Telechelic polymers preparation methods

Results and. Discussion. There are basically two approaches to the preparation of telechelic and semi-telechelic polymers by anionic procedures. One method involves terminating living anionic polymers with suitable electrophiles another technique utilizes functionally substituted anionic initiators. 

In a subsequent investigation, the author, (3), prepared the nitroxide-mediated polymerization agent, 4,4-dimethyl-2- [ 1 -(2,2,6,6-tetramethylpiperidin-1 -yloxy)-ethyl] -4H-oxazol-5-one, (I), as a method of preparing telechelic polymers. 

The use and limitations of Atom Transfer Radical Coupling (ATRC) reactions including polyrecombination reactions for the preparation of telechelic polymers, segmented block copolymers, and polycondensates are presented. Specifically, the preparation of telechelic polymers with hydroxyl, aldehyde, amino and carboxylic functionalities, poly(/i-xylylene) and its block copolymers, and polyesters via ATRC process is described. The method pertains to the generation of biradicals at high concentration from polymers prepared by ATRP or specially designed brfunctional ATRP initiators. The possibility of using silane radical atom abstraction (SRAA) reactions, that can be performed photochemically in the absence of metal catalysts, as an alternative process to ATRC is also discussed. 

In conclusion, it has been demonstrated that ATRC reactions are useful for preparing various macromolecular stmctures such as telechelics and certain polycondensates. The method preserves to the generation of biradicals at high concentration from polymers prepared by ATRP or specially designed bifimctional ATRP initiators. The radical generation process is not limited to the metal catalyzed atom transfer reactions. Silane radical atom abstraction reactions can also be used for the formation of reactive radicals. Aromatic carbonyl assisted photoinduced reactions seemed to a promising alternative route for silane radical generation since it can be performed at room temperature and does not require metal catalysts. 

In addition, there has been an increasing interest in new synthetic methods for the preparation of well-defined polymers with controlled chain-end functional groups [23], such as telechelic polymers, which are characterized by the presence of reactive functional groups placed at both chain ends. These materials can then be used as precursors in the synthesis of block copolymers, as modifiers of the thermal and mechanical properties of condensation polymers, as precursors in the preparation of polymer networks, and as compatibilizers in polymer blends [24]. 

Polycondensation reactions have also been used to develop an efficient and versatile one-step method for the preparation of some polymers by click processes, which has been applied for the synthesis of telechelic polymers [25]. 

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.. 

Three methods were developed to overcome transfer to monomer. These are (1) use of inifers (2) use of proton traps and (3) establishing conditions under which the rate of termination is much faster than the rate of transfer to monomer. The first one, the inifer method, is particularly useful in formation of block copolymers. It allows the preparation of head and end (a and to) functionalized telechelic polymers. Bifunctional initiators and transfer agents (inifers) are used. The following illustrates the concept ... 

Functionalized poly(vinyl ether)s can be prepared by the functionalized initiator method by the use of a HI/I2 initiating system and a functionalized vinyl ether, resulting in a-functionalized polymers. Carboxylic add- and amine-terminated polymers were prepared by this method with high degrees of functionality as determined by H NMR. This method can be extended to the preparation of telechelic polymers by quenching the polymerization with the appropriate nucleophile. Methacrylate-functionalized poly(vinyl ether) s... 

The most direct method of preparing telechelic polydienes utilizes a dilithium initiator which is soluble in hydrocarbon solution [220, 221]. The most expedient method of preparing such a dilithium initiator is to react two moles of an alkyllithium compound with a divinyl compound which will not homopolymerize. Unfortunately, because of the association behavior of organolithium compounds in hydrocarbon media [176-178], many potential systems fail because they associate to form an insoluble network-like structure [221]. Expediencies such as addition of Lewis bases can overcome solubility problems of dilithium initiators, however, such additives tend to produce high amounds of 1,2- and 3,4-microstructures (see Table IV). One exception is the adduct formed from the addition of two equivalents of sec-butyllithium to l,3- i5 (l-phenylethenyl)benzene as shown in Eq. (79) [222,223]. Although this is a hydrocarbon-soluble, dilithium initiator, it was found that biomodal molecular weight distributions are obtained monomodal distributions can be obtained in the presence of lithium alkoxides or by addition of Lewis base additives [224,225].This initiator has also been used to prepare telechelic polymers in high yields [226]. 

It is possible to prepare telechelic polymers by NMP procedure since it tolerates a wide variety of fiinctional groups (78). For the s5mthesis of telechelics by NMP there are two general methods, ie, fiinctional groups can be placed at the initiating chain end, F, or the nitroxide mediated chain end, F. ... 

Recently, new methods for preparation of telechelic polymers with better fimctionalization using alkyl aluminum initiated anionic polymerization were described (190-193). 

Preparations of macro-initiators or telechelic polymers by cationic methods have been executed primarily by polymerizing isobutylene in the presence of a co-initiator that also functions as a chain transfer agent. A typical reaction sequence is shown in Scheme 1, outlining the synthesis of difunctional polyisobutylene (PIB), which is then used to initiate the polymerization of a-methyl styrene (ffi-MS) to produce an A-B-A type block copolymer. By similar methods, polyisobutylenes with phenol, phenyl, cyclopentadiene, and olefin termini have been synthesized. 

Hetero-telechelic polymers are the most difficult to prepare, as both end groups carry different functionalities. Different approaches have been developed to address this very challenging synthetic task. Most of them are combinations of methods that were described earlier in this chapter. 

In general, the preparation of ionomers is a straightforward procedure. The particular acid group of interest can be introduced onto the hydrocarbon backbone either by direct copolymerization or post-synthesis reaction. The following five important groups of ionomers illustrate the various methods of preparation. These ionomer families are ethylene-based materials, ionic elastomers, modified polystyrenes, perfluorinated resins and halato-telechelic polymers. 

Medium-size members of homologous polymeric series such as dimers, trimers, etc. are called oligomers. They can be linear or cyclic and are often found as byproducts of polymer syntheses, e.g., in cationic polymerizations of trioxane or in polycondensations of e-aminocaproic acid (see Example 4-9). For the preparation of linear oligomers with two generally reactive end groups, the so-called telechelics, special methods, i.e., oligomerizations, were developed. 

Reed 332) has reported that reaction of ethylene oxide with the a,(a-dilithiumpoly-butadiene in predominantly hydrocarbon media (some residual ether from the dilithium initiator preparation was present) produced telechelic polybutadienes with hydroxyl functionalities (determined by infrared spectroscopy) of 2.0 + 0.1 in most cases. A recent report by Morton, et al.146) confirms the efficiency of the ethylene oxide termination reaction for a,ta-dilithiumpolyisoprene functionalities of 1.99, 1.92 and 2.0j were reported (determined by titration using Method B of ASTM method E222-66). It should be noted, however, that term of a, co-dilithium-polymers with ethylene oxide resulted in gel formation which required 1-4 days for completion. In general, epoxides are not polymerized by lithium bases 333,334), presumably because of the unreactivity of the strongly associated lithium alkoxides641 which are formed. With counter ions such as sodium or potassium, reaction of the polymeric anions with ethylene oxide will effect polymerization to form block copolymers (Eq. (80) 334 336>). 

Amino-terminated telechelic polybutadiene was prepared by LiAlH4 reduction of amidino end-group in polybutadiene, which was polymerised by a water-soluble initiator, 2,2 -azobis(amidinopropane)dihydrochloride. The structure was analysed by 1H- and 13C-NMR, but functionality of 2.0 was obtained by a titration method [70]. Synthesis of co-epoxy-functionalised polyisoprene was carried out by the reaction of 2-bromoethyloxirane with living polymer that was initiated with sec-butyl lithium. The functionality of the resulting polyisoprene was 1.04 by 1H-NMR and 1.00 by thin layer chromatography detected with flame ionisation detection [71]. 

The ABA-type block copolymers B-86 to B-88 were synthesized via termination of telechelic living poly-(THF) with sodium 2-bromoisopropionate followed by the copper-catalyzed radical polymerizations.387 A similar method has also been utilized for the synthesis of 4-arm star block polymers (arm B-82), where the transformation is done with /3-bromoacyl chloride and the hydroxyl terminal of poly(THF).388 The BAB-type block copolymers where polystyrene is the midsegment were prepared by copper-catalyzed radical polymerization of styrene from bifunctional initiators, followed by the transformation of the halogen terminal into a cationic species with silver perchlorate the resulting cation was for living cationic polymerization of THF.389 A similar transformation with Ph2I+PF6- was carried out for halogen-capped polystyrene and poly(/>methoxystyrene), and the resultant cationic species subsequently initiated cationic polymerization of cyclohexene oxide to produce... 

The functionalities of the telechelics prepared by iniferter method were reported to be close to 2, within experimental error. The formation of the nonfunctional polymers was claimed to be negligible because of the triple function of the iniferter. 

Polystyrene and Derivatives. Telechelic polystyrene, poly(2,4,6-trimethylst5Tene), poly(p-methylstyrene), and poly(p-chlorostyrene) can be prepared by living carbocationic polymerization (269-271) or by inifer method (272). While end-quenching the living carbocationic polsrmerization gave quantitatively polymers with sec-benzylic termini, the diciunyl chloride/BClg inifer system yielded a,with olefine end groups were also prepared by dehydrochlorination (272,273). 

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