Sequential anionic

The styrene-diene triblocks, the main subject of this section, are made by sequential anionic polymerisation (see Chapter 2). In a typical system cc-butyl-lithium is used to initiate styrene polymerisation in a solvent such as cyclohexane. This is a specific reaction of the type... 

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). 

Block copolymer—These copolymers are built of chemically dissimilar terminally connected segments. Block copolymers are generally prepared by sequential anionic addition or ring opening or step growth polymerization. 

The predictable stereochemistry of the ring closure of substituted 5-hexenyllithiums, coupled with the ease with which the product organolithium may be functionalized, permits rational design of synthetic routes to polycyclic systems by sequential anionic cyclizations of polyolefinic alkyllithiums.12 The preparation of stereoisomerically pure e do-2-substituted bicyclo[2.2.1]heptanes is illustrative of this approach to polycyclic systems.12... 

Preparation and Reactions of S-b-MM. As mentioned in the introduction, we were interested in block copolymers of styrene and alkali metal methacrylates with overall molecular weights of about 20,000 and methacrylate contents on the order of 10 mol%. The preparation of such copolymers by the usual anionic techniques is not feasible. An alternative is to prepare block copolymers of styrene and methacrylic esters by sequential anionic polymerization, followed by a post-polymerization reaction to produce the desired block copolymers. The obvious first choice of methacrylic esters is methyl methacrylate. It is inexpensive, readily available, and its block copolymers with styrene are well-known. In fact, Brown and White have reported the preparation and hydrolyses of a series of S-b-MM copolymers of varying MM content using p-toluenesulfonic acid (TsOH) (6). The resulting methacrylic acid copolymers were easily converted to their sodium carboxylates by neutralization with sodium hydroxide. 

Anionic polymerization techniques were also critical for the synthesis of a model cyclic triblock terpolymer [cyclic(S-fo-I-fr-MMA)] [196]. The linear cctw-amino acid precursor S-fr-I-fr-MMA was synthesized by the sequential anionic polymerization of St, I and MMA with 2,2,5,5-tetramethyl-l-(3-lithiopropyl)-l-aza-2,5-disilacyclopentane as the initiator and amine generator, and 4-bromo-l,l,l-trimethoxybutane as a terminator and carboxylic acid generator. Characterization studies of the intermediate materials as well as of the final cyclic terpolymer revealed high molecular and compositional homogeneity. Additional proof for the formation of the cyclic structure was provided by the lower intrinsic viscosity found for the cyclic terpolymer compared to that of the precursor. 

Similar block copolymers, i.e., poly(S- -CL), poly(BD- -CL) as well as po-ly(S-b-BD- -CL) ABC triblock copolymers have recently been prepared by Sta-dler et al. by sequential anionic polymerization (Scheme 20) [76]. Addition of... 

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. 

A variation of the sequential anionic polymerization is the use of dianions as initiator, like sodium naphthalene. One starts with the polymerization of monomer A. Then monomer B is fed to the reaction mixture which adds immediately to the living anions at each end of block A and thus leads to a triblock copolymer with an A-middle block and two B-outer blocks. This triblock copolymer is still alive and repetition of the above procedure results in a multiblock copolymer (see Example 3-49). 

Star polymers having several PS branches and only one poly(2-vinyl naphthalene), PVN branch were prepared by Takano et al. using anionic polymerization techniques [31]. Sequential anionic block copolymerization of (4-vinyl-phenyl) dimethylvinylsilane (VS) and VN was employed. The double bonds attached to silicon have to remain unaffected during the polymerization of VS. This was ac-... 

Scheme 2. Several examples of sequential anionic/anionic reactions promoted by Smij.

Scheme 3. An example of an intermolecular sequential anionic/anionie reaction promoted by Smij.

A nice extension of this chemistry to sequential anionic/radical/anionic sequence was also provided [5, 9]. Normally after acyl addition and radical cyclization onto a C = C bond, the newly formed carbon radical is reduced to an organosamarium intermediate which is subsequently protonated. However, as depicted in Scheme 5, this organosamarium may be trapped in the presence of a ketone substrate, thus terminating this three-step process. In another demonstration of how such anions may be further exploited, substrates possessing vinyl ethers as the radical acceptor were found to under-... 

Scheme 8.3 Synthesis of S-B block copolymers using sequential anionic/NMRP polymerization techniques...

Polymers and copolymers were laboratory-prepared samples. Samples W4 and W7 of the diblock copolymer AB poly(styrene-fo-tetramethylene oxide) (PS—PT) were synthesized by producing a polystyrene prepolymer whose terminal group was transformed to a macroinitiator for the polymerization of THF. Samples B13 and B16 of the diblock copolymer AB poly[styrene-h-(dimethyl siloxane)] (PS-PDMS) were prepared by sequential anionic polymerization. Samples of statistical copolymers of styrene and n-butyl methacrylate (PSBMA) were produced by radical copolymerization. Details of synthetic and characterization methods have been reported elsewhere (15, 17-19). 

Durkee and co-workers have prepared polyferrocene block copolymers for use in catalysis.223 The reported polymers were synthesized via sequential anionic polymerization of vinylferrocene and isoprene, followed by oxidation using silver triflate. The fraction of ferrocene converted to ferrocenium was directly proportional to the amount of Ag+ added. These materials were tested for their catalytic activity toward the Michael addition reaction of ethyl-2-oxycyclopentane barboxylate and methylvinylketone. These materials showed k values similar in magnitude to the rates of iron(III) chloride. 

Scheme 3.5 Sequential anionic polymerization of styrene and dimethylsila[l]ferrocenophane.

Scheme 3.6 Sequential anionic polymerization of dimethylsila[l]ferrocenophane and hexamethyl-cyclotrisiloxane.

The stereoeontrolled construction of hydroazulenones is achieved by sequential anionic oxy-Cope rearrangement/SN allylic ether displacement of Cope precursors 5 and 7y71. Optically active compounds 5-8 are prepared from (-)-(lR,6S)-A4(10)-caren-rrons-3-ol via intermediates 3 and 4 by individual exposure of 3 and 4 to 3 equivalents of vinylmagnesium bromide. This reagent promotes initial pinacol rearrangement with liberation of a cyclohexanone unit which is captured by a second equivalent of the organometallic. The divinyl alcohol pairs 5/6 and 7/8 are isolated in 2 1 ratios (43-82% combined yields after MPLC). 

It is also possible to form block co-polymers via the living anionic ROP of the phosphorus-bridged ferrocenophanes (Section 12.06.3.3.3). " Diblock co-polymers such as PI-/)-PFP 108 can be prepared by the sequential anionic polymerization of isoprene and ferrocenophane 107 (Scheme 10). These materials yield spherical micelles in -hexanes with an amorphous PFP core and a PI corona. The PI corona can be cross-linked via radical reactions to yield a permanently cross-linked shell, which retains its integrity even in good solvents for both blocks. With PFP block co-polymers the possibility of the coordination of various catalytically active transition metal moieties to the phosphorus centers may prove useful for catalysis and for materials science applications. 

Although polysiloxanes have superior properties compared to common organic polymers, the controlled formation of amphiphilic block copolymers consisting of a pure polysiloxane backbone remains essentially unexplored. There have been only a few reports of such polymers in the literature, for example one bearing pendant carboxyl groups on the polysiloxane chain,and another prepared by a one-pot sequential anionic ring-opening polymerization of two cyclotrisiloxanes. ... 

The polymerization of D3 was followed by GC/MS. After 36 h, the reaction was quenched by the introduction of trimethylchlorosilane. About 92% of the D3 had been converted, while the amount of unconverted D/ had not changed significantly. Si NMR analyses allowed the determination of the sequence distribution of repeat units, which showed no random copolymerization of D/ and D3 as in the case of diblock copolymers prepared by sequential anionic copolymerization of D3 extended with D/. Because the polymerization of the first monomer could not be carried out to completion (<100% conversion) without increasing the molecular weight distribution, the second monomer (with faster propagation rates) had to be introduced before the equilibration reaction became established. Therefore, unreacted monomer from the first step was still in solution when the second monomer was added. The risk of random copolymerization can be suppressed if the second monomer has far higher reactivity towards polymerization than the first monomer. The block formed in the second step contained only a few methylvinylsiloxane units, i.e. the block purity was very high. 

Polysiloxane-containing amphiphilic block copolymers have been prepared by different approaches. Coupling of an end-functionalized polydimethylsiloxane with a functionalized poly(ethylene oxide) led to the formation of PDMS- -PEO diblock copolymers. The sequential anionic ring-opening polymerization of tetramethyltetravinylcyclotetrasiloxane and hexamethylcyclotetrasiloxane resulted in the formation of a vinyl-substituted diblock copolymer, the vinyl groups of which could be modified by further reactions so as to import amphiphilic character. The phase behavior of short-chain PDMS-Z -PEO diblock copolymers revealed the preferred formation of lamellar phases by this type of amphiphile... 

The styrene-diene-styrene triblocks consist of a block of diene units joined at each end to a block of styrene units and are made by sequential anionic polymerization of styrene and a diene. In this way two important triblock copolymers have been produced—the styrene-butadiene-styrene (SBS) and styrene-isoprene—styrene (SIS) materials, developed by Shell. 

---