Step-growth polycondensation

The synthesis and the modification techniques concerning PES have been reviewed by Kricheldorf. Poly(ethersulfone)s can be obtained either by a conventional step-growth polycondensation, or by a chain-growth polycondensation, which is in fact a living polycondensation.  

A broad range of PES can be formed by the nucleophilic aromatic condensation reaction of an aromatic dihydroxy compound and a bis-(halophen- 

The condensation can be conducted in several ways. The most convenient way is to prepare the salt of aromatic dihydroxy compound in situ, using a high-boiling solvent that forms an azeotrope with water and then allowing it to react with the bis-(halophenyl)-sulfone. In this way, problems with residual water can be avoided. 

Limitations on polymer molecular weights may be expected when the aromatic dihydroxy compound or the corresponding alkali metal derivative contain strong electron-withdrawing groups. This may result in lower molecular weight polymers or slow reaction rates. Therefore, the aromatic dihydroxy compound should be a rather weakly acidic phenol, such as 2,2-bis-(4-hydroxyphenyl)-propane, l,l-bis-(4-hydroxyphenyl)-2-phenyl ethane, and bis-(4-hydroxyphenyl)-methane. 

Poly(biphenyl ether sulfone)s with improved polydispersity, a lower level of undesirable low-molecular-weight oligomeric components and improved melt flow properties have 4,4 -biphenylene, p-phenylene, 4,4 -di-phenyl sulfone and 2,2-diphenyl propane groups in the backbone.  

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A second example of step-growth polycondensations with formation of the ole-finic double-bond are Wittig- and Wittig-Horner-type condensations. The Wittig-type polycondensations involve AA/BB-type reactions of aromatic bisal-dehydes with bisphosphonium ylides [99,100] with formation of PPV derivatives (75) and lead to products of only moderate molecular weight (DP 10-20). 

Acyclic diene metathesis (ADMET) is a step-growth polycondensation reaction for the polymerization of o -dienes 729 The process is catalyzed by the same metal alkylidene initiators used for ROMP, and is driven by the removal of ethylene from the system (Scheme 13). Both molybdenum and ruthenium-based initiators have been used to prepare a variety of materials including functionalized polyethy-... 

The new polymers 9-28 were synthesized by the Pd-catalyzed reaction of (4,4 - or 5,5 -dibromo-2,2 -bipyridine)-bis(4,4 -terf-butyl-2,2 -bipyridine or 2,2 -bipyridine)ruthenium(II) complexes 5-8 and diethynylarenes (Table 2) in a step-growth polycondensation mechanism (Scheme 4). The typical reaction conditions used for the synthesis of the polymers involved stirring the argon-... 

Many researchers have investigated the use of amines and alcohols as initiators for the ROP of lactones. As a rule, amines and alcohols are not nucleophilic enough to be efficient initiators, and it is then mandatory to use catalysts to perform the polymerization successfully. Nevertheless, highly reactive p-lactones exhibit a particular behavior because their polymerization can be initiated by nucleophilic amines in the absence of any catalyst. As far as tertiary amines are concerned, the initiation step implies the formation of a zwitterion made up of an ammonium cation and a carboxylate anion, as shown in Fig. 20. Authors coined the name zwitterionic polymerization for this process [80]. Nevertheless, this polymerization is not really new because the mechanism is mainly anionic. Interestingly, Rticheldorf and coworkers did not exclude the possibility that, at least at some stage of the polymerization, chain extension takes place by step-growth polycondensation [81]. 

FIGURE 22.1 Relationship of the product number average molecular weight, Mn, to the percent monomer conversion for (a) a step-growth, polycondensation versus (b) a chain growth (vinyl-type) polymerization. 

Phenol blocked isocyanates also have been used to prepare hyperbranched polyurethanes by a step-growth polycondensation mechanism, using DBTDL as a catalyst. 4... 

ADMET has been shown to be a step-growth polycondensation reaction [31[. The kinetics of step-growth polymerization and consequences thereof are completely different than those of chain polymerizations. Since ROMP and many other single-site transition metal-catalyzed polymerizations discussed in this book proceed... 

An array of versatile and efficient step-growth polycondensation methodologies has been developed for the synthesis of transition metal-acetylide polymers. In this section, a representative review of the types of routes available and materials accessible is given. 

A step-growth polycondensation route has been succesfully devised to prepare novel nickel polymers 207 with arene spacer groups. The procedure involved the polycondensation of the fluorinated dilithiated species 206 and an Ni(ii) complex (Equation (75))." " The rod-like structure of these polymers was established by dilute-solution viscosity measurements, and the results were similar to those reported for the related platinum polyyne polymers 166 (M = Pt(P Bu3)2 x = 2) (Section 12.06.5.2.3). 

In 2001, an efficient step-growth polycondensation approach, reminiscent of that used to prepare metal-metal-bonded polymers 217 and 218, was successfully developed to yield polyurethanes 225 with Mo2lt2 clusters in the backbone. " Well-characterized polymers with molecular weights of up to 136,000 (PDI = 3.1), which correspond to a number average of around 30 clusters per polymer chain, were obtained by means of this method. It involved treatment of a diol of the Mo2lt2 cluster with the diisocyanate OCN-R-NGO (R = aliphatic or aromatic spacer) in the presence of a tin catalyst. The unusual optical properties of organometallic clusters will drive applications-oriented work on these materials. " ... 

A well-defined poly(ether sulfone) can be synthesized from 4-flu-oro-4 -hydroxydiphenyl sulfone, with (fluorophenyl)(trifluorophenyl) sulfone as an initiator, as shown in Figure 7.4. The condensation is performed with 18-crown-6 ether in sulfolane at 120°C. Flowever, when the polymerization is conducted at a higher feed ratio of monomer to initiator, both chain-growth and step-growth polycondensation occurs. 

This is the number distribution function for a linear step-growth polycondensation at the extent of reaction p. In this instance, X is given by the relation... 

Early attempts to prepare main-chain metal-containing polymers mainly focused on the use of step-growth polycondensation processes (Scheme 1.1, Route B). These routes work well for the synthesis of carbon-based polymers when difunctional organic monomers are used because the latter are generally easily accessible in a high degree of purity. This allows the stringent stoichiometry and conver-... 

Rod-like terpyridyl polymers 7.6 have also been accessed through step-growth polycondensation strategies (Eq. 7.3). Analogous routes those employed for the formation of 7.3-7.5 were successful in the preparation of high molecular weight materials with DPn>30 [12]. However, Pd-catalyzed polycondensation processes afforded only low molecular weight products. 

Figure 25. Step-growth polycondensation forming poly(ethylene terephthalate) (PET).

ADMET polymerization represents a versatile technique for the synthesis of unique, complex, and functional polymer stractures. ADMET is a step-growth polycondensation reaction that proceeds under mild conditions, whereby any molecule that can be functionalized with two terminal olefin groups has the potential to become an ADMET monomer. This, in turn, allows an almost Hmitless possibility to create interesting and useful polymer structures. Recently, ADMET has been used to synthesize functionalized PEs, silicon-containing elastomers, conductive polymers, and many other exotic and interesting macromolecules. Yet, this area of research is by no means exhausted rather, this simple and elegant reaction will continue to provide the means to explore the basic stracture-property relationships of complex functional materials. 

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