Block copolymers anionic polymerisation

Anionic polymerisation techniques aie one of many ways to synthesise a special class of block copolymers, lefeiied to as star block copolymers (eq. 25) (33). Specifically, a "living" SB block is coupled with a silyl haUde coupling agent. The term living polymerisation refers to a chain polymerisation that proceeds in the absence of termination or transfer reactions. 

In the absence of impurities there is frequently no termination step in anionic polymerisations. Hence the monomer will continue to grow until all the monomer is consumed. Under certain conditions addition of further monomer, even after an interval of several weeks, will eause the dormant polymerisation process to proceed. The process is known as living polymerisation and the products as living polymers. Of particular interest is the fact that the follow-up monomer may be of a different species and this enables block copolymers to be produced. This technique is important with certain types of thermoplastic elastomer and some rather specialised styrene-based plastics. 

Tbe system may be used for homopolymers and for block copolymers. Some commercial SBS triblock thermoplastic rubbers and the closely related K-resins produced by Phillips are of this type. Anionic polymerisation methods are of current interest in the preparation of certain diene rubbers. 

Closely related to these but thermoplastic rather than rubber-like in character are the K-resins developed hy Phillips. These resins comprise star-shaped butadiene-styrene block copolymers containing about 75% styrene and, like SBS thermoplastic elastomers, are produced by sequential anionic polymerisation (see Chapter 2). 

Synthesis of vinyl block copolymers is accomplished by living polymerisation, mostly by anionic polymerisation. Several strategies can be used, illustrated here by the example of the Styrene-Butadiene-Styrene (or SBS) triblock copolymer. 

It is possible to produce a block copolymer by the anionic polymerisation of styrene and butadiene as depicted below. The polystyrene and polybutadiene are mutually incompatible and hence phase separate to give the morphology also depicted below ... 

Fig. 30. Apparatus for purifying and manipulating reactants under high vacuum in order to prepare solutions of block copolymers via anionic polymerisation. Concentrations are determined spectro-photometrically in the optical cell and LS is measured on the solution in the Sofica cell120 ...

It is well known that AB diblock copolymers form micelles in solvents that are selective for one of the blocks. By varying the nature of the solvent, it is also possible to form micelles with the A block in the core or with the B block in the core. However, we have recently demonstrated that certain hydrophilic AB diblock copolymers can form either A-core micelles or B-core micelles in aqueous media. In the original example, both blocks were based on tertiary amine methacrylates and the diblock copolymer was prepared by group transfer polymerisation, a special type of anionic polymerisation which is particularly... 

In principle, aqueous ATRP offers the tantalising possibility of the direct synthesis of reasonably well-defined zwitterionic block copolymers in water without recourse to protecting group chemistry. However, ATRP in acidic media is generally unprofitable, hence the (co)polymerisation of acidic monomers such as methacrylic acid or 4-vinylbenzoic acid must be carried out in weakly alkaline solution, i.e. the monomer should be in its anionic carboxylate... 

We have some evidence that this theoretical problem is a genuine limitation in the case of a quaternised styrenic monomer which is block copolymerised with NaVBA. This problem can be circumvented in two ways. Firstly, the polymerisation sequence can be simply reversed so that the longer block is synthesised first. If this is the quaternised block, the resulting copolymer cannot exhibit an isoelectric point because the major block is permanently cationic, thus no charge compensation can occur. On the other hand, if the longer block is anionic, then addition of HCl will protonate the acidic monomer residues and at some point an isoelectric point will be attained (unless the acidic block is strongly acidic, e.g. 4-styrenesulfonic acid). 

Ionic polymerisation is a well-known technique for the preparation of graft copolymers but the fate of these reactions is determined by the reaction conditions. Since the discovery of living polymerisation , (anionic polymerisation) [67] it has become an excellent method for the synthesis of block and graft copolymers. In anionic polymerisation the graft copolymerisation is initiated by the anion generated by the reaction of bases with acidic protons in the polymer chain as shown in Scheme 2. 

Within the context of stable carbenium salts initiatron, we already examined a very interesting and successful study on the block copolymeriation of a-methylstyrene with cyclopentadiene performed by Vairon and Villesange (see Sect. V-A-4-b). The preparation of the product required three basic operations (i) the living anionic polymerisation of a-methylstyrene to give monodisperse macromolecules, (ii) transformation of their end groups into stable carbocationic moieties, and (iii) initiation of the polymerisation of cyclopentadiene from these active ends under conditions of minimal transfer and termination reactions. Thus, the macroinitiators in the second polymerisation were generated by a controlled anionic polymerisation and allowed tiie synthesis of a triblock near-isomolecular copolymer. 

Starting Material. 1,2-polybutadiene (PB) and the A-polystyrene-B-1,2-poly-butadiene block copolymer (PSPB) were obtained by "living 1 anionic polymerisation under high-vacuum conditions at -78°C with sec-buthyllithium as initiator. 

The present volume is particularly concerned with the use of the different modes of controlled radical polymerisation for the preparation of copolymers such as random copolymers, linear block copolymers, as well as graft copolymers and star-shaped copolymers. It also presents the combination of controlled radical polymerisation with non-controlled radical copolymerisation, cationic and anionic polymerisation,both of vinyl monomers and cyclic monomers, and ringopening metathesis polymerisation. 

In Section 4.1.1, the general mechanistic aspects of anionic polymerisation of alkylene oxides (especially PO) were discussed. The anionic polymerisation of PO initiated by hydroxyl groups is considered as a pseudo living polymerisation. This type of polymerisation has some important aspects of living polymerisations the active centre (alcoholate type) is stable and active, and during the polymerisation reaction the number of active alcoholate centres remains constant. This characteristic of living polymerisations is very important for the synthesis of block copolymers. For example if after the addition of PO to the living polymer EO (or BO) are added, then block copolymers are obtained. 

The pseudoliving character of PO anionic polymerisation produces a large variety of block copolymers, by simply changing the nature of the oxirane monomer because the catalytic species (potassium alcoholate) remains active during and after the polymerisation reaction. Thus, if a polyether is synthesised first by anionic polymerisation of PO and the polymerisation continues with another monomer, such as EO, a block copolyether PO-EO with a terminal poly[EO] block is obtained. Another synthetic variant is to obtain a polyethoxylated polyether first by the anionic polymerisation of EO initiated by glycerol [108], followed by the addition of PO to the resulting polyethoxylated triol. A block copolyether PO-EO is obtained with internal poly[EO] block linked to the starter. Another possibility is to add the monomers in three steps first PO is added to glycerol, followed by EO addition and finally by the addition of PO. A copolyether triol block copolymer PO-EO with the internal poly[EO] block situated inside the polyetheric chain between two poly[PO] blocks is obtained [4, 100, 101]. 

Equation 4.20 shows that mass transfer is a determining factor in anionic polymerisation of PO, a high surface area of the liquid reaction mass giving high rates of PO consumption. On the other hand, due to the very high efficiency of stirring, the gas-liquid contactor reactor type assures a very narrow MW distribution of the resulting polyether. For the ethoxylation of intermediate propoxylated polyethers (in block copolymers PO-EO... 

The advantage of phosphazenium catalysts, compared to DMC, is their capability to catalyse the anionic polymerisation of PO and EO and to be used successfully in the synthesis of PO-EO block copolymers with terminal poly[EO] block, without intermediate change of the catalyst nature. 

The nomenclature poly (M1-6-M2) is used where Mj and M2 are the monomer names for example poly (styrene-b-butadiene). To make block copolymers, the polymer chains must have the ability to propagate [living polymers) when the first monomer is replaced by the second. In conventional addition polymerisation the chain termination and transfer processes make the lifetime of a growing polymer chain too short. Consequently, special ionic polymerisation catalysts were developed. A fixed number of di-anions such as [C6H5CHCH2CH2CHC6H5] are introduced into an inert solvent. These propagate from both ends if a suitable monomer is introduced. As there are no termination or transfer reactions, once the first monomer has been consumed, a second monomer can be introduced to produce a triblock copolymer such as styrene-butadiene-styrene. Each block has a precisely defined molecular weight. These materials undergo phase separation (Chapter 4) and act as thermoplastic rubbers. 

The cure mechanism for curing triglycidyl -amino phenol with diaminodiphenylsulfone was studied. The concentration of primary and secondary amine and epoxide groups were monitored directly as a function of cure with NIR spectroscopy. In similar fashion, monitoring of the percent conversion of methyl methacrylate to PMMA in situ in a mould used short-wavelength NIR spectroscopy. NIR spectroscopy was also used to monitor conversion during conventional, anionic solution polymerisation of styrene and isoprene to homopolymers and block copolymers (314). 

FTIR spectroscopy with photoacoustic detection and micro-Raman confocal spectroscopy were used to study the conformations of poIy(epsilon caprolactam) and poly(epsilon-caprolactam)-polybutadiene block copolymers. In the block copolymers prepared by anionic polymerisation, the fraction of the planar conformation of poly(epsilon-caprolactam) chains decreased with increasing polybutadiene content. In the surface layers formed by rapid saw cutting and in the islands formed by microtome cutting, the content of the planar conformation was lowered. This was substantially increased by water treatment, especially at elevated temperatures. 15 refs. 

Near-IR spectroscopy (10000-4000/cm) was successfully used to monitor conversion dining conventional, anionic solution polymerisation of styrene and isoprene to homopolymers and block copolymers. The conversion of the vinyl protons in the monomer to methylene protons in the polymer was easily monitored under conventional (10-20% solids) solution polymerisation conditions. In addition to the need for an inert probe, high sampling frequencies were required since polymerisation times ranged from 5s in tetrahydrofuran to 20 minutes in cyclohexane. Preliminary data indicate that near IR is capable of detecting sequence distribution for tapered block copolymers, geometric isomer content, and reactivity ratios for free-radical copolymerisation. 20 refs. USA... 

Synthesis of block copolymers with well-defined structure has received considerable attention, as their properties are potentially of great interest (see Section 4.1). Until recently, the possibilities were limited to the use of either sequential addition of monomers in living anionic polymerisation systems, or coupling of polymers possessing reactive ends, e.g. telechelic polymers. Advances in radical controlled polymerisation have opened new perspectives. 

In addition to the interconversion of polymerisation processes, combination of two polymerisations offer new routes to macromolecular architecture. Recent work has demonstrated that ROMP was the method of choice to prepare polymacromonomers within high yields and with precise control of the size. Based on the combination of ROMP and anionic polymerisation random and block copolymacromonomers and graft copolymers have been prepared using the Schrock well defined molybdenum initiators (Schemes 1 and 2. ... 

Diblock copolymers were synthesised by two stepwise anionic polymerisation methods. One method produced diblock copolymer plus 30% of poly(2-vinylpyridine) homopolymer. The copolymers were dissolved in O.IM hydrochloric acid. When the pH was increased by the dropwise addition of 0.1 M sodium hydroxide, micelles with well-defined hydrodynamic diameters formed spontaneously at around pH 5. Further basification produced stable micelle structures and reacidification produced the mirror image of this titration curve. Blue swirls were observed when sodium hydroxide was added at pH4 or pH5. The micelle sizes were measured by quasielastic light scattering. It is shown that (1) it is possible to control micelUsation by pH and (2) formation of well-behaved micelles of variable hydrodynamic diameter is possible by titration of different ratios and different total polymer concentrations of poly(2-vinylpyridine/poly(2-vinylpyridine-block-PEO). Relevance to drug release systems that can remain intact and circulate for long periods within the vascular system is suggested. 17 refs. 

By the same method of living radical polymerisation, a series of block copolymers of poly(ethylene oxide-styrene) with narrow polydispersity were synthesised by the following two-step approach [96]. Initially, living anionic polymerisation of ethylene oxide with sodium-4-oxy-2,2,6,6-tetramethyl-l-piperidinoxyl as initiator yields polyethylene oxide with ARs at the chain end ... 

Although anionic polymerisation remains an important technique today for the preparation of block copolymers and other controlled polymer architecture, recent developments in controlled free-radical polymerisation has presented an alternative approach which may complement this methodology. 

Fig. 10.6 NMR spectrum of a high molecular weight block copolymer of styrene and isoprene produced by anionic polymerisation...

Functionalised PO as block and graft copolymers used as compatibilisers or to increase interactions with other materials are prepared by free radical grafting (the simplest method), metallocene-catalysed copolymerisation of olefins with functional monomers, or anionic polymerisation (silane-containing PO). They are also produced by controlled/living polymerisation techniques such as nitroxide-mediated controlled radical polymerisation, atom transfer radical polymerisation (ATRP), and reversible addition-fragmentation chain transfer (RAFT). 

It was pointed out in Section 2.16.9 that anionic living polymerisation can be used to prepare ABA tri>block copolymers suitable for use as thermoplastic elastomers. In such copolymers the A blocks are normally of a homopolymer which is glassy and the B block is of a rubbery homopolymer (e.g. a polydiene such as polybutadiene or polyisoprene). The characteristic properties of these materials stems from the fact that two polymers which contain repeat units of a different chemical type tend to be incompatible on the molecular level. Thus the block copolymers phase separate into domains which are rich in one or the other type of repeat unit. In the case of the polystyrene-polydiene-polystyrene types of tri-block copolymers used for thermoplastic elastomers (with about 25% by weight polystyrene blocks), the structure is phase-separated at ambient temperature into approximately spherical polystyrene-rich domains which are dispersed in a matrix of the polydiene chains. This type of structure is shown schematically in Fig. 4.36 where it can be seen that the polystyrene blocks are anchored in the spherical domains. At ambient temperature the polystyrene is below its Tg whereas the polydiene is above its Tg. Hence the material consists of a rubbery matrix containing a rigid dispersed phase. 

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