Polymers end-reactive

The polymerization of 2-methyl-2-oxazoline is a clean reaction, which is not disturbed by chain transfer and termination. In this polymerization, the propagating species having the structure of an oxazolinium salt is not fragile, which is conveniently utilized for syntheses of block copolymers and end-reactive polymers [28],... 

By controlled thermal degradation of PIB, end-reactive oligomers can be prepared (53). For example, the resulting end-reactive polymers having one or two tertiary chloro groups can be converted to terminal-unsaturated polymers bearing an isopropenyl group by dehydrochlorination... 

For PIB, a method resulting in end-reactive polymers, however, based on chain transfer reactions during polymerization, was addressed as the cationic inifer method (54). 

Synthesis of branched and network state polymers. End-reactive polymer [Y. Tezuka, K. Imai, Kohunshi 1991, 40(5), 314-317],... 

Diblock copolymers are known to be the most effective compatibilizers for improving the interfacial interactions between two polymers that are immiscible. This is particularly interesting for iPP, since its lack of functionality and the poor compatibility between iPP and other materials have imposed limitations for iPP applications in many areas, including polymer blends and composites. The synthesis of iPP with terminal functional groups (OH, NH2, etc.) offers a good opportunity to carry out chain extensions through simple coupling reactions with suitable polymers. These may be carried out in solution or in the polymer melt. Reactive extrusion of two chain-end reactive polymers... 

Star copolymers can be prepared by two main synthetic routes the arm-first and core-first methodologies. In the first case, living, or end-reactive, polymer chains are coupled to a multifunctional core. In the second case, a multifunctional core molecule is used to initiate the polymerization of the arms. Most of the star polypeptides reported so far have been obtained following the core-first strategy and were prepared using conventional primary amine-initiated NCA polymerization. In this way, Daly successfully prepared a series of... 

Some monomers give the polymers with narrow molecular weight distribution even in the presence of a protic compound as a chain transfer agent. This is different from living polymerization, and a new concept of immortal polymerization has been presented . Living and immortal polymerizations initiated with metalloporphyrins can provide various types of block copolymers and end reactive polymers . ... 

Japan-US Seminar on Synthesis and Reactions of Oligomers and End-Reactive Polymers, J. MacromoL ScL, Chem, 1984,21 (8 9). 

Such functionality can also be of great practical importance since functional initiators, transfer agents, etc. are applied to prepare end-functional polymers (see Section 7.5) or block or graft copolymers (Section 7.6). In these cases the need to maximize the fraction of chains that contain the reactive or other desired functionality is obvious. However, there are also well-documented cases where weak links formed by initiation, termination, or abnormal propagation processes impair the thermal or photochemical stability of polymers. 

The synthesis of end functional polymers by NMP, ATRP and RAFT has already been discussed in Section 9.7. The "grafting to approach involves the covalent attachment of an end-funetionalized polymer with reactive surface groups on the substrate. The approach is inherently limited by the crowding of chains at the surface and the limit this places on the final graft density. 

Tethering may be a reversible or an irreversible process. Irreversible grafting is typically accomplished by chemical bonding. The number of grafted chains is controlled by the number of grafting sites and their functionality, and then ultimately by the extent of the chemical reaction. The reaction kinetics may reflect the potential barrier confronting reactive chains which try to penetrate the tethered layer. Reversible grafting is accomplished via the self-assembly of polymeric surfactants and end-functionalized polymers [59]. In this case, the surface density and all other characteristic dimensions of the structure are controlled by thermodynamic equilibrium, albeit with possible kinetic effects. In this instance, the equilibrium condition involves the penalties due to the deformation of tethered chains. 

Another approach uses reactive alkyl halogen compounds containing a terminal carboxylate group on the other end to form spacer arms off the dextran polymer from each available hydroxyl. In this manner, Brunswick et al. (1988) used chloroacetic acid to modify the hydroxyl groups to form the carboxymethyl derivative. The carboxylates then were aminated with ethylene diamine to create an amine-terminal derivative (Inman, 1985). Finally, the amines were modified with iodoacetate to form a sulfhydryl-reactive polymer (Figure 25.14). 

Figure 25.19 Polyaldehyde dextran may be modified with the hydrazide end of M2C2H to create a thiol-reactive polymer.

In the co-end of the chain, the dissociation always occurs at the bond which is indicated by the arrow A. The dissociation of this C-S bond at the A position gives a more-reactive carbon-centered radical and a less-reactive polymer thiyl radical, which leads to the termination of the active chain ends. In the case of the a-chain end, however, there is a possibility that the bond at the C position dissociates to produce a diethylaminothiocarbonyl radical and a thiyl radical in addition to the preferable bond scission at B. Such dissociation at C may not induce living radical polymerization [76]. 

The dissociation of model compounds for co-chain ends of polymers obtained using iniferters with the DC group was examined by the spin-trapping technique, similar to the disso dation of 7 and 8 previously mentioned [174,175]. From the results of the trapping experiments, it was concluded that 46,47, and 48 as model compounds for poly(MA), poly(MMA), and poly(VAc), respectively, dissociated at the appropriate position to produce a reactive carbon-centered radical and a stable DC radical. In fact, these compounds were found to induce the living radical polymerization of St when they were used as photoiniferters. 

If one is able to control precisely the formation of end-functionalized polymers carrying groups of the proper reactivity and philicity, this scheme should open efficient and diversified new ways of access to interesting block copolymers. The basic problem remains accordingly a catalytic one, i.e. the quantitative end-functionali-zation of growing homopolymer chains by efficient termination or transfer reactions that is fortunately a rapidly improving field. 

New Telechelic Polymers and Sequential Copolymers by Polyfunctional Initiator-Transfer Agents (Inifers) End Reactive Polyisobutylenes by Semicontinuous Polymerization... 

In this method, a reactive group on the surface initiates the polymerization, and the propagating polymer chain grows from the surface (Fig. 9.19b). In principle, it can be employed with all polymerization types, and a number of papers have reported high amounts of immobihzed polymer using surface-initiated polymerization with various initiator/monomer systems. If controlled or Hving polymerization techniques are used, block copolymer or end-functionahzed polymer brush systems can be prepared in consecutive reaction steps (Fig. 9.19c). 

While it can be expected that a number of physical properties of hyperbranched and dendritic macromolecules will be similar, it should not be assumed that all properties found for dendrimers will apply to hyperbranched macromolecules. This difference has clearly been observed in a number of different areas. As would be expected for a material intermediate between dendrimers and linear polymers, the reactivity of the chain ends is lower for hyperbranched macromolecules than for dendrimers [125]. Dendritic macromolecules would therefore possess a clear advantage in processes, which require maximum chain end reactivity such as novel catalysts. A dramatic difference is also observed when the intrinsic viscosity behavior of hyperbranched macromolecules is compared with regular dendrimers. While dendrimers are found to be the only materials that do not obey the Mark-Houwink-Sakurada relationship, hyperbranched macromolecules are found to follow this relationship, albeit with extremely low a values when compared to linear and branched polymers [126]. 

T. Sawaguchi and M. Seno, Preparation and characterization of end-reactive oligomers by thermal degradation of polyisobutylene, Polymer, 37(16) 3697-3706, August 1996. 

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