End groups hydroxyl

Polyurethanes. About 3% of the U.S. polyurethanes market in 1988 was derived from the condensation product of polyisocyanates with low molecular weight polyadipates having hydroxyl end groups (195). In 1986 this amounted to 29,000 t, or 4% of total adipic acid consumption. The percentage was similar in Western Europe. About 90% of these adipic acid containing polyurethanes are used in flexible or semirigid foams and elastomers, with the remainder used in adhesives, coatings, and spandex fibers. 

Preparation of the polymer can be carried out in glass equipment at atmospheric pressure at temperatures typically above 100°C, but the higher pressures in an autoclave result in much faster reaction rates. Each polymer molecule which used butanol as a starter contains one hydroxyl end group as it comes from the reactor diol-started polymers contain two terminal hydroxyls. Whereas a variety of reactions can be carried out at this remaining hydroxyl to form esters, ethers, or urethanes, this is normally not done and therefore lubricant fluids contain at least one terminal hydroxyl group (36). 

Propylene oxide and other epoxides undergo homopolymerization to form polyethers. In industry the polymerization is started with multihinctional compounds to give a polyether stmcture having hydroxyl end groups. The hydroxyl end groups are utilized in a polyurethane forming reaction. This article is mainly concerned with propylene oxide (PO) and its various homopolymers that are used in the urethane industry. 

Chain Transfer. A number of materials act as tme transfer agents in THF polymerization notable examples are dialkyl ethers and orthoformates. In low concentrations, water behaves as a transfer agent, and hydroxyl end groups are produced. The oxygen of dialkyl ethers are rather poor nucleophiles compared to THF and are therefore not very effective as transfer agents. On the other hand, orthoformates are effective transfer agents and can be used to produce alkoxy-ended PTHFs of any desired molecular weight (169). 

Early examples of such branched polysulphides, e.g. Thiokol FA, are believed to possess hydroxyl end groups and are coupled by means of zinc compounds such as the oxide, hydroxide, borate and stearate by a mechanism which is not understood. Later elastomers, e.g. Thiokol ST, have been modified by a restricted reductive cleavage (see below) and this generates thiol (mercaptan) end groups. These may be vulcanised by oxidative coupling as illustrated below with lead peroxide ... 

A urethane is typically prepared by nucleophilic addition reaction between an alcohol and an isocyanate (R—N = C=0), so a polyurethane is prepared by reaction between a cliol and a diisocyanate. The diol is usually a low-molecular-weight polymer (MW 1000 amu) with hydroxyl end-groups the diisocyanate is often toluene-2,4-diisocyanate. 

As it was shown in73, 74), methods that can be used to synthesize these copolymers of PAN are those of radical AN block copolymerization in the presence of an oxidation-reduction system in which the hydroxyl end groups of polyethylene oxide) (PEO)73) and polypropylene oxide) (PPO)74- oligomers serve as the reducing agents and tetravalent cerium salts as the oxidizing agents. 

Hydrogenolysis. Hydrogenation of polycyclopropanone was carried out using supported catalysts. Before hydrogenation was attempted the polymers were end-capped in a refluxing mixture of acetic anhydride and pyridine. This was done to convert thermally unstable hydroxyl end groups, which may have been present in the polymer, to more thermally stable acetyl end groups as shown in Equation 8. This was... 

Resonances e and f in Figure 14 arise from the hydroxyl end group in PET. Integration of resonance f versus resonance b (or the resonance from the... 

The presence of hydroxyl end groups in PPG can be assigned from 13C NMR data (Figure 24), with resonances (g) observed between 65-68 ppm in the spectrum shown in Figure 24. There are two different resonances here (g) due tacticity effects in the polymer (meso/racemic (m/r) isomerisation of the terminal dyad). Estimations of the amount of these ends in the polymer, along with the average molecular weight of the PPG, can be made from the 13C NMR data. 

Figure 24 BC NMR spectrum from di-hydroxyl end-capped PPG. Inset shows expansion of spectrum indicating assignments for hydroxyl end groups of polymer.

Confirmation of the modification of the hydroxyl end groups from the PPG can be accomplished by analysis using MALDI-TOF MS [59]. An example MALDI-TOF mass spectrum of a prepolymer based on PPG 2000 is shown in Figure 28. End group and average molecular weight data can be gleaned from the spectrum. The dominant distribution of peaks centred at approximately m/z 2,500... 

A combination of anionic and ATRP was employed for the synthesis of (PEO-b-PS) , n = 3, 4 star-block copolymers [148]. 2-Hydroxymethyl-l,3-propanediol was used as the initiator for the synthesis of the 3-arm PEO star. The hydroxyl functions were activated by diphenylmethyl potassium, DPMK in DMSO as the solvent. Only 20% of the stoichiometric quantity of DPMK was used to prevent a very fast polymerization of EO. Employing pentaerythritol as the multifunctional initiator a 4-arm PEO star was obtained. Well-defined products were provided in both cases. The hydroxyl end groups of the star polymers were activated with D PM K and reacted with an excess of 2-bromopropionylbro-mide at room temperature. Using these 2-bromopropionate-ended PEO stars in the presence of CuBr/bpy the ATRP of styrene was conducted in bulk at 100 °C, leading to the synthesis of the star-block copolymers with relatively narrow molecular weight distributions (Scheme 72). 

PS-b-PEO) , n = 3, 4 star-block copolymers were synthesized by ATRP and anionic polymerization techniques [149]. Three- or four-arm PS stars were prepared using tri- or tetrafunctional benzylbromide initiators in the presence of CuBr/bipy. The polymerization was conducted in bulk at 110 °C. The end bromine groups were reacted with ethanolamine in order to generate the PS stars with hydroxyl end groups. These functions were then activated by DPMK to promote the polymerization of ethylene oxide and afford the desired well-defined products (Scheme 73). 

Bipyridine-centered triblock copolymers of the type BA-bpy-AB were prepared by a combination of ATRP and ROMP [159]. 4,4 -Bis(hydroxymelhyl)-2,2/-bipyridine was employed for the polymerization of lactic acid, LA or CL in the presence of Sn(Oct)2 in bulk at 130 and 110°C, respectively. The hydroxyl end groups were converted to tertiary or secondary bromo esters by reaction with 2-bromoisobutyryl bromide or 2-bromopropionyl bromide. The reaction yields were very high (> 80%) but not quantitative. These products were used as macroinitiators for the ATRP of MMA or tBuA in the presence of CuBr/HMTETA. 4,4/-bis(Chloromethyl)-2,2 -bipyridine was employed to promote the ATRP of MMA or styrene followed by the addition... 

The first type was produced from ethylene dichloride and sodium polysulphide. A high molecular weight polymer is obtained with predominantly hydroxyl end groups on the polymer chain. This type of polysulphide rubber was the first commercial grade but has, to a large extent, been superseded. 

The first systematic study of the structure of polydioxolan by Okada, Yamashita, and Ishii [5] proved that the polymer formed by bulk polymerisation with various catalysts at 0 °C had a completely regular structure resulting from the breaking of the O-l-C-2 link -0-CH2-0-(CH2)2-0-CH2-0-(CH2)2-. In polymers made with aqueous perchloric acid and acetic anhydride they identified acetoxy and hydroxyl end-groups. 

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