 styrene and 119 MPCS repeating units. The rod—coil copolymer was observed to self-assemble into a core—shell nanostructure in a selective solvent for polystyrene... 

Addition polymers. The polymerization proceeds by a sequential incorporation of monomeric molecules into the growing polymer chain, without the release of any molecules or fragments to the reaction medium. As a consequence, the repeating units of addition polymers have the same... 

That the repeating units are normally polymerized while still attached to P-ACL, and then transferred to the core, was demonstrated in two ways. (1) Pulse-chase experiments that used a phosphoman-noisomerase mutant of S. typhimurium, where D-mannose- C is incorporated only into O side-chains, indicated a precursor-product type of relationship between free O side-chains (presumably linked to P-ACL before extraction) and LPS-linked, O side-chains. (2) O Side-chains were shown to be elongated at the reducing end, rather than at the nonreducing end. This mechanism, which is later discussed in detail, tends to rule out the possibility that the O side-chain grows by the sequential addition of repeating units on the core LPS. 

In the polymers synthesized, the repeating units are joined together by phosphoric diester linkages between C-1 of D-glycerol and C-6 of the hexose. For these reactions, there is a question as to whether the repeating unit is preassembled and then polymerized, or whether each component of the chain is added in a strictly sequential way. Burger and Glaser could find no evidence for the presence of a nucleo-... 

The polymerization of D3 was followed by GC/MS. After 36 h, the reaction was quenched by the introduction of trimethylchlorosilane. About 92% of the D3 had been converted, while the amount of unconverted D/ had not changed significantly. Si NMR analyses allowed the determination of the sequence distribution of repeat units, which showed no random copolymerization of D/ and D3 as in the case of diblock copolymers prepared by sequential anionic copolymerization of D3 extended with D/. Because the polymerization of the first monomer could not be carried out to completion (<100% conversion) without increasing the molecular weight distribution, the second monomer (with faster propagation rates) had to be introduced before the equilibration reaction became established. Therefore, unreacted monomer from the first step was still in solution when the second monomer was added. The risk of random copolymerization can be suppressed if the second monomer has far higher reactivity towards polymerization than the first monomer. The block formed in the second step contained only a few methylvinylsiloxane units, i.e. the block purity was very high. 

Thus, the degree of polymerization derives neither from the number of constitutional repeating units nor from the number of constitutional units, but from the number of monomeric units. Only in exceptional cases is the degree of polymerization identical with the chain link number. The chain link number is the number of atoms joined sequentially together in the chain. The chain link number is equal to N for poly(methylenes), to IN for poly(dimethyl siloxanes), and to 3A for poly(oxyethylenes). 

GTP is a controlled Tiving polymerization method suitable for the preparation of block copolymers composed of meth (acrylic) repeating units and it is supposed to proceed through a dissociative (anionic) mechanism (proposed by Quirk and Kim in the presence of catalysts in the form of anions). One of the major advantages of GTP when compared with the classical anionic polymerization is the fact that sequential monomer addition can be used without taking into consideration the order of monomer addition and monomer reactivity. 

Irregular block n. A block (in a polymer structure) that cannot be described by only one species of constitutional repeating unit in a single sequential arrangement. Odian GC (2004) Principles of polymerization. John Wiley and Sons Inc., New York. 

The physical and mechanical properties of a polymeric material critically depend on many factors, one of which is stereochemistry. Polymers that have chiral centers in the repeated unit can exhibit two structures of maximum order, isotactic and syndiotactic [27]. Sequential stereocenters of isotactic polymers are of same relative stereochemistry whereas those of syndiotactic polymers are of opposite relative configuration. Due to their stereoregularity, isotactic and syndiotactic polymers are typically crystalline, which is an important feature for many applications. Isotactic polymers are used in a wide range of applications. Typical examples include isotactic polyolefins and almost all natural polymers. In contrast, syndiotactic polymers have limited applications mainly due to their hard productivity and inherently alternating stereochemistry. The properties of syndiotactic polymers are usually similar to or in some cases better than isotactic counterparts according to the studies on syndiotactic polystyrene and other syndiotactic polyolefins [28]. Syndiotactic PLA is expected to be a versatile polymer with controllable stereochemistry. 

Sequential reactions can be used for post-polymerization modifications. In this case, two general approaches are commonly used the polymer chain ends can be modified or the modification can take place at each repeating unit. We first focus on side chain modifications. Some of these reactions, which can be subdivided according to their functional groups, are discussed next. 

L. lelpi, R.O. Couso, and M.A. Dankert, Sequential assembly and polymerization of the polyprenol-linked pentasaccharide repeating unit of the Xanthan polysaccharide in Xanthoma-nas campestris, J. Bacterial. 1993, 175, 2490 - 2500. 

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