Chains extenders

The chain extenders play a very important role. Without a chain extender, a PU formed by directly reacting diisocyanate and polyol generally has very poor physical properties and often does not exhibit microphase separation. 

The introduction of a chain extender may increase the HS length to allow HS segregation, which results in good mechanical properties, such as an increase in the modulus and an increase in the HS glass transition temperature of the polymer. 

Examples of diol chain extenders (Table 1.1) include 1,6-hexanediol (HG) [20], ethylene glycol (EG) [58, 60, 61], 1,4-butanediol (BDO, BD or BG) [53, 75, 77, 78], diethylene glycol (DEG) [61], diacetylene diols [50] such as 2,4-hexadiyne-1,6-diol or 5,7-dodecadiyne-l,12-diol and some aromatic diols triazine diols [72, 85] or 4,4 -(ethane-l,2-diyl)bis(benzenethiohexanol) [86]. 

Examples of diamine chain extenders (Fig. 1.10) include 2,5-bis-(4-amino-phenylene-l,3,4-oxadiazole) (DAPO), 4,4 -diamino-dibenzyl (DAB), 2,6-diamino-pyridine (DAPy) or 4,4 -methylene-diamine (MDA). 

The chain extender structure strongly influences the PUs mechanical performance. Modifying the ratio between the polyol and chain extender, PUs may result in a change from a hard, brittle material to a rubbery elastomer, as a result of the variation of the HS concentration (defined as the ratio of the mass of the non-polyol components to the total mass of the polymer) [2-4]. 

Several studies have been carried out on the chain extension of aromatic polyesters through the use of various additive species over the last 20-30 years [162-190] many patents have also been applied for in this area [191-210]. 

The basic concept for a chain extender is a molecule which is polyfunctional (preferably difunctional), and where functionalities 

Early attempts at chain extension took the route of using esters of dicarboxylic acids which had greater reactivity towards the polyester chain ends than simplistic additives such as dimethyl terephthalate [191, 193, 194], but many of the more reactive species gave nonvolatile small-molecule by-products such as phenol, which were difficult to remove. Another early attempt [192] used diisocyanates, but this approach can give imdesirable branching, and the new linkage formed was thermally unstable. Later studies used diisocyanates to chain-extend recycled PET [178]. 

Scientists at Teijin Ltd., patented [196,197] and published extensive studies on [162-167] the use of bis(cyclic imino ethers) and bis(cyclic imino esters) as chain extenders for PET. 

Cylic imino ethers such as 2,2 -bis(2-oxazoline) (BOZ) react with carboxylic acid chain ends without production of a leaving group  

To develop low melting and highly processable aromatic diol extenders from resorcinol and ethylene carbonate (HER materials). 

To determine the nature of products or by-products formed by the resorcinol-ethylene carbonate reaction. 

To investigate how the high MW aromatic diol monomer affects the phase separation behavior of polyurethane elastomers. 

To examine the effect of various HER chain extenders on the physical, mechanical and dynamic mechanical properties of cast polyurethanes made with MDI-terminated prepolymers. 

To study both polyether- and polyester-based polyurethane systems. 

The two main groups used as chain extenders are diamines and hydroxyl compounds. Triols are also used where some cross-linking is required. The choice of chain extender depends on the properties required and the process conditions. Diols are the most commonly used hydroxyl compound. In the normal course of events, diols provide good properties and processing speed with MDI-based prepolymers and diamines with TDI-terminated prepolymers. 

Aromatic diamines are the most commercially used chain extenders with TDI-based polyurethanes. The rate of reaction of a simple aromatic diamine is too great for normal use. Thus the rate of reaction is commonly controlled by having substitutes on the aromatic ring. An example is the simple diisocyanate MDA and MOCA with the chlorine atoms in MOCA slowing the reaction to a useable rate. 

A chlorine or methyl group ortho (positions 1,2 = ortho) to the amine group gives optimum properties in a diamine-cured compound. It has been found that a methoxy group next to the aromatic amine gives polyurethane with inferior properties. 

Some aromatic diamines used as curing agents, such as 4,4 diaminodi-phenyl derivatives or 3,3 Dichloro 4,4 diaminodiphenylmethane (MOCA), have been investigated and have been declared to have suspect carcinogenic properties. All local rules and regulations must be adhered to if these materials are used. 

Amines produce polyurethanes with better mechanical properties than when diols are used for curing. Amines produce polyurethanes with a lower temperature resistance than when diols are used. The use of catalysts has been found to direct the cross-linking reactions away from the biuret to the allophanate reactions. 

Polyester polyols and sometimes polyether polyols are reacted with TDI to give better compression set and high-temperature properties than might be achieved with MDI. Better properties from a TDI-terminated prepolymer can be achieved more effectively when it is cured with a diamine. 

TDI-terminated prepolymers can impart unique properties to a PUR application, especially if a diamine curative is being used. MDI-terminated prepolymers have difficulty matching certain physical properties. 

Chain extenders are used by polyurethane fabricators to cure a liquid isocyanate-terminated prepolymer into a useful product. Typically this chain extender can be a 

Diol curatives can be used as chain extenders to cure the isocyanate-terminated 

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With electrons flowing from ethylene to zirconium the Zr—CH3 bond weakens the carbons of ethylene become positively polarized and the methyl group migrates from zirconium to one of the carbons of ethylene Cleavage of the Zr—CH3 bond is accom panied by formation of a ct bond between zirconium and one of the carbons of ethylene m Step 3 The product of this step is a chain extended form of the active catalyst ready to accept another ethylene ligand and repeat the chain extending steps... 

Section 27 18 In the Memfield method the carboxyl group of an ammo acid is anchored to a solid support and the chain extended one ammo acid at a time When all the ammo acid residues have been added the polypeptide is removed from the solid support... 

Polyurethanes. The hard portion of polyurethane consists of a chain extender and polyisocyanate. The soft component is composed of polyol segments. 

Uses. The largest uses of butanediol are internal consumption in manufacture of tetrahydrofuran and butyrolactone (145). The largest merchant uses are for poly(butylene terephthalate) resins (see Polyesters,thermoplastic) and in polyurethanes, both as a chain extender and as an ingredient in a hydroxyl-terminated polyester used as a macroglycol. Butanediol is also used as a solvent, as a monomer for vadous condensation polymers, and as an intermediate in the manufacture of other chemicals. 

The reaction of OF2 and various unsaturated fluorocarbons has been examined (35,36) and it is claimed that OF2 can be used to chain-extend fluoropolyenes, convert functional perfluorovinyl groups to acyl fluorides and/or epoxide groups, and act as a monomer for an addition-type copolymerization with diolefins. 

Fillers (qv) are occasionally used in flexible slab foams the two most commonly used are calcium carbonate (whiting) and barium sulfate (barytes). Their use level may range up to 150 parts per 100 parts of polyol. Various other ingredients may also be used to modify a flexible foam formulation. Cross-linkers, chain extenders, ignition modifiers, auxiHary blowing agents, etc, are all used to some extent depending on the final product characteristics desired. 

In the second step, a papermaking method is also used for the fine fibers, less than 0.1 tex (1 den). This process is usually followed by a high pressure water jet process instead of the third step. In the fourth step, to obtain the required properties in specific appHcations, a polyurethane is selected out of the segmented polyurethanes, which comprises a polymer diol, a diisocyanate, and a chain extender (see Urethane polymers). A DMF—water bath for coagulation is also controlled to create the adequate pore stmcture in combination with fibers. 

After polymeri2ation is carried out by blending mono- and difunctional chlorosilanes ia excess water, the siloxanes are separated from the water and neutraH2ed. Ratio of the mono-chain stopper to di-chain extender controls the length of the polymer. Once an equiHbrium mixture of chain lengths is catalyticaHy formed, volatile light ends are removed and the desired product results. 

The synthetic scheme typically involves chain-extending addition of protected mononucleotides to a nucleoside bound covalentiy at the 3 -hydroxyl to an inert siUca-based soHd support, such as controlled pore glass (Fig. 11). The initial base-protected 5 -O-dimethoxytrityl (DMT) deoxynucleoside is linked to the soHd support via the reaction of a siUca-bound amino-silane and the -nitrophenylester of the 3 -succinylated nucleoside, yielding a 3 -terminal nucleoside attached to the soHd support (1) (Fig. 11). Chain elongation requites the removal of the 5 -DMT protecting group. 

Polymerization by Gycloaddition. Bisimides and oligoimides capped with reactive unsaturations such as maleimide, acetylene, and xylylene groups, can be chain-extended by a cycloaddition reaction with proper bisdienes. 

Another subclass of substituted amides that is of great commercial value is the ethoxylated amides. They can be synthesized from alkanolamides by chain extending with ethylene or propylene oxide or by ethoxylation directly from the primary amide (46—48). It was originally beheved that the stepwise addition of ethylene oxide (EO) would produce the monoethano1 amide and then the diethanolamide when sufficient ethylene oxide was added (49), but it has been discovered that only one hydrogen of the amide is substituted with ethylene oxide (50—53). As is typical of most ethylene oxide adducts, a wide distribution of polyethylene oxide chain length is seen as more EO is added. A catalyst is necessary to add ethylene oxide or propylene oxide to a primary or an ethoxylated amide or to ethoxylate a diethoxy alkanolamide synthesized from diethanolamine (54). 

Only a few commercial uses for TDA per se have been found. In epoxy curing appHcations, 2,4- I DA has been used as a component of a eutectic mixture with short chain aUphatic glycidal ether resins (46) as well as by itself (46,47) TDA (46) and single isomers (47) are also used as amine curatives. TDA can be used as a chain extender in polyurethanes (48,49). TDA is cited as a monomer in making aromatic polymers with unique properties, eg, amorphous polyamides (50), powdered polyamides (51), and low melting, whoUy aromatic polyamides (52). 

Almost all TDA use is as a chemical intermediate, mosdy in polyurethanes. Toluenediamine derivatives are found as all three components of urethanes isocyanates, chain extenders, and polyols (see Isocyanates, organic Urethane polymers). 

Almost all IDA derived chain extenders are made through ortho-alkylation. Diethyltoluenediamine (DE I DA) (C H gN2) (53), with a market of about 33,000 t, is the most common. Many uses for /-B I DA have been cited (1,12). Both DE I DA and /-B I DA are especially useful in RIM appHcations (49,53—55). Di(methylthio)-TDA, made by dithioalkylation of TDA, is used in cast urethanes and with other TDI prepolymers (56). Styrenic alkylation products of TDA are said to be useful, eg, as in the formation of novel polyurethane—polyurea polymers (57,58). Progress in understanding aromatic diamine stmcture—activity relationships for polyurethane chain extenders should allow progress in developing new materials (59). Chlorinated IDA is used in polyurethane—polyurea polymers of low hysteresis (48) and in reinforced polyurethane tires (60). The chloro-TDA is made by hydrolysis of chloro-TDI, derived from TDA (61). 

Fig. 2. Protein secondary stmcture (a) the right-handed a-helix, stabilized by intrasegmental hydrogen-bonding between the backbone CO of residue i and the NH of residue t + 4 along the polypeptide chain. Each turn of the helix requires 3.6 residues. Translation along the hehcal axis is 0.15 nm per residue, or 0.54 nm per turn and (b) the -pleated sheet where the polypeptide is in an extended conformation and backbone hydrogen-bonding occurs between residues on adjacent strands. Here, the backbone CO and NH atoms are in the plane of the page and the amino acid side chains extend from C ...

Multilayers of Diphosphates. One way to find surface reactions that may lead to the formation of SAMs is to look for reactions that result in an insoluble salt. This is the case for phosphate monolayers, based on their highly insoluble salts with tetravalent transition metal ions. In these salts, the phosphates form layer stmctures, one OH group sticking to either side. Thus, replacing the OH with an alkyl chain to form the alkyl phosphonic acid was expected to result in a bilayer stmcture with alkyl chains extending from both sides of the metal phosphate sheet (335). When zirconium (TV) is used the distance between next neighbor alkyl chains is - 0.53 nm, which forces either chain disorder or chain tilt so that VDW attractive interactions can be reestablished. 

The Michael addition reaction of amines and thiols with bismaleimides or functionalized monomaleimides is a versatile tool ia the synthesis of chain-extended maleimide-terroinated prepolymers. These prepolymers generally are soluble ia organic solvents from which they can be processed to prepreg and molded to high quaUty, void-free laminates. 

Step-Growth Gopolymerization. A sample of a block copolymer prepared by condensation polymerisation is shown in equation 30 (37). In this process, a prepolymer diol (HO—Z—OH) is capped with isocyanate end groups and chain extended with a low molecular-weight diol (HO—E—OH) to give a so-called segmented block copolymer, containing polyurethane hard blocks and O—Z—O soft blocks. 

Derivatives of CPD have also been incorporated into these resins. CPD and 2-butene-l,4-diol have been condensed in ethanol and catalyticaHy hydrogenated in situ to give 2,3-bis(hydroxymethyl)bicyclo[2.2.1]heptane (51). This latter compound is used as a chain extender in polyesters for engineering plastics (52). 

An entirely new concept was iatroduced iato mbber technology with the idea of "castable" elastomers, ie, the use of Hquid, low molecular-weight polymers that could be linked together (chain-extended) and cross-linked iato mbbery networks. This was an appealing idea because it avoided the use of heavy machinery to masticate and mix a high viscosity mbber prior to mol ding and vulcanization. In this development three types of polymers have played a dominant role, ie, polyurethanes, polysulftdes, and thermoplastic elastomers. 

Advancement Process. In the advancement process, sometimes referred to as the fusion method, Hquid epoxy resin (cmde diglycidyl ether of bisphenol A) is chain-extended with bisphenol A in the presence of a catalyst to yield higher polymerized products. The advancement reaction is conducted at elevated temperatures (175—200°C) and is monitored for epoxy value and viscosity specifications. The finished product is isolated by cooling and cmshing or flaking the molten resin or by allowing it to soHdify in containers. 

Polyamide-imides may also be produced by reacting a diacid chloride with an excess of diamine to produce a low molecular mass polyamide with amine end groups. This may then be chain extended by reaction with pyromellitic dianhydride to produce imide linkages. Alternatively the dianhydride, diamine and diacid chloride may be reacted all together. 

In the 1940s ICI introduced a material marketed as Vulcaprene made by condensing ethylene glycol, adipic acid and ethanolamine to a molecular weight of about 5000 and then chain extending this with a diisocyanate. This rubbery material found some use as a leathercloth and is dealt with further in Chapter 25. 

The resulting prepolymer can then be chain extended with water, glycols or amines by linking cross terminal isocyanate groups (Figure 27.4). 

More recently, storage systems based on MDI have become available which pose less of a health hazard than MDI/MOCA systems. Both polyethers and polyesters are used, with glycols being the usual chain extenders. 

If a branched polyol, usually either castor oil or a simple polyester, is heated with an isocyanate but without chain extenders soft and weak rubbery products are obtained with very low resilience. These materials are useful for encapsulation of electronic components and for printer rollers. 

Similar reactions occur when an amine is used instead of a glycol as chain extender ... 

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