Thiol-terminated polymer

The reaction of a,co-dilithiumpolyisoprene with ethylene sulfide crosses over very rapidly (4-5 min) at -40 °C in a 50/50 hydrocarbon/tetrahydrofuran mixture, while the subsequent polymerization requires several days 339 These results suggest that it might be possible to prepare ethane thiol terminated polymers under certain conditions. 

Scheme 33 Various Michael acceptors used to end-functionalize thiol-terminated polymers.

In the absence of hydrogen, metallocene-based catalyst systems produce well-defined polymers which are olefin- or aluminum-terminated. Miilhaupt has polymerized propylene with a chiral metallocene and MAO under conditions where P-hydrogen elimination was the predominant chain transfer process. In a post-polymerization functionalization, the olefin endgroups of the highly isotactic polypropylene chains were converted to bromo-, epoxy-, anhydride-, ester-, amine-, carboxylic acid-, silane-, borane-, hydroxy-, thiol-terminated polymers as intermediates for the preparation of block copolymers. Using olefin-terminated atactic and isotactic polypropylene formed with MAO-activated Cp2ZrCl2 and (EBTHI)ZrCl2 Shiono has synthesized amine- and aluminum-terminated polymers." ... 

Two types of solid product are prepared commercially, viz hydroxy- and thiol-terminated polymers. The significance of the nature of the end-groups lies in the means whereby the polymer may be cured. (See Section 17.2.5.)... 

Thiol-terminated polymers are by far the most important type of polysulphide polymer manufactured. They may be readily cured by oxidative coupling of the thiol terminals ... 

An important difference between solid hydroxy- and thiol-terminated polymers is that since the latter are of relatively low molecular weight it is possible to prepare base polymers which are slightly branched (by including a trihalide in the polymerization system) but still easily handled. When the branched material is cured a cross-linked product is obtained and this has impoved compression set resistance. In contrast, if a trihalide is included in the polymerization system and the resulting polymer is not cleaved by a hydrosulphide/sulphite treatment, the product is too tough to be worked on a rubber mill. 

Liquid thiol-terminated polymers are also frequently used in conjunction with epoxy resins (Chapter 16). Interaction of the two materials leads to chain... 

The use of the base-catalyzed thiol-isocyanate reaction in the quantitative preparation of m-end functional polymers was also recently reported (Li et al, 2009). Here, RAFT polymerization was used to prepare poly (A, N- diethylacrylamide), the dithioester end group of which was transformed into a thiol moiety by aminolysis. Subsequent end-group modification of the resulting thiol-terminated polymer was achieved in an overnight, ambient temperature and oxygen-free reaction with one of a collection of commercially available isocyanates in the presence of triethylamine (Figure 2.11a). 

Although a twofold excess of isocyanate was used and the reactions were allowed to proceed overnight, a real-time reaction-kinetics experiment was performed. Hexyl isocyanate, thiol-terminated polymer and triethylamine were added in the ratio of 1 1 1.75 in THF and the reaction that followed was monitored at 20 °C. As can clearly be observed in Figure 2.1 lb, 94% conversion of the isocyanate is achieved within 15 min. 

A polymer made by the RAFT process with a dithioester results in a dithioester end group. This ester can also be cleaved by aminolysis, resulting in a thiol terminated polymer. In an analogous way to the bis-maleimide compound, divinyl sulfone can be used, thereby creating a vinyl sulfone end group. This group is then able to react with cysteine groups by Michael addition (13) [135]. 

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]. 

Similarly, RAFT polymer poly[Af-(2-hydroxypropyl)methacrylamide], after being converted to semitelechelic thiol-terminated polymers, can be conjugated... 

For example, polymerizing inorganic nanoparticles with cross-linker divinyl-benzene yields composite polymer-inorganic latex particles functionalized with vinyl groups. In the process of composite miniemulsion polymerization, vinyl groups were easily introduced to the particle surface by using cross-linker divinylbenzene, thiol-terminated polymers can be further clicked on the particle surface via a thiol-ene approach catalyzed by V-50 as a free-radical source (Figure 8.27) [48]. 

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