Living styrene

The requirements for a polymerisation to be truly living are that the propagating chain ends must not terminate during polymerisation. If the initiation, propagation, and termination steps are sequential, ie, all of the chains are initiated and then propagate at the same time without any termination, then monodisperse (ie, = 1.0) polymer is produced. In general, anionic polymerisation is the only mechanism that yields truly living styrene... 

In 1988 Faust and Kennedy reported that controlled/living styrene polymerization would be possible with a combination of l-(p-methylphe-nyl)ethyl acetate [CH3ChH4CH(CH3)-OCOCH3 the adduct of acetic acid with p-methylstyrene] and BCI3 in CH3CI solvent below -30° C [214]. Similar systems were also reported by Matyjaszewski and Lin [27,117,208], Although the M of the polymers increases linearly with conversion, the controlled/living nature of these polymerizations is rather obscured by very broad MWDs where the MJM ratios sometimes exceed 6. 

PS. However, significant research has been conducted with the goal of achieving living styrene polymerization nsing cationic, free-radical, and Ziegler-Natta mechanisms. 

A comprehensive list of the grafting reactions of (a) and (b) onto different polymer backbones is given in Ref 543. The methods summarized there include radiation techniques [623], grafting by radical transfer [624], and grafting initiated by functional groups in backbone polymers [625,626]. Macromonomers of (a) have been synthesized by means of anionic polymerization techniques and have been copolymerized with styrene [627,628]. Only a few examples are known in which polymers from (a) and (b) were used as backbone [629]. Star shaped block copolymers with four arms were prepared by coupling living styrene/(a) block copolymers with 1,2,4,5-tetrakis-bromomethyl-benzene [626]. 

Anionic polymerization, if carried out properly, can be truly a living polymerization (160). Addition of a second monomer to polystyryl anion results in the formation of a block polymer with no detectable free PS. This technique is of considerable importance in the commercial preparation of styrene—butadiene block copolymers, which are used either alone or blended with PS as thermoplastics. 

Considerable advances have taken place in the 1990s with regard to cationic polymerisation of styrene. Its uses to make block copolymers and even living cationic polymerisation have been reported (171). 

Most of the LFRP research ia the 1990s is focused on the use of nitroxides as the stable free radical. The main problems associated with nitroxide-mediated styrene polymerizations are slow polymerization rate and the iaability to make high molecular weight narrow-polydispersity PS. This iaability is likely to be the result of side reactions of the living end lea ding to termination rather than propagation (183). The polymerization rate can be accelerated by the addition of acids to the process (184). The mechanism of the accelerative effect of the acid is not certain. 

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. 

When the styrene has been consumed, to give living polymers of narrow molecular mass distribution, more styrene and more catalyst is added. The styrene adds to the existing chains and also forms new polymer molecules initiated by the additional sec-butyl-lithium. 

When the replenishing styrene had also been consumed butadiene is added to give a living diblock and when the monomer has been consumed the diblocks will have two modal molecular weights. 

The earliest SIS block copolymers used in PSAs were nominally 15 wt% styrene, with an overall molecular weight on the order of 200,000 Da. The preparation by living anionic polymerization starts with the formation of polystyryl lithium, followed by isoprene addition to form the diblock anion, which is then coupled with a difunctional agent, such as 1,2-dibromoethane to form the triblock (Fig. 5a, path i). Some diblock material is inherently present in the final polymer due to inefficient coupling. The diblock is compatible with the triblock and acts... 

The addition of living poly(styrene) to AIBN leads finally, especially for high coupling efficiencies, to the elimination of one nitrile group [72]. More recently, Ren et al. [73] have used bis(2-chloroethyl)2,2 -azodiisobu-tyrate (see scheme 19) to terminate anionically initiated poly(butadiene) chains. Since the azo transfer agent possesses two functional groups (Cl) that are able to termi-... 

Similarly, two living polystyrene chains were terminated with the azobisacid chloride ACPC yielding poly-(styrene) with one central azo group [74]. The yield of dimerization was found to be higher when the living chain was treated with 1,1-diphenyl ethylene before reacting it with ACPC. 

Weathering Many plastics has short lives when exposed to outdoor conditions. The better materials include acrylic, chlorotri-fluorethylene, vinylidene fluoride, chlorinated polyether, polyester, alkyd, and black linear poly-ethylene. Black materials are best for outdoor service. Some of the styrene copolymers are suitable for certain outdoor uses (Chapter 2, WEATHERING/ ENVIRONMENT). 

---