Mono-telechelic polymers

While end-functional polymers are clearly important industrial products for materials synthesis, they are also interesting from an academic point of view. Many complex macromolecular architectures can be realized from end-functional polymeric starting materials. Two mono-telechelic polymers can, for example, be joined to form a diblock copolymer, or several such polymers can be joined to form star-like polymers. Mono-telechelic polymers can also be attached to a linear, multifunctional polymer to yield graft copolymers. Depending on the functional end group, such polymers can attach to macroscopic or nanoparticle surfaces or form conjugates with bio-oligomers [5] or biomacromolecules [6]. 

Synthetic procedures are in place today that allow the polymer end-functionalization for all commonly used carbene complexes based on ruthenium, tungsten, and molybdenum. From a practical and applications point of view, both ends of a polymer chain are equally useful. From a mechanistic point of view, the functionalization of the polymer chain end using the reactivity of the propagating carbene complex is much more readily achieved than functionalization of the focal group. Nonetheless, functional initiation is an attractive way to ensure complete end (start) functionalization, which has received comparatively little attention so far. Reliable methods for the functional derivatization of commercially available carbene catalysts will allow not only the synthesis of mono-telechelic polymers with high degrees of end-functionalization but also polymerization from surfaces or solid supports and open up more synthetic pathways to hetero-telechelic polymers. 

The problem of molecular weight distribution (MWD) and functionality type distribution (FTD) belongs by definition to an extensive problem of molecular heterogeneity of polymers. In the synthesis of a polymer with the requested properties, e.g. a telechelic polymer, one is always faced with different types of polydispersity the macromolecules can be of different length, they can have a different number of functional groups, i.e. be mono-, bifunctional, etc., they can be branched (star-, comb- or tree-like) and, finally, they can be cyclic. 

These heterogeneities, which can be called elementary , can be superimposed one on the other, i.e. bifunctional molecules can be linear or branched, linear molecules can be mono- and bifunctional, etc. In order to characterize in an ideal way a telechelic polymer with respect to its subsequent transformation, it is necessary to know a set of functions (fj(M), the molecular weight distributions within each heterogeneity type. Clearly, it is very difficult in a general case to solve this characterization problem. 

Although several telechelic polymers of 1,3-dioxolane have been prepared by cationic polymerization, their application is limited due to their susceptibility to acid-catalyzed hydrolysis and/or depolymerization. By termination of living mono- and difunctional poly(l,3-dioxolane) with amines or phosphines, polymers containing one or two stable ionic (ammonium, phosphonium) end groups has been prepared [129,274],... 

ATRC, is based on the application of Cu(0) to an ATRP system, leading to the reduction of any Cu(ll) in the reaction medium to form Cu(I). The increase of Cu(l) in the reaction medium dramatically shifts the equilibrium between active and dormant species to the side of the active radical species. So the high concentration of radicals presents in the system favor the recombination reactions between two macroradicals. This method was performed to obtain telechelic polymer from monotelechelic polystyrene and results showed that it is very effective way with high efficiency in a short reaction time. Figure 1 shows a,co-telechelic polymer with double molecular weights as compared with the mono functional polystyrene obtained by ATRP using aldehyde functional initiator. 

Figure 1. Gel Permeation Chromatography (GPC) traces of mono-aldehyde functional starting polymer and a,co-telechelic polymer obtained by ATRC.

More recently, a convenient, one-pot S5mthesis of telechelic polymers with imsaturated end groups was developed (130). Addition of excess ethyl(2-bromomethyl)acrylate to ATRP of acrylate monomers after 80-90% conversion resulted in the formation of mono- and bifunctional polymers. The average degree of end functionality was almost quantitative (/" = 1 for a monofiinctional and/ = 2 for a bifiinctional initiator). 

Depending on the choice of transfer agent, mono- or di-cnd-functional polymers may be produced. Addition-fragmentation transfer agents such as functional allyl sulfides (Scheme 7.16), benzyl ethers and macromonomers have application in this context (Section 6.2.3).212 216 The synthesis of PEG-block copolymers by making use of PEO functional allyl peroxides (and other transfer agents has been described by Businelli et al. Boutevin et al. have described the telomerization of unsaturated alcohols with mercaptoethanol or dithiols to produce telechelic diols in high yield. 

For comparison, a telechelic sulfonated polystyrene with a functionality f = 1.95 was prepared. In cyclohexane the material forms a gel independent of the concentration. At high concentrations the sample swells. When lower concentrations were prepared, separation to a gel and sol phase was observed. Thus, dilution in cyclohexane does not result in dissolution of the gel even at elevated temperatures. Given the high equilibrium constant determined for the association of the mono functional sample, the amount of polymer in the sol phase can be neglected. Hence, the volume fraction of polymer in the gel phase can be calculated from the volume ratio of the sol and gel phases and the total polymer concentration. The plot in Figure 9 shows that the polymer volume fraction in the gel is constant over a wide range of concentrations. 

A polymer can be considered to be telechelic if it contains end gronps that react selectively to give a bond with another molecule. Depending on the fimction-ality, which mnst be distingnished from the fnnctionality of the end gronp itself, telechelics can be classified as mono-, di-, tri-, and multifunctional telechelics (polytelechelics) (9). The fnnctionality is defined as... 

ADMET has been used to prepare unsaturated telechelic oligomers of PBD by reacting 1,5-hexadiene with an appropriate mono or difunctional olefin [176b, 178]. Alternatively, 1,5-hexadiene may undergo ADMET polymerization to form ADMET PBD and subsequently be reacted with an appropriate functionalized olefin. This reaction is believed to proceed through a cyclic intermediate of the PBD followed by CM of the cyclic polymer with the functionalized olefins (Figure 13.23) [179]. In some cases, the formation of telechelics is incomplete. 

Tg measurements have been performed on many other polymers and copolymers including phenol bark resins [71], PS [72-74], p-nitrobenzene substituted polymethacrylates [75], PC [76], polyimines [77], polyurethanes (PU) [78], Novolac resins [71], polyisoprene, polybutadiene, polychloroprene, nitrile rubber, ethylene-propylene-diene terpolymer and butyl rubber [79], bisphenol-A epoxy diacrylate-trimethylolpropane triacrylate [80], mono and dipolyphosphazenes [81], polyethylene glycol-polylactic acid entrapment polymers [82], polyether nitrile copolymers [83], polyacrylate-polyoxyethylene grafts [84], Novolak type thermosets [71], polyester carbonates [85], polyethylene naphthalene, 2,6, dicarboxylate [86], PET-polyethylene 2,6-naphthalone carboxylate blends [87], a-phenyl substituted aromatic-aliphatic polyamides [88], sodium acrylate-methyl methacrylate multiblock copolymers [89], telechelic sulfonate polyester ionomers [90], aromatic polyamides [91], polyimides [91], 4,4"-bis(4-oxyphenoxy)benzophenone diglycidyl ether - 3,4 epoxycyclohexyl methyl 3,4 epoxy cyclohexane carboxylate blends [92], PET [93], polyhydroxybutyrate [94], polyetherimides [95], macrocyclic aromatic disulfide oligomers [96], acrylics [97], PU urea elastomers [97], glass reinforced epoxy resin composites [98], PVOH [99], polymethyl methacrylate-N-phenyl maleimide, styrene copolymers [100], chiral... 

Because initiation of vinyl ethers with HI/I2 yields polymers with extremely narrow poly-dispersities," " bifunctional vinyl ethers can be used in combination with HI/I2 to polymerize vinyl ethers and produce a,co-diiodo telechelics (Scheme 39). This method has been used to produce telechelics from ethyl vinyl ether, methyl vinyl ether and hexadecyl (cetyl) vinyl ether. Treatment of the living poly(ethyl vinyl ether) with mono- and di-amines yields telechelics with amino end groups (Scheme 39, equation 41). More jecently, telechelic poly(vinyl ether)s with terminal malonate or carboxy groups were synthesized using both a functional initiator and a functional terminator according to Scheme 40." ... 

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