Anionic polymerization molar mass distribution

A second route is termed sequential anionic polymerization. More recently, also controlled radical techniques can be applied successfully for the sequential preparation of block copolymers but still with a less narrow molar mass distribution of the segments and the final product. In both cases, one starts with the polymerization of monomer A. After it is finished, monomer B is added and after this monomer is polymerized completely again monomer A is fed into the reaction mixture. This procedure is applied for the production of styrene/buta-diene/styrene and styrene/isoprene/styrene triblock copolymers on industrial scale. It can also be used for the preparation of multiblock copolymers. 

As explained in Sections 16.3.4, 6.4.1, and 16.4.2, SEC is a nonabsolute method, which needs calibration. The most popular calibration materials are narrow molar mass distribution polystyrenes (PS). Their molar mass averages are determined by the classical absolute methods—or by SEC applying either the absolute detection or the previously calibrated equipment. The latter approach may bring about the transfer and even the augmentation of errors. Therefore, it is recommended to apply exclusively the certified well-characterized materials for calibrations. These are often called PS calibration standards and are readily available from numerous companies in the molar mass range from about 600 to over 30,000,000g moL. Their prices are reasonable and on average (much) lower than the cost of other narrow MMD polymers. Other available homopolymer calibration materials include various poly(acrylate)s and poly(methacrylate)s. They are, similar to PS, synthesized by anionic polymerization. Some calibration materials are prepared by the methods of preparative fractionation, for example, poly(isobutylene)s and poly(vinylchloride)s. 

So far we have discovered very few polymerization techniques for making macromolecules with narrow molar mass distributions and for preparing di-and triblock copolymers. These types of polymers are usually made by anionic or cationic techniques, which require special equipment, ultrapure reagents, and low temperatures. In contrast, most of the commodity polymers in the world such as LDPE, poly(methyl methacrylate), polystyrene, poly(vinyl chloride), vinyl latexes, and so on are prepared by free radical chain polymerization. Free radical polymerizations are relatively safe and easy to perform, even on very large scales, tolerate a wide variety of solvents, including water, and are suitable for a large number of monomers. However, most free radical polymerizations are unsuitable for preparing block copolymers or polymers with narrow molar mass distributions. 

Many addition polymerization reactions with very low concentrations of impurities have propagation rates much faster than initiation rates and have essentially no termination. Such reactions produce narrow molar mass distributions that can be approximated by the Poisson distribution. Comparison of the polydispersity index of anionically polymerized butadiene with Eq. (1.69) is shown in Fig. 1.20. 

Propagation can then occur at both carbanion centres. Polymers synthesized by an anionic route have characteristic features of a narrow molar-mass distribution and controlled degree of polymerization, and these follow from the mechanism and kinetics of polymerization. 

The particular features of anionic polymerization that made the polymer chains living were discussed above. The main requirement for a living polymerization is the absence of any process for spontaneous termination so that the degree of polymerization is controlled by the ratio of monomer to initiator concentrations. The molar-mass of the polymer therefore increases linearly with monomer conversion. On exhaustion of the monomer, the initiation centres remain, so chains may be re-initiated by addition of further monomer. Termination or chain transfer is controlled by the delibemte addition of a reagent to remove the living end. The resulting polymers will also have very narrow molar-mass distributions since rapid initiation ensures that all chains are initiated at the same time. 

Since the discovery of living polymerizations by Swarc in 1956 [1], the area of synthesis and application of well-defined polymer structures has been developed. The livingness of a polymerization is defined as the absence of termination and transfer reactions during the course of the polymerization. If there is also fast initiation and chain-end fidelity, which are prerequisites for the so-called controlled polymerization, well-defined polymers are obtained that have a narrow molar mass distribution as well as defined end groups. Such well-defined polymers can be prepared by various types of living and controlled polymerization techniques, including anionic polymerization [2], controlled radical polymerization [3-5], and cationic polymerization [6, 7]. 

A variety of important monomers are polymerized via anionic polymerization including cyclic ones like ethylene oxide as base for poly(ethylene oxide) (PEO) or poly(ethylene glycol) (PEG) and A-carboxyanhydrides (NCAs) used for making polypeptides and caprolactam, the monomer of nylon 6 (Eig. 3.9). Whereas in all cases rather stringent conditions have to be used to allow at least an efficient polymerization, for technical products often not fully living conditions are achieved, which leads to broader molar mass distributions. 

In the past decade, several new approaches for controlled NCA polymerization based on the classical primary amine initiation have been reported. In 2003, Dimitrov and Schlaad reported the controlled ( ammonium mediated ) polymerization of ZLL-NCA at elevated temperature using primary amine hydrochloride salts as initiators (Figure 4.3). The initiator reactivity most likely is due to the formation of a small amount of free amine by reversible dissociation of HCl. This equilibrium is strongly shifted toward the dormant amine hydrochloride species. Consequently, as soon as a free amine reacts with an NCA, the resulting adduct is immediately protonated and prevented from further reaction. The presence of protons in the system suppresses formation of unwanted NCA anions ( activated monomers ). The obtained polypeptide blocks exhibit a very narrow, close to a Poisson, molar-mass distribution. 

As a typical example, Frey etal. [16, 27] described the anionic polymerization of glycidol, which was considered also as a latent AB2 (= ABB ) monomer (Scheme 24.3). The polymerization proved to be very versatile and led to hb polymers with a rather narrow molar mass distribution (Mu,/M = 1.1-1.4) due to a chain growth-hke character of the reaction when only partial deprotonation to the initiating alkoxide (initiating site, triol in Scheme 24.3) was performed. This led to a more or less simultaneous growth of all chain ends, and allowed control of both the molar mass and polydispersity. By use of the trifunctional initiator (core molecule) and slow monomer addition, cychzation was suppressed such that the molar mass and polydispersity could be controlled. 

The number-average molar mass of living polymerizations via macroions is completely determined by the monomer and initiator concentrations initially and at equilibrium. However, the molar mass distribution additionally depends on whether all the initiator anions or monomer anions do actually initiate polymerization simultaneously. If, for example, the initiator... 

A peculiar situation is met in the polymerization at temperatures below the melting point of the polyamide when depolymerization and side reactions are largely reduced and the disproportionation reaction is appreciably slower. In this case, even at equimolar concentrations of initiator and activator, the polymerization proceeds essentially by the reaction of lactam anions with a constant number of growth centers, resulting in a narrower molar mass distribution (M /Mn<2). Moreover, since bifunctional activators may be safely used under specific conditions without any formation of cioss-linked structures, very high molar masses can he obtained. ° ... 

The existence of several kinds of propagating species (e.g. free ions and ion-pairs) with vastly different rates of propagation could lead to the formation of polymers with broad or complex molar mass distributions. However, the rates of interconversion between the different species normally are greater than their rates of propagation and so there is no significant effect upon the molar mass distribution. Thus polymers prepared by anionic polymerization using fast initiation have narrow molar mass distributions typically with 1.05. 

The ability to produce polymers of well-defined structure using anionic polymerization is of great importance and despite the above difficulties it is widely used for this purpose. Thus polymers with narrow molar mass distributions, terminally-functionalized polymers, and perhaps most important of all, well-defined block copolymers (Section 2.16.9) can be prepared using anionic polymerization. 

Anionic living polymerization provides the most important chain polymerization route to block copolymers. By careful control of the reaction conditions it is possible to produce block copolymers with blocks of pre-defined molar mass, narrow molar mass distribution and controlled stereochemistry. In particular, anionic living polymerization is used to prepare block copolymers suitable for use as thermoplastic elastomers (Section 4.5.1), e.g. ABA tri-block copolymers in which homopolymer A is glassy (e.g. polystyrene) and homopolymer B is rubbery (e.g. polyiso-... 

Anionic polymerization methods have been used to synthesize a wide variety of macromolecules including linear [1] and cyclic [2] homopolymers, linear copolymers [1], and functional polymers such as macromonomers [3]. These macromolecules are well defined with predetermined molar masses, sharp molar mass distributions, and low compositional heterogeneity. They serve as ideal compounds to establish the relation between the structure, the properties, and theory. 

Burchard [65] was the pioneer for the preparation of functional starshaped pol)miers in nonpolar solvents by an anionic core-first method. To build the cores pure / -DVB was reacted with /i-butyllithium in dilute cyclohexane solution. Suspensions of small crosslinked poly(DVB) noduli were obtained that contained numerous lithium organic sites. In a second step, styrene (or isoprene) was added to the living cores and polymerized. The polymeric species obtained exhibit huge molar mass distribution and rather large polydispersity indices. Even if these star-shaped polymers could exhibit active sites at the outer end of the branches, the efficiency of initiation of a second generation of monomers or of hmctionalization was never given by the authors. 

The discovery of the living character of the anionic polymerization by M. Szwarc [93] has contributed tremendously to the domain of macromolecular engineering and especially to the synthesis of star-shaped polymers. Not only can the molar mass and the molar mass distribution of the branches be controlled in advance, but anionic polymerization has also demonstrated its efficiency in controlling the functionality of the star-shaped polymers. Earlier attempts to apply anionic polymerization to build star-shaped structures were focused essentially on arm-first methods. Well-defined star-shaped polymers exhibiting homopolymeric branches could thus be obtained. These samples have been studied to confirm expected structures or have served as models. Two decisive improvements have been made in the domain of application of anionic polymerization to the synthesis of well-defined star-shaped polymers ... 

Besides the Wurtz-coupling reaction, other synthetic routes towards non-branched polysilanes have been reported, which overcome the drawbacks of polymodal molar weight distribution and low yield of the high molar mass fraction. However, so far the application of these routes has been very limited. The anionic ring opening polymerization of the strained 1,2,3,4-tetramethyl-1,2,3,4-tetraphenylcyclotetrasilane leads to poly(methylphenylsilane) in high yield but the starting cyclotetrasilane has to be prepared in three steps from diphenyldichlorosilane via octaphenylcyclotetrasilane [84] ... 

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