Unsaturated end group

End Groups and Branching. Both saturated and unsaturated end groups can be formed during polymerization by chain transfer to monomer or polymer and by disproportionation. Some of the possible chain end groups are... 

Polyisobutylene has the chemical properties of a saturated hydrocarbon. The unsaturated end groups undergo reactions typical of a hindered olefin and are used, particularly in the case of low mol wt materials, as a route to modification eg, the introduction of amine groups to produce dispersants for lubricating oils. The in-chain unsaturation in butyl mbber is attacked by atmospheric ozone, and unless protected can lead to cracking of strained vulcanizates. Oxidative degradation, which leads to chain cleavage, is slow, and the polymers are protected by antioxidants (75). 

Polybutenes enjoy extensive use as adhesives, caulks, sealants, and glaring compounds. They are used as plasticizers in mbber formulations with butyl mbber, SBR, and natural mbber. In linear low density polyethylene (LLDPE) blends they induce cling to stretch-wrap films. Polybutenes when modified at their unsaturated end groups with polar fiinctionahty are widely employed in lubricants as dispersants. Blends of polybutene with polyolefins produce semisoHd gels that can be used as potting and electrical cable filling materials. 

The resins tend to be dark in colour and it has been suggested that this is due to a fulvenation process involving the unsaturated end group of a polymer molecule. Hydrogenation of the polymer molecule, thus eliminating unsaturation, helps to reduce discolouration. 

Peroxide initiators may also undergo primary radical transfer to produce unsaturated end groups, which may result in a less stable polymer. In the case of benzoyl peroxide, an additional possibility is initiation by phenyl radicals to give a polymer with terminal phenyl groups [Eq. (17)] ... 

A method for reaction conditions. Monomers that have been used include MAH and maleimide derivatives such as NPMI (Scheme 9.51).582 In these cases, elimination of hydroxylamine under the reaction conditions provides an unsaturated end group. 

A side reaction in NMP is loss of nilroxide functionality by thermal elimination. This may occur by disproportionation of the propagating radical with nitroxide or direct elimination of hydroxy lam ine as discussed in Section 9.3.6.3. In the case of methacrylate polymerization this leaves an unsaturated end group.1" The chemistry has also been used to prepare macromonomers from PMMA prepared by ATRP (Section 9.7.2.1),... 

Termination in such a system occurs in what is essentially a reversal of the initiation reaction, i.e, protonation of the (BF30H) species to yield a polymer containing an unsaturated end group (2.1). 

Transfer reaction to the monomer, leading to the insertion of an unsaturated end group, is an important reaction in cationic chain polymerisation. As the activation energies of both termination and transfer reactions are higher than that of the propagation step, cationic chain polymerisation can only lead to high molecular masses when undertaken at low temperatures, typically — 100°C. 

The cationic polymerization of arylenes differs in some respects from that of alkenes. The most notable features are that the degree of polymerization is generally lower and that the proportion of unsaturated end groups is always small [21, 22] and often very variable [10]. In the system styrene-TiCl4-CCl3COOH-toluene low polymers are formed which have tolyl end groups [11]. It is not possible to decide at present whether the transfer reaction involved in this is (VIII) or (IX) ... 

The kinetic chain length for termination by disproportionation is DP = v (compared with the relationship for coupling of DP = 2v. The extent of the two types of termination is experimentally found by determining the number of head-to-head sites [coupling] and unsaturated end groups [disproportionation]). 

There are two different types of P-protons, and two different unsaturated end groups are possible for isobutylene as well as some other monomers such as indene and a-methylstyr-ene. The relative amounts of the two end groups depend on the counterion, identity of the propagating center, and other reaction conditions. Only one type of unsaturated end group (internal) is possible for other monomers such as styrene, ethyl vinyl ether, and coumarone. 

The two types of chain transfer to monomer are kinetically indistinguishable, but one (Eq. 5-17) results in unsaturated end groups, while the other (Eq. 5-18) results in saturated end groups. 

Termination is even more complicated than described, especially when polymerizations by metallocene initiators are considered [Beach and Kissin, 1986 Kissin et al., 1999b Lehmus et al., 2000 Liu et al., 2001b Resconi et al., 2000 Rossi et al., 1995, 1996 Thorshaug et al., 1998 Weng et al., 2000 Zhao et al., 2000]. A variety of different unsaturated end groups have been found in different polymerizations. Isomerization of propagating chain ends followed by P-hydride transfer (Eq. 8-41) results in trisubstituted double bond end... 

Another mechanism, conceivable with most monomers, and believed to occur in vinyl acetate polymerization (see Section 10), is transfer with the monomer forming a polymer with an unsaturated end-group capable of copolymerizing, e.g. ... 

E. Walch and R. Gaymans, Telechelic polyisobutylene with unsaturated end groups and with anhydride end groups, Polymer, 35(8) 1774-1778, April 1994. 

Infrared Spectroscopy. The following bands are seen in the ir spectrum of PPG 2970, 2940, 2880 cm-1 (C—H stretch, m) 1460,1375 cm-1 (C—H bend, m) 1100,1015 cm-1 (C—O stretch, m) of which the 2940 and 1015 band are specific. The latter are also present in copolymers of EO and PO. Absorptions due to unsaturated end groups are found at 1650 cm-1 (allyl ether) and 1672 cm-1 (1-propenyl ether). The O—H stretching band at 3470 cm-1 shows the greatest variation for different hydroxyl number polyols and has been used to estimate the hydroxyl number (169). 

The similarity between polymers and homologous low molecular weight compounds may be sometimes disturbed on account of the presence of some highly reactive sites, e.g. unsaturated end groups due to chain termination by disproportionation (a) or to chain transfer with monomer (6) ... 

---