EO block

The term poloxamer is widely used to describe a series of ABA block coploymers of polyethylene oxide and polypropylene oxide, extensively used in industry as antifoams, emulsifiers, wetting agents, rinse aids, and in numerous other applications [1-5]. Poloxamers are amphiphilic in character, being comprised of a central polypropylene oxide (PO) block, which is hydrophobic, sandwiched between two hydrophilic polyethylene oxide (EO) blocks as shown below ... 

For the central PO block to serve as an effective hydrophobe, the value of n must be at least 15 the value of m in commercially manufactured poloxamers is such that the EO blocks constitute between 10-80% of the total polymer mass. The absolute and relative masses of the hydrophilic and hydrophobic blocks, on which the physico-chemical properties of the polymers depend, can be controlled during manufacture, enabling the production of poloxamers tailored to specific applications. 

The initiator usually constitutes less than 1% of the final product, and since starting the process with such a small amount of material in the reaction vessel may be difficult, it is often reacted with propylene oxide to produce a precursor compound, which may be stored until required [6]. The yield of poloxamer is essentially stoichiometric the lengths of the PO and EO blocks are determined by the amount of epoxide fed into the reactor at each stage. Upon completion of the reaction, the mixture is cooled and the alkaline catalyst neutralized. The neutral salt may then be removed or allowed to remain in the product, in which case it is present at a level of 0.5-1.0%. The catalyst may, alternatively, be removed by adsorption on acidic clays or with ion exchangers [7]. Exact maintenance of temperature, pressure, agitation speed, and other parameters are required if the products are to be reproducible, thus poloxamers from different suppliers may exhibit some difference in properties. 

Santos et al. [128,129] and Guerrero et al. [130] prepared segmented poly(EO) containing bistrialkylbenzopinacolate moieties to synthesize poly(St)-poly(EO) block copolymers. The St polymerization with the polymeric iniferter 27 was comparable to that initiated with small molecular benzopinacolates. 

It turned out that for all the polymeric amphiphiles of the (EO) -(PO)m-(EO) type there was an increase in enantioselectivity compared with the reaction without amphiphile. Moreover, the ratio of the length of the (PO) block compared with the (EO) block seemed to determine enantioselectivity and activity and not the cmc (critical micelle concentration). A (PO) block length of 56 units works best with different length of the (EO)n block in this type of hydrogenation [30]. for the work-up of the experiments, G. Oehme et al. used the extraction method, but initial experiments failed and the catalyst could not be recycled that way. To solve this problem the authors applied a membrane reactor in combination with the amphiphile (EO)37-(PO)5g-(EO)37 (Tab. 6.1, entry 9) [31]. By doing so, the poly-mer/Rh-catalyst was retained and could be reused several times without loss of activity and enantioselectivity by more than 99%. 

Recently, carboxyl- and amino-functionalized polystyrene latex particles were synthesized by the miniemulsion copolymerization of styrene and acrylic acid or 2-aminoethyl methacrylate hydrochloride (AEMH) [70, 71]. The reaction was started by using an oil-soluble initiator, 2,2 -azobis(2-methylbutyronitrile) (V-59). Two types of surfactant, i.e., ionic negatively charged SDS or positively charged CTMA-Cl, and nonionic Lutensol AT50 (which is a PEO hexadecyl ether with an EO block length of about 50 units) were used to stabilize the initial droplets and final particles. 

Unfortunately, DMC catalysts are not efficient for EO polymerisation, and it is practically impossible to obtain PO-EO block copolymers with this catalyst. Acidic catalysts are not used on an industrial scale for alkylene oxide polymerisation due to the formation of substantial amounts of cyclic ethers as side products. Acidic catalysts are used industrially only for the synthesis of polytetrahydrofuran polyols or, to a lesser extent, for tetrahydrofuran - alkylene oxide copolyether polyol fabrication (see Sections 7.1, 7.2 and 7.3) Other catalysts have a minor importance for large scale polyether polyol production. 

By using two oxiranic monomers, such as PO and EO, it is possible to obtain a great variety of polyether polyols homopolymers of PO, block copolymers PO-EO (with terminal or internal poly[EO] block) or random copolymers (heteropolyethers) of PO-EO, diols or triols of different MW. 

The polyether triols are the most important class of polyether polyols and they are used in flexible PU foam fabrication. The majority of polyether triols used in flexible foams are copolymers of PO-EO. Random copolymers are used in continuous slabstock flexible foams and block copolymers (PO-EO), with terminal poly[EO] block, are used in moulded foams (hot moulding and cold cure moulding processes). 

Polyether triols with poly[EO] block linked to the starter (MW = 3000-3600)... 

It can be seen that moulded flexible PU foams using EO capped polyether polyols (block copolymers PO-EO with terminal poly[EO] block) represent only 22% of total worldwide consumption and that the majority of foams are flexible slabstock PU foams which use random copolyethers of PO-EO. It can therefore be concluded that the most important polyols for flexible PU foams production are in fact the random copolyethers PO-EO. 

A similar polyether polyol, having the same EO and PO content but in the form of a block copolymer (internal poly[EO] block and terminal poly[PO] block), gives a solid polyether in the form of a wax, probably due to the crystallisation of poly[EO] chains. 

This polyether triol hybrid structure block [PO] - random [PO - EO]- block EO, with a MW of 3000 daltons, was used successfully for hot moulded flexible PU foams with... 

An interesting hybrid structure [97] was obtained by an alternate PO and EO addition without any intermediary digestion or degassing. The resulting structure, block [PO]-random [PO-EO] - block [EO] - random [PO-EO] - block [PO], is shown in Figure 4.22. 

The pseudoliving character of PO anionic polymerisation produces a large variety of block copolymers, by simply changing the nature of the oxirane monomer because the catalytic species (potassium alcoholate) remains active during and after the polymerisation reaction. Thus, if a polyether is synthesised first by anionic polymerisation of PO and the polymerisation continues with another monomer, such as EO, a block copolyether PO-EO with a terminal poly[EO] block is obtained. Another synthetic variant is to obtain a polyethoxylated polyether first by the anionic polymerisation of EO initiated by glycerol [108], followed by the addition of PO to the resulting polyethoxylated triol. A block copolyether PO-EO is obtained with internal poly[EO] block linked to the starter. Another possibility is to add the monomers in three steps first PO is added to glycerol, followed by EO addition and finally by the addition of PO. A copolyether triol block copolymer PO-EO with the internal poly[EO] block situated inside the polyetheric chain between two poly[PO] blocks is obtained [4, 100, 101]. 

The synthesis of polyether triols, block copolymers with terminal poly[EO] block is relatively simple in the first step a propoxylated intermediate polyether is synthesised by the polyaddition of PO to the starter (glycerol or propylene glycol). After the addition of the required quantity of PO, the unreacted monomer is eliminated by vacuum distillation and the polymerisation continues by the addition of EO, the second monomer. 

Polyether triol, block copolymer PO-EO, with terminal poly[EO] block... 

Figure 4.23 Variation of primary hydroxyl content as a function of EO content in polyether triols block copolymers [PO-EO] with terminal poly[EO] block MW = 5000 daltons catalyst KOH - 0.0056 mol%...

Figure 4.24 Graphical representation of Equation 4.17 in the form of a straight line, at polyether triols of MW of 5000 daltons, block copolymers PO-EO with terminal poly[EO] block Distribution constant K = 14...

The polyether diols, block copolymers of PO-EO with terminal poly[EO] block are obtained absolutely identically to the previously described EO capped polyether triols, the difference being that the propoxylated intermediate is a propoxylated polyether diol. 

The most important polyether, PO-EO block copolymer structures, having terminal poly[EO] block (structure a) and internal poly[EO] block (structures b and c), are presented in Figure 4.28. 

Figure 4.28 The structures of polyether triol block copolymers PO-EO a) terminal poly[EO)] block b) poly[EO] block linked to the starter c) internal poly[EO] block...

The most important polyether triol, PO-EO block copolymers with poly[EO] block, used in practice, are ... 

Table 4.10 Characteristics of polyether triol, based on glycerol PO-EO block copolymers (terminal block) with a MW of 3000 daltons ...

The polyether triols (PO-EO block copolymers with terminal poly[EO] block) are very reactive polyols due to the presence of a high percentage of primary hydroxyls. These polyether polyols with terminal poly[EO] block, are used preferentially for moulded flexible PEI foams. 

As an example, a group of polyether diols (block PO-EO copolymers with terminal poly[EO] block) are the polyethers derived from propylene glycol (or DPG), PO and EO of MW of 2000 daltons and around 15-20% EO as a terminal block (Figure 4.29). 

In Sections 4.1, 4.1.1, 4.1.2, 4.1.3 and 4.1.4, the chemistry of polyether polyol synthesis, the mechanism and kinetics of alkylene oxide polyaddition to hydroxyl groups and the most important structures of polyalkylene oxide polyether polyols for elastic polyurethanes - PO homopolymers, random PO-EO copolymers and PO-EO block copolymers - were discussed. 

Tables 4.9-4.14 show some general characteristics of polyether polyol PO-EO block copolymers, such as MW, hydroxyl number, functionality, viscosity and colour, but also some other characteristics such as unsaturation, EO content, and potassium and sodium content which are specific for polyether polyols.

The advantage of phosphazenium catalysts, compared to DMC, is their capability to catalyse the anionic polymerisation of PO and EO and to be used successfully in the synthesis of PO-EO block copolymers with terminal poly[EO] block, without intermediate change of the catalyst nature. 

The synthesis of block copolymers PO-EO with a terminal poly[EO] block is practically impossible, cloudy polyols always being formed, with a very low ethoxylation rate. The formation of cloudy polyols is because of an unfavourable (nonuniform) distribution of... 

Excellent PO-EO block copolyether polyols with terminal poly[EO] block, are formed by the addition, to the intermediate propoxylated polyether obtained with DMC catalysts, of an anionic catalyst (KOH or potassium alcoholates) followed by the addition of EO by classical technology, via an anionic mechanism. By this relatively complicated route, it is possible to obtain PO-EO block copolymers with high primary hydroxyl content and very low unsaturation. 

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