Chain-end modification

Macromonomers have been synthesized by living cationic polymerization by three different techniques by the use of a functional initiator, employing functional capping agent or by chain end modification. 

Numbers 30-35 = functional initiator 36-43 = functional terminator - chain end modification... 

The use of thiol compounds as transfer agents exclusively leads to monofunctional oligomers. To achieve bifunctionality requires a chain-end modification. Fock et al. [20-23] developed a new method based on the chemical chain-end modification of polymethacrylate telomers. These were previously obtained by telomerization reaction in the presence of mercaptoethanol or thioglycolic acid as transfer agents. The chemical modification can be summarized in two strategies ... 

Polymer properties are dependent on many factors, including chain end interactions with substrates such as carbon black or silica fillers, as well as clay and calcium carbonate. At this point, there is a large volume of work that has been done on chain end modification, particularly those made by anionic polymerization with group 1 or group II metals (Bielinsk et al., 1995 Yamato and Oahu, 1996 Schulz et al., 1974), as seen later. 

Figure 2 Chain end modification of PNIPAAm by CuAAC can alter LCST by as much as 10° C7...

FIGURE 5.5. The idea of the pivot aigorithm (A), and the iocai moves aigorithm (B). The biack contours indicate the initiai structures, the iighter bonds show the accepted modifications. The iocai moves inciude (from top to the bottom of B) random chain end modification, a crankshaft move and a corner move. 

As seen in Scheme 5.1, preparation of the IG polymer in this synthesis involved the ROP of cCL and subsequent chain-end modification. Conversion of the terminal hydroxyl group to two hydroxyl functions enabled further ROP to the 2G polymer. The IG polymer synthesized by this procedure was a 6-arm star-branched PcCL. The target dendrimer-like star-branched polymer was obtained as a 2G polymer by the second iteration and possessed a minimum architectural unit. One more repetition of the synthetic sequence involving the two reaction steps resulted in a 3G dendrimer-like star-branched PaCL. The 3G polymer possessed six branches at the core and two branches at the junctions in both the 2G- and 3G-based layers, composed of 42 arm segments (6 (IG) + 12 (2G) + 24 (3G) = 42). The observed M value was 96 000 g/mol, close to the theoretical value, and the molecular-weight distribution was not narrow, but an acceptable value of 1.14. 

The methodology using living ROP of EO and a chain-end modification reaction to increase the number of the hydroxyl reaction sites twofold was developed to allow the synthesis of a series of dendrimer-like star-branched (PEO)s up to the 8G stage (Feng et al, 2005). The 8G polymer possessed three branches at the core and two branches at the junctions in all generations having an value of 6.50 x lO g/mol IAS) with 384 external... 

Given the appropriate choice of hving polymerization and chain-end modification in divergent synthesis, isolation of the polymer in each iterative process is not necessary and... 

Abstract In this chapter, selected examples of sequential post-polymerization modifications are highlighted. Initially, we focus on side chain and chain end modifications in solution and at surface bounded polymers. Afterwards, the usage of this modifications as powerful tools in the synthesis of polymer structures such as graft and star polymers are discussed. 

As summarized in Sect. 3, the synthesis of grafted polymers requires side chain functionalities on the main chain as well as chain end functionalities on the polymers forming the arms. Besides, chain end modified polymers themselves exhibit highly interesting properties. To obtain chain end modificatimis, weU-defined chain ends are a primary requirement and can be obtained via controlled polymerizatiOTi techniques and subsequent chain end modification. For example, Sumerlin and coworkers reduced the thioester of poly(iV-isopropylacrylamide) (PNIPAM prepared by RAFT polymerization) using 1-hexylamine in the presence of tributyl-phosphine to yield a thiol-terminated polymer (Scheme 13). Subsequently, a bismaleimide was used to coimect the PNIPAM and other thiols. Thereby, small organic molecules could be used as well as other polymers or thiol-containing proteins [20, 21]. 

So far, we have focused on modifications with small organic molecules. In this section, we focus on macromolecules that can be joined in sequential reactions to construct star-shaped polymers. Generally, all arm-first approaches, which require prior chain end modification, can be considered as sequential reactions. Therefore, we highlight some selected examples and recommend that the interested reader looks at the reviews of Hirao et al. [48], Matsuo and coworkers [49], and Lowe [50] for further information. 

The DPE scheme can also be used to generate a controlled version of heteroarm stars produced by using DVB. A2B2 star polymers of I or Bd and MMA or nBMA were obtained by sequential sBuLi-initiated polymerisation of I in hexane at RT followed by solvent exchange to THE, chain-end modification by a compound of the DPE-X-DPE type and polymerisation of MMA at —78°C in the presence of LiCl. (38 < Mn/(kg/mol) < 70 1.01 < M / Mn < 1.06) [131,132]. 

Yamaguchi N, Wang JS, Hewitt JM, Lenhart WC, Monrey TH (2002) Acid chloride-functionalized hyperbranched polyester for facile and quantitative chain-end modification one-pot synthesis and structure characterization. J Polym Sci A Polym Chem 40 2855-2867... 

Is the network destroyed when the strain is increased [26] Does the bound rubber play any role in flow Rubber molecules with long branches and gels usually accept a higher loading of carbon black. At the same loading level the mixed compound with branched rubber stretches more than that made with less branched rubber [27]. Evidently, the rubber-carbon black interaction varies with polymer chain structure. A recent development of the chain-end modification also affects this interaction. How do these rubber-carbon black interactions affect the flow mechanism ... 

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