Polyisoprene polymerisation

Free-Radical Polymerization. The best method for polymerising isoprene by a free-radical process is emulsion polymerisation. Using potassium persulfate [7727-21-1] as initiator at 50°C, a 75% conversion to polyisoprene in 15 h was obtained (76). A typical emulsion polymerisation recipe is given as follows (77). 

One other variant in double-bond polymerisations may be mentioned here. Many conjugated dienes may be polymerised in such a way as to generate long chain molecules with residual double bonds in the chain. Well-known examples of such dienes are buta-1,3-diene and isoprene, which yield 1,4-polybutadiene and 1,4-polyisoprene respectively (Figure 2.4). Natural rubber has a formula corresponding to the 1,4-polyisoprene. 

The close structural similarities between polychloroprene and the natural rubber molecule will be noted. However, whilst the methyl group activates the double bond in the polyisoprene molecule the chlorine atom has the opposite effect in polychloroprene. Thus the polymer is less liable to oxygen and ozone attack. At the same time the a-methylene groups are also deactivated so that accelerated sulphur vulcanisation is not a feasible proposition and alternative curing systems, often involving the pendant vinyl groups arising from 1,2-polymerisation modes, are necessary. 

Polymerised isobutylene, a non-vulcanisable, rubber-like polymer. See Butyl Rubber. Polyisoprene... 

Polymerised isoprene. Naturally-occurring polyisoprenes are natural rubber (cA-form) and gutta percha (trans- form). The use of stereo specific catalysts has made possible the manufacture of synthetic cA-polyisoprene and fraws-polyisoprene both of which are now available commercially. 

Materials that are constructed from organic polymers such as polyethylene, polystyrene, polyisoprene (natural rubber and a synthetic elastomer) and poly(vinyl chloride) are common features of our daily lives. Most of these and related organic polymers are generated from acyclic precursors by free radical, anionic, cationic or organometallic polymerisation processes or by condensation reactions. Cyclic precursors are rarely used for the production of organic polymers. 

Amino-terminated telechelic polybutadiene was prepared by LiAlH4 reduction of amidino end-group in polybutadiene, which was polymerised by a water-soluble initiator, 2,2 -azobis(amidinopropane)dihydrochloride. The structure was analysed by 1H- and 13C-NMR, but functionality of 2.0 was obtained by a titration method [70]. Synthesis of co-epoxy-functionalised polyisoprene was carried out by the reaction of 2-bromoethyloxirane with living polymer that was initiated with sec-butyl lithium. The functionality of the resulting polyisoprene was 1.04 by 1H-NMR and 1.00 by thin layer chromatography detected with flame ionisation detection [71]. 

In 1954, 1,4-cA-polyisoprene, the synthetic equivalent of natural rubber, was obtained in the laboratories of Goodrich-Gulf [22] by isoprene polymerisation with new catalysts developed by Natta, and later on 1,4-trans-polyisoprene, a synthetic analogue of gutta percha, was obtained by Natta et al. [23]. 

The Ziegler-Natta catalysts have acquired practical importance particularly as heterogeneous systems, mostly owing to the commercial production of linear high- and low-density polyethylenes and isotactic polypropylene. Elastomers based on ethylene-propylene copolymers (with the use of vanadium-based catalysts) as well as 1,4-cz s-and 1,4-tran.y-poly(l, 3-butadiene) and polyisoprene are also produced. These catalysts are extremely versatile and can be used in many other polymerisations of various hydrocarbon monomers, leading very often to polymers of different stereoregularity. In 1963, both Ziegler and Natta were awarded the Nobel Prize in chemistry. 

Conjugated dienes have been polymerised using supported half-sandwich metallocene catalysts. For instance, catalysts derived by supporting CpTiCl3 on alumina-silica gels, containing—0-Ti(Cp)Cl2 species, displayed activity in isoprene polymerisation without the addition of any other activator. Depending on the alumina-silica gel composition, the kind of polymerisation medium and the temperature, these catalysts exhibited various activities and selectivities polyisoprenes with a predominant 3,4 structure and mixed 1,2/ trans-1,4 structure were obtained [118,119],... 

Hierzu gehoren auch die Grignard-Verbindungen (RMgX). Sie spielen jedoch nur in Kombination mit Obergangsmetallverbindungen (vgl. Ab-schnitt 4.1) eine Rolle. liber die Polymerisation des Isoprens mit Barium und Strontium berichteten Kistler, Friedmann und Kaempf fl7>. Aus Isopren erhielten sie bei 40 °C Polyisoprene der folgenden Struktur ... 

Diese mit Ather komplexierten Katalysatoren liefern auch Polymere mit besseren physikalischen und anwendungstechnischen Eigenschaften. Als LGsungsmittel fur die Polymerisation eignen sich Aliphate und Aromaten. Bemerkenswert ist die Tatsache, daB man mit dem Katalysatorsystem Dialkylaluminiumchlorid/Kobaltverbindung, das Polybutadiene mit den hochsten 1.4-cis-Anteilen liefert, nur Polyisoprene mit max. 44% 1.4-cis-Anteilen erhalt, der Rest besteht iiberwiegend aus 3.4-Anteilen 83>. 

The theory proposed for equilibrium swelling and diffusion is based on the assun tlon that the hydrophilic impurities are present in particulate form and are dispersed throughout the rubber. The precise nature of this impurity in natural rubber is not known so it was decided to make a model rubber by adding 0.17. of a hydrophilic Impurity (sodium chloride) to a solution polymerised synthetic rubber (cis-polyisoprene) Ich is chemically the same as natural rubber. Using this model rubber it is possible to check the theory more precisely since both the nature and concentration of the hydrophilic lgq>urlty in the model rubber are known. It is proposed that the water diffuses through the rubber and forms droplets of solution inside the rubber where there are particles of the hydrophilic impurity thereby causing a non-uniform distribution of water in the rubber. The... 

Polyisoprenes (IR) polymerised with a Ziegler catalyst have cis contents of about 98%. The last few per cents of non-cis polymer, which makes IR different from natural rubber (NR), are very important for the physical properties of the vulcanizates. The uncertainty in the NMR analysis, amounting up to l %wt. was the reason to investigate the possibilities of dilatometric crystallisation rate measurements. 

The presence of double bonds in the mainchain of a polymer increases the number of appearance forms of such a polymer. Well known examples of such systems are polybutadiene rubber (BR) and polyisoprene rubber (IR). BR for instance, can be polymerised into the following configurations ... 

The best-known elastomer is natural rubber, poly-isoprene (Scheme 6.10). Isoprene (Scheme 6.10a) is a liquid at room temperature, which polymerises readily to give the elastomer polyisoprene (Scheme 6.10b). The polymerisation produces two main geometrical isomers (see Section S2.1.) Natural rubber is the aU-c form of polyisoprene (Scheme 6.10c), in which the methyl (—CH3) groups and hydrogen (H) atoms are on the same side of the carbon-carbon double bond. Rubber latex, a milky liquid, is a suspension of rubber in water. It is found in many plants (e.g. in dandelions) as well as mbber... 

---