Chain Extension with Diisocyanates

For use in plastics, this might require chain extension with diisocyanates, bisoxazolines, carbodiimides, bis(an-hydrides), and such. One use of the oligomers would be to form polyurethanes by reaction with isocyanates. The use of acid chlorides can be avoided if the polymers are made by ester exchange or made enzymatically, with compounds such as divinyl adipate. Poly(butylene sebacate) with a molecular weight of 46,400 has been made has been made from bis(2,2,2-trifluoroethyl)sebacate and 1,4-butane diol.150 One polyester with a molecular weight of 24,000 has been made by ester exchange from isosorbide and a... 

Chain extension with diisocyanates is undoubtedly the most commonly applied chain-extension approach for aliphatic polyesters (Figure 3.4). A large number of different diisocyanates have been used as linking molecules in the preparation of aliphatic poly(ester-urethane)s. Various isocyanates are listed in Table 3.1 together with the t) e of prepolymer used. 

In addition to linear chain extension, excess diisocyanate leads to cross-linking iato a network because the diisocyanate groups can also react with the hydrogen atoms of the —NH— groups ia the chains. Furthermore, the weU-known polyurethane foam mbber can be made by a dding water to the mixture because the isocyanate groups react vigorously with water to Hberate carbon dioxide gas as follows ... 

In a recent patent, Reuter (110) describes a polyurethane prepared from PTHF (mol. wt. 1000 to 3000), 1,4-butanediol, and OCN(CH2)6CN0. In another case Murbach and Adicoff (67) interrupted the regularity of PTHF by copolymerization with ethylene oxide before chain extension with diphenyl-methane-4,4 -disiocyanate. Dickinson (99) prepared a series of polyurethane elastomers from THF-PO copolymer diols and 2,4-tolylene diisocyanate. He found that the use of copolymers with approximately 75 wt.-% THF led to polyurethanes with very good properties relative to the use of propylene oxide homopolymer. 

Following the early developments using NDI, it was found that by using TDI instead, a far more stable prepolymer could be made. Stable prepolymers are normally made using either polyesters or polyethers that have been reacted with a slight excess of a diisocyanate such as toluene diisocyanate (TDI) or methylene diisocyanate (MDI). Provided the storage is moisture free, the stable prepolymer may be kept for months before use. The polyurethane is prepared by chain extension with diols or diamines. 

In contrast in the two-step polymerization technique (Fig. 1.6), in the first step of the reaction a prepolymer is produced through the reaction of a SS oligomer with an excess of diisocyanate, followed by chain extension with a short diol(urethane or ester) or a diamine (urea, urethaneurea, amide, or ester-amide) to form the HS and also to increase the overall molecular weight of the polymer. 

Polyurethane fibres of a kind different to those described above have become important within the last decade these are elastomeric fibres, which are commonly called spandex fibres. These products are made either by solution spinning or by reaction spinning. In the first process, a hydroxy-terminated polyester (e.g., an adipate) or polyether (e.g., poly(oxytetramethylene) glycol) is treated with an excess of diisocyanate (e.g., tolylene diisocyanate) to give an isocyanate-terminated pre-polymer similar to those used for cast elastomers (Section 14.6.1). The pre-polymer is dissolved in a strongly polar solvent (e.g., dimethylformamide) and treated with an aliphatic diamine or hydrazine to effect chain extension with hydrazine the following reaction occurs ... 

The synthesis of multiblock cyclic carbonate copolymers was presented by Kricheldorf and Rost. First, telechelic random copolymers were prepared by copolymerization of CL and TMC in bulk using bismuth(III) hexanoate [Bi(OHex)3] as an initiator. A-B-A triblock copolymers were synthesized by chain extension of these random copolymers with l-LA. Finally, the triblock copolymers were transformed into multiblock copolymers by chain extension with 1,6-hexamethylene diisocyanate. It is worth mentioning that all three synthetic steps were performed in a one-pot procedure. ... 

Stabilizers and pigments are normally slurried with macroglycol and added to the polymeric glycol charge, prior to diisocyanate addition. Therefore, care must be taken to avoid additives that react significantly with diisocyanates or diamines under processing conditions. Also, stabilizers should be chosen that have no adverse catalytic effect on the prepolymer or chain-extension reactions. 

Another class of hydrocarbon binders used in propints are the carboxy-terminated polybutadiene polymers which are cross-linked with either tris[l-(2-methyl)aziridinyl] phosphine oxide (MAPO) or combinations with phenyl bis [l -(2-methyl)aziridinyl] phosphine oxide (Phenyl MAPO). Phenyl MAPO is a difunctional counterpart of MAPO which makes possible chain extension of polymers with two carboxylic acid groups. A typical propint formulation with ballistic properties is in Table 11 (Ref 83) Another class of composites includes those using hydroxy-terminated polybutadienes cross-linked with toluene diisocyanate as binders. The following simplified equations illustrate typical reactions involved in binder formation... 

We create polyurethanes from prepolymers by chain extension. In the case of hydroxyl-terminated prepolymers the chain extender is an isocyanate. If we use a diisocyanate, the resulting polymer is linear. If we substitute some or all of the diisocyanate with a tri- or... 

According to O. Bayer, the latter procedure, which is used especially for the preparation of elastomeric polyurethanes, is carried out in two separate stages. First, a carefully dried, relatively low-molecular-weight, aliphatic polyester or polyether with hydroxy end groups is reacted with an excess of diisocyanate. A chain extension reaction occurs in which two to three linear diol molecules are coupled with diisocyanate, so as to yield a linear polymer with some in-chain urethane groups and with isocyanate end groups. 

In the polyurethane industry, the polymeric glycols are prepared by anionic polymerization of epoxides such as ethylene oxide and propylene oxide. Poly(tetra-methylene glycol), which was prepared by polymerization of tetrahydrofuran, was subjected to chain extension by reaction with diisocyanate (polyurethane formation) and with dimethyl terephthalate (polyester by alcoholysis). 

Infrared and functional group analyses showed the presence of hydroxyl end groups in the products. In addition, the results of chain extension and cross-linking reactions with diisocyanates (vide infra) confirmed the presence of two terminal hydroxyl groups. 

Elastomers are prepared by chain extension of hydroxyl-terminated low-molecular-weight polymers followed by vulcanization 180). The most important work concerns the use of hydroxy telechelic polybutadienes and polyisoprenes in the tire industry 249 252>. The hydroxylated polydienes of molecular weight 1000-20000 are mixed with a diisocyanate, a catalyst, vulcanization agent (sulfur), and accelerator, reinforcing additives (carbon black), and surface-active agents. The reaction takes place in two steps simultaneously or consecutively ... 

The two-phase polyurethanes were synthesized in the usual way starting from a polyester diol which is reacted with a diisocyanate and finally the chain extension reaction is achieved by... 

Segmented polyurethanes were synthesized from the a, to diol polyethers listed in Table I and the diisocyanates IV, V, and VI by the two-step process shown in Figure 1 or the three-step process in Figure 2. In all cases, chain extension of isocyanate-terminated prepolymers was accomplished with ethylenediamine. The synthesis took place in a 2 1 (v/v) mixture of dimethyl sulfoxide and 4-methyl-2-pentanone at 60°C. 

The first commercial thermoplastic elastomers (TPE) were the thermoplastic urethanes (TPU). Their general structure is A-B-A-B, where A represents a hard crystalline block derived by chain extension of a diisocyanate with a glycol. The soft block is represented by B and can be derived from either a polyester or a polyether. Figure 5.1 shows typical TPU structures, both polyester and polyether types. 

Third the chain extension through reacting with chain extenders such as 1,6-hexamethylene diisocyanate (HDl) was also developed. Such strategy has resulted in cross-linked urethane-doped polyester elastomers (CUPEs) based on POC [19], cross-linked urethane-doped BPLP (CUBPLP)... 

Diisocyanate is often used in the chain extension reactions of biopolymers such as PEA, PCL, and their copolymers [97-101]. The combination of hard segment and soft segment may confer the resulting polyurethanes with shape-memory property [100,101]. Polyurethane is an important type of elastomeric polymer for biomedical applications [1,9,11]. Chain extension or cross-linking by diisocyanate can be adapted to many -OH-terminated or H-containing polymers or prepolymers [102]. The convenience of the urethane chemistry has made it into a very popular way of polymer chain extension method in biomaterial designs. 

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