Polystyrene lithium

Hyperbranched polymers have also been prepared via living anionic polymerization. The reaction of poly(4-methylstyrene)-fo-polystyrene lithium with a small amount of divinylbenzene, afforded a star-block copolymer with 4-methylstyrene units in the periphery [200]. The methyl groups were subsequently metalated with s-butyllithium/tetramethylethylenediamine. The produced anions initiated the polymerization of a-methylstyrene (Scheme 109). From the radius of gyration to hydrodynamic radius ratio (0.96-1.1) it was concluded that the second generation polymers behaved like soft spheres. 

In this technique, used, e.g., for the synthesis of block copolymers of poly(styrene-h-butadiene-h-styrene) (SBS), a polystyrene block is formed by employing n-butyllithium as the initiator in an aromatic solvent. Butadiene monomer is then added to react with the polystyrene-lithium chain end to form the poly(butadiene) block. If the reaction was terminated at this stage, a poly(styrene-h-butadiene) copolymer would result, which has no thermoplastic properties. Therefore, styrene monomer is added to produce the triblock SBS. The process for the preparation of SBS is very carefully controlled to avoid the formation of a diblock, as the presence of any appreciable amount of SB dramatically reduces the thermoplastic properties of SBS. 

Macroinitiators such as polymeric Li, Na, and K alkoxides can also be used for the initiation of the six-membered cyclic carbonate polymerization. Thus, besides living vinyl polymers, hydroxyl group-terminated polymers of poly (tetrahydrofuran) (PTHF), poly(oxyethylene), and poly(dimethylsiloxane) (PDMS) were transformed to alkoxides by treatment with scc-BuLi or K-naphthalene and used as initiators. The use of these macroinitiators enables the identification of side reactions, as shown by Keul and Hocker for polystyrene lithium (PS Li ). The addition of the macro initiator to the monomer, to maintain a high excess of monomer, minimizes side reactions. Transformation of the polystyryl... 

Typical examples of initiators for cycHc carbonates-as shown for DTC-are alkali metal organic compounds such as sec-butyUithium (seoBuLi), sodium- and potassium naphthalene, and Hthium-, sodium- and potassium alkoxides or polymeric living vinyl or diene polymers with alkah metal counterions, as weU as polymeric alcoholates. The use of these macroinitiators enables the identification of side reactions, as will be shown exemplarily for polystyrene lithium (PS li ) [25]. Besides the initiation reaction of PS"Li, which represents a site transformation of... 

Scheme 12.3 Initiation, side reactions and site transformation in ring-opening polymerization of 2,2-dimethyltrimethylene carbonate initiated by polystyrene lithium.

Polystyrene produced by free-radical polymerisation techniques is part syndio-tactic and part atactic in structure and therefore amorphous. In 1955 Natta and his co-workers reported the preparation of substantially isotactic polystyrene using aluminium alkyl-titanium halide catalyst complexes. Similar systems were also patented by Ziegler at about the same time. The use of n-butyl-lithium as a catalyst has been described. Whereas at room temperature atactic polymers are produced, polymerisation at -30°C leads to isotactic polymer, with a narrow molecular weight distribution. 

The Laboratory prepn for amorph styrene involves thermal conditions or the use of butyl-lithium at 50°. Crystalline polystyrene can be... 

Amos prepared his polymer-supported reagent in two steps from commercially available polystyrene beads (bromination, then condensation with lithium diphenylphosphide). He found that a useful range of sulphoxides could be reduced effectively, in good yields and in a few hours, to give clean samples of sulphides. 

Bywater 68) concerned with the polymerization of lithium polystyrene in benzene and published in 1960. 

The kinetics of polymerization and conductometric studies of barium polystyrene with one active end-group per chain were reported by De Groof et al. 79,80). Formation of an unconventional anionic species, Ba2 +, (CH(Ph)CH2—)j, had to be postulated to account for the results. The existence of such a species is supported by the recent study of the kinetics of polymerization of lithium polystyrene performed in the presence of barium polystyrene endowed with two active endgroups 78). The polymerization of the lithium salt is retarded by the presence of the barium salt, and the retardation is accounted for by the formation of the inert aggregated anions,... 

The heat of dissociation in hexane solution of lithium polyisoprene, erroneously assumed to be dimeric, was reported in a 1984 review 71) to be 154.7 KJ/mole. This value, taken from the paperl05> published in 1964 by one of its authors, was based on a viscometric study. The reported viscometric data were shown i06) to yield greatly divergent AH values, depending on what value of a, the exponent relating the viscosity p of a concentrated polymer solution to DPW of the polymer (q DP ), is used in calculation. As shown by a recent compilation 1071 the experimental a values vary from 3.3 to 3.5, and another recent paper 108) reports its variation from 3.14 to 4. Even a minute variation of oe results in an enormous change of the computed AH, namely from 104.5 KJ/mole for oe = 3.38 to 209 KJ/mole for oe = 3.42. Hence, the AH = 154.7 KJ/mole, computed for a = 3.40, is meaningless. For the same reasons the value of 99.5 KJ/mole for the dissociation of the dimeric lithium polystyrene reported in the same review and obtained by the viscometric procedure is without foundation. 

The order of reactivities could be also reversed by a change of solvent. For example, in THF styrene is more reactive than butadiene towards salts of polystyryl anions, whereas in hydrocarbon solvents butadiene is more reactive than styrene towards lithium polystyrene. This reversal of reactivities presumably is caused by a change in the mechanism of propagation. The monomers react directly with carbanions in THF, but become coordinated to Li+ in hydrocarbon solvents. 

The correct explanation of the peculiar behaviour of the butadiene-styrene system was provided by O Driscoll and Kuntz 144). As stated previously, under conditions of these experiments butadiene is indeed more reactive than styrene, whether towards lithium polystyrene or polybutadiene, contrary to a naive expectation. This was verified by Ells and Morton 1451 and by Worsfold 146,147) who determined the respective cross-propagation rate constants. It is germane to stress here that the coordination of the monomers with Li4, assumed to be the cause for this gradation of reactivities, takes place in the transition state of the addition and should be distinguished from the formation of an intermediate complex. The formation of a complex ... 

The results reported by Helary and Fontanille 84) provide an illustration of the above principles. Coordination of lithium polystyrene in cyclohexane by TMEDA increases the propagation rate for c = 8.3 mM but decreases for c = 0.92 mM. This is seen in the plots shown in Fig. 22. 

Say that at c = 1 mM the propagation rate is not affected by the addition of TMEDA, a reasonable assumption based on the data of Helary and Fontanille. This leads to ktKigs = 0.84 10-2 M1/2 sec-1 as derived from the equation c = k2Kdiss/8k, whereas its value determined from the kinetics of lithium polystyrene polymerization in cyclohexane is 0.7 102 M1/2 sec-1. The agreement is fair. Note, the results are independent of the value of Kso). 

The effect of tetramethyl tetraaza cyclotetradecane, TMTCT, on the behaviour of lithium polystyrene in cyclohexane was investigated recently 149). 

The kinetics of lithium polystyrene polymerization obeys a first order law at constant concentration of TMTCT. The first order constant increases linearly with the concentration of this complexing agent149) and becomes constant for [TMTCT] [lithium polystyrene] as shown in Fig. 23. 

Under these conditions the maximum propagation constant, kpc = 750 M-1 sec-1, gives the absolute rate constant of the monomer addition to the complexed unassociated lithium polystyrene, a value obviously larger than that of the unassociated but also uncomplexed polymer. 

Polymerisations triggered by butyl lithium have often led to detonations, which destroyed the installations. The reaction can be best controlled by first incorporating polystyrene with a low molecular mass. 

Polystyrene was prepared by the anionic polymerisation of styrene in toluene plus THF mixtures (4 1 volume ratio) using n-butyl lithium as initiator. After removing a sample for analysis at this stage, the remainder of the living polystyrene was reacted with a five molar excess of trichloromethylsilane for 15 min and then excess methanol introduced. The methoxy-terminated polystyrene was freeze-dried from dioxan. The method described here essentially follows the route proposed by Laible and Hamann (6). 

The conversion of the polystyrene-supported selenyl bromide 289 into the corresponding acid 290 allowed dicyclohexylcarbodiimide (DCC)-mediated coupling with an amidoxime to give the 1,2,4-oxadiazolyl-substituted selenium resin 291 (Scheme 48). Reaction with lithium diisopropylamide (LDA) and allylation gave the a-sub-stituted selenium resin 292, which was then used as an alkene substrate for 1,3-dipolar cycloaddition with nitrile oxides. Cleavage of heterocycles 293 from the resin was executed in an elegant manner via selenoxide syn-elimination from the resin <2005JC0726>. 

As polystyrene obtained by free radical polymerisation technique is atactic it is therefore non-crystalline. The isotactic polystyrene is obtained by the use of Ziegler-Natta catalysts and n-butyl lithium. Isotactic polystyrene is having a high crystalline Melting point of 250°C. It is transparent. It is more brittle than the atactic polymer. 

Amino acids are separated in their native form on a sulphonated polystyrene resin using a system of sodium or lithium based buffers. Separation is effected by stepwise, rather than gradient elution, and the chromatography can be further optimized by carefully controlling the temperature of the analytical column. 

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