Polymers with functional end groups

Most of the methods for synthesizing block copolymers were described previously. Block copolymers are obtained by step copolymerization of polymers with functional end groups capable of reacting with each other (Sec. 2-13c-2). Sequential polymerization methods by living radical, anionic, cationic, and group transfer propagation were described in Secs. 3-15b-4, 5-4a, and 7-12e. The use of telechelic polymers, coupling and transformations reactions were described in Secs. 5-4b, 5-4c, and 5-4d. A few methods not previously described are considered here. 

A double-side adhesive tape was prepared using a mixture of a hydroxylterminated poly butadiene, a polyol, and an isocyanate compound containing an oxadiazinetrione ring 241 A compound suitable for sealing spaced glass panels comprises a reaction product of an unsaturated polymer with functional end groups with HTPB, a decane thiol adduct, and tolylene diisocyanate 242). 

Heterogeneous Graft Copolymerization Polymers with Functional End Groups... 

Formation of polymers with functional end groups by selective termination with appropriate reagents. 

The free radical polymerization of HPMA in the presence of mercaptans involves two different initiation mechanisms (Scheme 2) [26]. One is the initiation by RS radicals from chain transfer agent the other appears to be the direct initiation by the primary isobutyronitrile (IBN) radicals formed by the decomposition of AIBN [27]. The RS are formed by either the free radical transfer reaction of alkyl mercaptans with the IBN radicals or the chain transfer reaction of an active polymer chain with the mercaptans. The initiation by the RS radicals produces the ST polymers with a functional group at one end of the polymer chain. The initiation by IBN radicals leads to nonfunctional polymer chains with an IBN end group. The presence of the polymers with IBN end groups effects the purity and the functionality of ST polymers. As expected, the production of nonfunctionalized polymer chains is affected by reaction conditions. The polymerization is mainly terminated by chain transfer reaction with the mercaptans, but other termination mechanisms, such as disproportionation and recombination, take place depending on the reaction conditions [26]. 

The preparation of prepolymers [111] or macromers with functional end groups, so called telechelic polymers, is another approach to structurally unconventional architecture. The functional end groups are introduced either by functional initiation or end-capping of living polymers, or by a combination of the two. In this way, monomers that are not able to copolymerize can be incorporated in a copolymer. Telechelic prepolymers can be linked together using chain extenders such as diisocyanates [112]. In this process, it is essential that the structure and end groups of the prepolymers can be quantitatively and qualitatively controlled [113]. 

Diene carbanions exhibit a reactivity similar to that of styryl carbanions. Carban-ionic sites of lower reactivity can be functionalized under certain conditions32). Living polymers with alkoxide end groups exhibit the same reactivity as alcoholates with respect to proton-donating substances and activated organic halides. Protonation yields terminal hydroxy groups which are often used in the two-step macromonomer synthesis. 

Block copolymers are also formed by condensation of two chain types with functional end groups. Some kinds of polyurethanes are examples. Dissimilar and mutually immiscible polymers can be connected in this way. Henderson and Szwarc condensed a, co-polystyrenedicarboxylic acid with a polyamide carrying terminal amino groups [272], producing the copolymer poly-(styrene)-6/ocfc-poly(amide). The condensation was slow the probability of finding functional groups at an effective distance is small. 

Polymers may be made with functionalized end-groups, leading to block copolymers with controlled structures, in parallel with the anionic systems described in more detail in Section 9.2.6.2. Also, as in living anionic polymerizations, of the polymer is directly proportional to the monomer conversion, and the polymerization may be restarted by adding more monomer after the initial monomer charge has been consumed. 

Polymers containing LM at the chain ends can be obtained by the reaction of anthracene-containing reagents with functional end groups of macromolecules such as carboxy groups or living ends of macromolecules generated in anionic poly-merization ... 

The direct synthesis of block copoljmiers by radical polymerization is an alternative to using preformed blocks with functional endgroups. Polymers with initiator end-groups (3a,3b), with end groups accessible to radical activation polyinitiators (6, 7.)... 

A telechelic polymer is defined as a relatively low-molar-mass spedes (M < 20,000), with functional end groups that can be used for further reaction to synthesize block copolymers or for network formation. Cationic polymerization methods can be used to prepare these fimctionalized polymers using the initiator-transfer, or Inifer, technique perfected by Kennedy. If the initiating catalyst-cocatalyst system is prepared from a Lewis acid and an alkyl or aryl halide, i.e.. 

Fig. 5.2 Schematic representation of functional polymer chains configured on a cubic lattice. The darker cubes indicate a lattice site occupied by a functional end group, and the lighter cubes are occupied by polymer chain segments (a) illustrates a chain with a low-energy attractive end group, (b) depicts a nonfunctional polymer with neutral end groups. Reproduced with permission from [54]...

An early study by Meijer and co-workers used poly(propy-lene oxide-co-ethylene oxide) three-arm star polymers with hydroxyl end-group functionality that could be converted into UPy functional termini in two steps using bis-isocyanate and methylisocytosine. The materials were determined to be viscoelastic in nature and were easily degraded through the addition of a small monovalent molecule. Moreover, the addition of a small amount of water led to large decreases in mechanical properties through competitive H-bonding with the UPy units. 

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