Crosslinking post-polymerization

Other more complex linear block co-, ter- and quarterpolymers, such as ABC, ABCD, ABABA can be prepared using the previously mentioned methods. An important tool in the synthesis of block copolymers involves the use of post-polymerization chemical modification reactions. These reactions must be performed under mild conditions to avoid chain scission, crosslinking, or degradation, but facile enough to give quantitative conversions. Hydrogenation, hydrolysis, hydrosilylation and quaternization reactions are among the most important post-polymerization reactions used for the preparation of block copolymers. 

The question may arise whether the same changes in material properties can also be obtained by stretching the filaments after the polymerization has taken place. In our experience the anisotropy was never so great in such cases, whereas the isotropic state was almost completely recovered when the samples were heated above the melting temperature. At this temperature the post-polymerization drawn filaments retained the original dimensions they possessed before stretching. Apparently some strain-induced crystallization yielded a metastable anisotropy which was lost under the combined action of entropy and strained crosslinks when the crystalline areas were melted. 

The significance of intramolecular crosslinking was clearly demonstrated in the post-polymerization of DAP prepolymer (Fig. 8) and its post-copolymerization with ABz (Fig. 7). This was also checked by SEC-LALLS the occurrence of... 

A thermopolastic elastomer based on sulfonated-EPDM, S-EPDM, was developed in the 1970 s by Exxon and more recently by Uniroyal. Unlike the synthesis of the carboxylate ionomers described above, S-EPDM is prepared by a post-polymerization sulfonatlon reaction(28). Compared to the metal neutralized S-EPDM, the sulfonic acid derivative is not highly associated. The free acid materials possess low strengths and are less thermally stable. The metal salts of S-EPDM have properties comparable to crosslInked elastomers, but they do exhibit viscous flow at elevated temperatures. In the absence of a polar cosolvent, such as methanol, hydrocarbon solutions of the metal salts of S-EPDM are solid gels at polymer concentrations above several percent(31). With the addition of 1 to 5% alcohol the polymer solution becomes fluid with solution viscosities of the order of 10 to 100 poise. 

Poly butadiene rapidly becomes crosslinked when irradiated in vacuo at 253.7 nm [54]. A decrease amounting to 80% of the original unsaturation is observed by infrared spectroscopy. Since no new unsaturation has been detected this decrease has been accounted for by cyclization. However, the absence of absorption at 1020 cm-1 implies that formation of cyclopropyl groups does not occur. Formation of a ladder polymer is also unlikely since all attempts to accomplish the free radical post-polymerization of 1,2-poly butadiene have been unsuccessful namely,... 

Post-polymerization modification is the most expensive method for preparing functional polymers. It is expensive because it means added steps. However, it is the method of choice when one wants to directly compare structure and reactivity or properties of functional with nonfunctional polymers. The only caveat is that chemical mo fication of polymers must not introduce adventitious chain scission or crosslinking. The other benefit of post-polymerization modification is that it can be accomplished for bulk polymers, as well as polymer surfaces. For some applications surface modification wUl be enough to achieve the desired effect... 

There are a wide range of functional monomers which can be copolymerized with the principal monomers described in the previous section. These functional monomers are often used in very small amounts (typically 1-3% in a formulation) and provide reactive sites for crosslinking, surface modification, and post-polymerization processing of latex particles [21]. Examples of the roles that these functional monomers play during the interfacial crosslinking process are also given in this section. There are several major classes of these functional monomers, based on the type of reactive moiety which is introduced into the latex particle. These moieties include ... 

Spirocyclic [Ijferrocenophanes have also been shown to thermally polymerize and these species function as crosslinking agents that allow access to polyferrocenylsilanes with controlled crosslink densities [67]. Amber, solvent-swellable gels are available by this route (see Section 3.3.6.4). The [l]dichlorosilaferrocenophane 3.23 represents a very useful precursor to [Ijferrocenophanes 3.24 with alkoxy (or amino) substituents, and subsequent ROP allows access to, for example, polyferrocenylalkoxy-silanes 3.25 (Eq. 3.11) [68]. In addition, polymers with Si-H or Si-Cl groups have been prepared, and these provide opportunities for post-polymerization modification by hydrosilylation and nucleophilic substitution, respectively [66]. 

Fig. 2 The general synthetic procedure to prepare amine-functionalized hydrogels by a post-polymerization crosslinking reaction...

It has also been noted that in these polymerizations, some further degrees of crosslinking can occur due to post reaction of isocyanate with already formed ureas or urethane. Isocyanate can react with substituted ureas to form biuret ... 

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