Living polystyrene type

Various types of well-defined block copolymers containing polypropylene segments have been synthesized by Doi et al. on the basis of three methods (i) sequential coordination polymerization of propylene and ethylene 83-m>, (ii) transformation of living polypropylene ends to radical or cationic ones which initiate the polymerization of polar monomers 104, u2i, and (iii) coupling reaction between iodine-terminated monodisperse polypropylene and living polystyrene anion 84). In particular, the well-defined block copolymers consisting of polypropylene blocks and polar monomer unit blocks are expected to exhibit new characteristic properties owing to the effect of microphase separation. 

Doi et al. 84) have synthesized a new type of diblock copolymer of propylene and styrene by the coupling reaction between the iodine-terminated polypropylene (4) and monofunctional living polystyrene anion (11) in toluene at 50 °C, as represented by Eq. (47). 

This is an example of the preparation of ABA-type thermoplastic elastomer. Styrene is polymerized first since styryl initiation of isoprene is faster than the reverse reaction. The reaction is carried out in a nonpolar solvent with Li" " as the counterion to enable predominantly cis-l,4-polyisoprene to be formed in the second growth stage. The living polystyrene-6/ocfc-polyisoprene AB di-block copolymer resulting from the second stage is then coupled by a double nucleophilic displacement of Cl ions from a stoichiometric equivalent of dichloromethane to give a polystyrene-61ock-polyisoprene-/)/ock-polystyrene triblock copolymer. 

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. 

In an analogous way, living polystyrene has been deactivated with 2,2 -azobisisobutyronitrile (AIBN). The corresponding new azo compounds generate radical species for the polymerization of methyl methacrylate and vinyl chloride. The reaction between monofunctional living polystyrene and AIBN may be of two types, shown in Scheme 2. 

The rate of electrophilic substitution also depends upon the carbanionic chain end under identical conditions, living polyisoprene (PI) is much more reactive than living polystyrene (PS). As mentioned later in this discussion, Hadjicristidis has taken advantage of this to prepare a new type of star-shaped pol)mier where the same nodulus carries arms of different chemical nature. 

Anionic polymerization of polystyrene takes place very rapidly- much faster than free radical polymerization. When practiced on a large scale, this gives rise to heat transfer problems and limits its commercial practice to special cases, such as block copolymerization by living reactions. We employ anionic polymerization to make tri-block copolymer rubbers such as polystyrene-polybutadiene-polystyrene. This type of synthetic rubber is widely used in the handles of power tools, the soft grips of pens, and the elastic side panels of disposable diapers. 

The living nature of the poly(styryl)anion allows one to prepare block copolymers with a great deal of control of the block copolymer structure. The preparation of diblock, triblock, and other types of multiblock copolymers has been reviewed [29-32]. Several of these block copolymers are in commercial use. The basic concept involves first preparing polystyrene block [RSt StLi—see Eq. (2)] and then adding a new monomer that can be added to start another growing segment. 

When a tetra-chain, star-shaped polystyrene is prepared by a coupling reaction between living polymer and coupler (e.g 1,2,4,5-tetrachloromethyl benzene), the reaction is often carried out with the polystyryl anion in slight excess in order to avoid by-production of types of branched polystyrene other than the tetra-chain. 

It should be pointed out that the same type of graft copolymers has been obtained by Asami130) in a quite different manner involving deactivation of living poly-THF on a backbone chain of polystyrene with some randomly distributed p-hydroxystyrene units which have been metalated beforehand. 

The radical nature of nitroxide-mediated processes also allows novel types of block copolymers to be prepared in which copolymers, not homopolymer, are employed as one of the blocks. One of the simplest examples incorporate random copolymers124 and the novelty of these structures is based on the inability to prepare random copolymers by living anionic or cationic procedures. This is in direct contrast to the facile synthesis of well-defined random copolymers by nitroxide-mediated systems. While similar in concept, random block copolymers are more like traditional block copolymers than random copolymers in that there are two discrete blocks, the main difference being one or more of these blocks is composed of a random copolymer segment. For example, homopolystyrene starting blocks can be used to initiate the copolymerization of styrene and 4-vi-nylpyridine to give a block copolymer consisting of a polystyrene block and a random copolymer of styrene and 4-vinylpyridine as the second block.166... 

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... 

Polystyrene B-114401 and poly(acrylate) B-115402 are connected to a dendrimer at its focal core. These are prepared with dendrimer-type macroinitiators with a benzyl bromide at the focal point, from which are initiated the copper-catalyzed living radical polymerizations of styrene and acrylates, respectively. For B-114, various functional groups (R) were introduced into the periphery. 

A half-rufhenocene complex, Ru(indenyl)Cl(PPh3)2 was similarly used for polymerization of MMA in conjunction wifh an MMA-dimer-type initiator, H(MMA)2C1 (Scheme 6.181) [233]. Addition of Al(Oi-Pr)3 accelerated the polymerization rate to afford similar living polymers (M /M = 1.1). The behavior of the Ru-Ind complex was similar in the polymerization of styrene. Polystyrene with narrow MWD (M /Mn= 1.1) was obtained by use of a suitable radical initiator -Me2C(CO2Et)Br. 

Two different types of (PS-b-PBDh) diblock can be presently synthesized. The first one by classical anionic initiation (s-buty1-lithium) and "living" propagation of the (PS-b-PBD) copolymer (8), followed by the hydrogenation procedure described here as discussed above, the resulting product will be close to a (PS-b-LLDPE) copolymer. The second one came from the discovery (9) of a "living" polymerization of butadiene into a pure (99 %) 1,4 polymer by a bis n allylnickel-tri-fluoroacetate) coordination catalyst, followed by styrene polymerization unfortunately, the length of the polystyrene block is limited (to a M.W. of ca. 20,000) by transfer reactions. 

In order to make use of the dual nature of the growing sites of living polyacetals we have carried out grafting experiments by reacting the latter onto phenyl containing backbones such as polystyrene. One thus expects a Friedel and Crafts type reaction to proceed which will produce the desired graft copolymers. 

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