Additive chain extender

By variation of the proportion of organic segments the mechanical properties can be tuned between very soft and very hard (Fig. 3). Those materials display tensile strengths up to 12 MPa and do not need additional frllers. The mechanical properties can also be modifted by introducing additional chain extenders or mineral fillers. 

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. 

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. 

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). 

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. 

This reaction is reported to proceed at a rapid rate, with over 25% conversion in less than 0.001 s [3]. It can also proceed at very low temperatures, as in the middle of winter. Most primary substituted urea linkages, referred to as urea bonds, are more thermally stable than urethane bonds, by 20-30°C, but not in all cases. Polyamines based on aromatic amines are normally somewhat slower, especially if there are additional electron withdrawing moieties on the aromatic ring, such as chlorine or ester linkages [4]. Use of aliphatic isocyanates, such as methylene bis-4,4 -(cyclohexylisocyanate) (HnMDI), in place of MDI, has been shown to slow the gelation rate to about 60 s, with an amine chain extender present. Sterically hindered secondary amine-terminated polyols, in conjunction with certain aliphatic isocyanates, are reported to have slower gelation times, in some cases as long as 24 h [4]. 

Alle the deformation zones contain a finite and equal number of extended chains in their most highly stretched strands. This surprising conformity of the deformation zones may well be the consequence of the imposed plane-strain fracture condition which impedes lateral contraction of the material. However, no quantitative explanation has been presented as yet. A plausible explanation would be to assume that due to the hindered lateral contraction additional tensile stresses are transferred to the most extended strand with each additional chain pulled out of the matrix [112]. 

An alternative is to carry out a normal Michael addition and extend the chain bv cyanide displacement, though this does not strictly produce a reagent for either (42) or (43). 

Addition of a core molecule, or any other comonomers (chain extending AB, terminating A or B). 

The future direction of polyester R D efforts is likely to involve further progress in polyester synthesis given the wide range of potential monomer combinations, new blending technology and the use of advanced functional additives such as nanoclay reinforcements, reactive impact modifiers, anti-hydrolysis agents and chain extenders. 

There is a wide range of chain extenders commercially available for PET (Table 14.2). Bifunctional chain extenders promote linear chain extension while tri- and tetra-functional chain extenders promote chain branching. Such reactive compounds are also known as repair additives since they can reverse the molecular weight loss of hydrolytically damaged PET. 

Aromatic carboxylic dianhydride chain extenders (e.g. PMDA) are a low-cost way of converting recycled PET flakes into high-IV crystalline pellets that can be used in high-value applications (e.g. bottles, strapping, foam, engineering alloys/compounds, etc.) (see Figure 14.2). PMDA is an effective chain extension additive for thermoplastic polyesters such as PET and PBT. It is suitable for the following applications ... 

Figure 14.4(a) shows the structure for a common diepoxide chain extender. Haralabakopoulos et al. [12] have reported that the molecular weight of PET increases via chain extension reactions with commercially available diepoxides. Low concentrations of extender and short reaction times generally favoured chain extension. In addition, purging with nitrogen resulted in chain extended polymers having the highest values of intrinsic viscosity (e.g. 0.82 dL/g). 

Clearly, care would need to be exercised in selecting stable protecting groups for both reactants. By appropriate construction of the two partners, a wide variety of chain-extended carbohydrates would become available. The double bond produced by the condensation reaction would, of course, be amenable to suitable manipulation in order to Incorporate additional needed or desired functionality. Such manipulation would be facilitated if the proposed reaction produced only one configuration about the double bond, though separation of the two Isomers would not be a major stumbling block. 

Application of metal salts and well-defined metal complexes in ROP has enabled the exploitation of a three-step coordination-insertion mechanism, first formulated in 1971 by Dittrich and Schulz [17]. This proceeds through coordination of lactide by the carbonyl oxygen to the Lewis acidic metal center, leading to the initiation and subsequent propagation by a metal alkoxide species. This species can be either isolated or generated in situ by addition of an alcohol to a suitable metal precursor to result in the formation of a new chain-extended metal alkoxide, as shown in Scheme 3 [16]. 

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