Polymerization, living

The living chains can be terminated when desired by adding suitably reactive materials, such as water, alcohol, or ammonia. The unique features of living polymer systems described above provide fascinating possibilities of polymer syntheses, which include making monodisperse polymers (by controlled addition of monomer), structures with specific end groups (by chain termination with ap- 

Water is an especially effective chain terminating agent. For example, its Cir,s value is approximately 10 in the polymerization of styrene at 25°C with sodium naphthalene. So the presence of 

The hydroxide ion formed is not suf dently nucleophilic to reinitiate polymerization and the active center is thus effectively destroyed. In contrast, the Q. s value for ethanol being very small ( 10 ), its presence in small amounts does not limit the molecular weight. 

Oxygen and carbon dioxide from the atmosphere add to propagating carbanions to form peroxy and carboxyl anions  

As these anions are not reactive enough to continue propagation, the chains are effectively terminated. By adding a proton donor subsequently to the polymerization system, the peroxy and carboxyl anions are converted to OH and COOH groups. A notable example of the appUcation of the latter reaction is the preparation of carboxyl ion terminated polybutadiene by anionic polymerization of butadiene with bifunctional initiators, followed by termination with CO2  

They are fulfilled in the case of the polymerization of VFc in THF initiated by n-BuLi. Typical results are shown in Figs. 15.2 and 15.3. 

Molar masses of the samples were determined by field-desorption mass spectrometry (FD-MS), vapour pressure osmometry (VPO), and GPC. GPC was calibrated with PVFc. Both, calibration via an oligomer sample in which oligomers from = 3 through n = 11 could be identified and via polymers produced by living anionic polymerization (characterized by VPO or FD-MS), gave similar results and excellent agreement with theoretical values. 

The FD-MS did not show only the oligomers from = 3 through n = 11 but satellite peaks of M + 79 for each molar mass. 

This again would strongly support the view of complexation of the gegenion by THF. All attempts to start an anionic polymerization of VFc initiated with naphthyl failed. However, it was possible to apply distyryl dianion as initiator. The conversion vs. 

The molar masses are again controlled by the ratio of monomer initiator and the molar mass distribution is narrow. 

An essential feature of a strictly living polymerization is the absence of transfer reactions [652]. This requirement was found to be valid for the polymerization of IP catalyzed by NdCl3 TBP/TIBA in hexane. The lack of chain termination reactions, the second requirement for a living polymerization [652], was confirmed by the application of a mathematical model to the experimental data [279]. Bruzzone et al. realized that transfer reactions occur in the polymerization of BD. In spite of this observation the pseudo-living character of the polymerization was assigned to the superposition of chain growth and chain transfer both of which exhibit a different dependence on monomer conversion [87]. 

Another indicator in favor of a living polymerization is a linear increase of number average molar masses (Mn) on monomer conversion. Hsieh et al. reported on linear plots for the dependence of inherent viscosities on monomer conversion. These plots were highly Unear and Hsieh et al. assigned the term quasi-living to these polymerizations [134,139]. Kwag et al. attributed the living character of Nd-catalyzed diene polymerizations to the ionic character of the Nd allyl bond and the stable oxidation state of Nd. In addition, theoretical frontier orbital analysis confirmed these results [653]. 

A serious contradiction with the requirements of a strictly Uving polymerization are broad or even bimodal MMDs which are in the focus of many studies, e.g. [87,178,620]. This observation of broad and at least bimodal MMDs is the result of the presence of at least two active catalyst species which show different activities. This feature is in contradiction with a strictly living polymerization. Wilson attributed the polymer fraction with a high molar mass to insoluble catalyst species which are invisible to the naked eye whereas the low molar mass fraction of the polymer is supposedly produced by soluble sites which operate in a quasi-living manner [89]. In his study Wilson used catalyst systems of the type Nd(carboxylate)3/DIBAH/tBuCl. 

First order kinetics with respect to monomer conversion (no irreversible 

A number of linear isotactic copolymers of 4-methyl-l-pentene with 1-hexene and functionalized olefins, such as 5-(trialkylsiloxy)-1-pentene, could be prepared under similar conditions (9). 

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Diisocyanobenzene (130) undergoes living polymerization to form the poly(quinoxaline-2,3-diyl)s 131, and the optically active helical polyfquinoxa-line-2,3-diyl) 132 is prepared from 131[123]. 

Anionic polymerization, if carried out properly, can be truly a living polymerization (160). Addition of a second monomer to polystyryl anion results in the formation of a block polymer with no detectable free PS. This technique is of considerable importance in the commercial preparation of styrene—butadiene block copolymers, which are used either alone or blended with PS as thermoplastics. 

Details of the mechanism of living polymerization have been refined continually (42), and a dormant species has been shown to be cmcial for such polymerization. The dormant species is in equiUbrium with the active species and the ratio can determine MWD and hence the quaUty of the living process (43). 

The living polymerization process offers enormous flexibiUty in the design of polymers (40). It is possible to control terminal functional groups, pendant groups, monomer sequencing along the main chain (including the order of addition and blockiness), steric stmcture, and spatial shape. 

VEs do not readily enter into copolymerization by simple cationic polymerization techniques instead, they can be mixed randomly or in blocks with the aid of living polymerization methods. This is on account of the differences in reactivity, resulting in significant rate differentials. Consequendy, reactivity ratios must be taken into account if random copolymers, instead of mixtures of homopolymers, are to be obtained by standard cationic polymeriza tion (50,51). Table 5 illustrates this situation for butyl vinyl ether (BVE) copolymerized with other VEs. The rate constants of polymerization (kp) can differ by one or two orders of magnitude, resulting in homopolymerization of each monomer or incorporation of the faster monomer, followed by the slower (assuming no chain transfer). 

Group-Transfer Polymerization. Living polymerization of acrylic monomers has been carried out using ketene silyl acetals as initiators. This chemistry can be used to make random, block, or graft copolymers of polar monomers. The following scheme demonstrates the synthesis of a methyl methacrylate—lauryl methacrylate (MMA—LMA) AB block copolymer (38). LMA is CH2=C(CH2)COO(CH2) CH2. 

The second front originates in the polymer synthesis community. Efforts are mainly directed toward production of monodisperse block copolymers by living polymerizations. These stmctures typically result in microphase separated systems if one block is a high T material and the other is elastomeric in... 

The formation of polymer can be considered as a quasi-living polymerization. After the polymerization is complete, it can be reinitiated with the addition of more monomer to the unquenched polymer. However, the degree of polymerization cannot be predicted by the monomer/initiator molar ratio, the polydispersity is 1.5-2.0, and water, or even carboxylic acids, act as inhibitors and do not terminate the polymerization [10]. 

A number of techniques for the preparation of block copolymers have been developed. Living polymerization is an elegant method for the controlled synthesis of block copolymers. However, this technique requires extraordinarily high purity and is limited to ionically polymerizable monomers. The synthesis of block copolymers by a radical reaction is less sensitive toward impurities present in the reaction mixture and is applicable to a great number of monomers. 

A potential drawback of all the routes discussed thus far is that there is little control over polydispersity and molecular weight of the resultant polymer. Ringopening metathesis polymerization (ROMP) is a living polymerization method and, in theory, affords materials with low polydispersities and predictable molecular weights. This methodology has been applied to the synthesis of polyacctylcne by Feast [23], and has recently been exploited in the synthesis of PPV. Bicyclic monomer 12 [24] and cyclophane 13 [25) afford well-defined precursor polymers which may be converted into PPV 1 by thermal elimination as described in Scheme 1-4. 

The reaction of radicals with nitroxides is reversible. 09 This means that the highest temperature that the technique can reasonably be employed at is ca 80 °C for tertiary propagating species and ca 120 °C for secondary propagating species.22 These maximum temperatures are only guidelines. The stability of alkoxyamines is also dependent on solvent (polar solvents favor decomposition) and the structure of the trapped species. This chemistry has led to certain alkoxyamines being useful as initiators of living polymerization (Section 9.3.6). At elevated temperatures nitroxides are observed to add to monomer albeit slowly. 3IS 5" 523... 

Microemulsion and miniemulsion polymerization processes differ from emulsion polymerization in that the particle sizes are smaller (10-30 and 30-100 nm respectively vs 50-300 ran)77 and there is no discrete monomer droplet phase. All monomer is in solution or in the particle phase. Initiation usually takes place by the same process as conventional emulsion polymerization. As particle sizes reduce, the probability of particle entry is lowered and so is the probability of radical-radical termination. This knowledge has been used to advantage in designing living polymerizations based on reversible chain transfer (e.g. RAFT, Section 9.5.2)." 2... 

Systems that give reversible chain transfer can display the characteristics of living polymerization. Such systems are discussed in Section 9.5. 

In this chapter, we restrict discussion to approaches based on conventional radical polymerization. Living polymerization processes offer greater scope for controlling polymerization kinetics and the composition and architecture of the resultant polymer. These processes are discussed in Chapter 9. 

Living polymerization processes lend themselves to the synthesis of end functional polymers their use in this context is described in Chapter 9. In this section we limit discussion to processes based on conventional radical polymerization,... 

For this book, we have decided to entitle this chapter Living Radical Polymerization and use the term throughout, it is a chapter describing various approaches to living radical polymerization. We do not intend to imply that termination is absent from all or, indeed, any of the polymerizations described, only that the polymerizations display at least some of the observable characteristics normally associated with living polymerization. 

Following on from the above, various methods have been described to test and/or rank the livingness of polymerization processes." Ul7 20 All of these tests have limitations.. The following list paraphrases a set of criteria for living polymerization set out by Quirk and Lee11 who also critically assessed their applicability primarily in the context of living anionic polymerization. 

Figure 9.1 Predicted evolution of molecular weight (arbitrary units) with monomer conversion for a conventional radical polymerization with a constant rate of initiation (---------------) and a living polymerization (--).

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