Mercaptan-terminated polymer

Polysulfide rubber manufacturing processes are described in more detail by Bertozzi [26]. Briefly, the processes involve preparation of sodium polysulfide (S + NaOH), and reaction with bis(2-chloroethyl)formal along with 2% of a branching agent such as 1,2,3-trichloropropane to give a mercaptan terminated polymer of MW approximately 80,000 similar to that described in Preparation 2-2, Method A. 

Use of a mixed dihalide monomer feed in a conventional polymerization will produce random copolymers. A block copolymer cannot be prepared by a stepwise addition of the monomers. During copolymerization interchange takes place, resulting in randomization. To prepare block copolymers, it is best to prepare individual mercaptan-terminated polymers, blend them in the desired proportion, and then process the blended system by conventional techniques. 

Between 1960 and 1976 several new mercaptan terminated polymers having varied polymer backbones were introduced. These are covered in the section entitled Other Mercaptan Terminated Polymers. The chemical resistance of these various polymers can vary depending upon their backbone structure and, on this basis, should be evaluated on their own relative merits. 

Erickson reacted diacrylate-terminated esters with H2S to obtain mercaptan-terminated polymers. These polymers can be cured using di-cumyl peroxide and MgO to give rubbery compounds. Cameron and Duke disclosed mercaptan-terminated polyesters made by reacting glycols with mercapto-organic acids. 

Petersen, E.A., 1986, "Formulating Sealants with Mercaptan Terminated Polymers", Caulks and Sealants Short Course, Adhesive and Sealant Council, Washington, DC, Dec., pp 87-132. 

Two of the more recendy developed polysulftde polymers are the mercaptan-terminated polyoxypropylene urethane polymer and the polythioether polymer. The urethane-backbone-based polymer is used in many sealant formulations for insulating glass appHcations. The thioether backbone contains sulfur, but no S—S bonds, which are the weakest part of the conventional polysulftde polymer. This polymer improves the thermal stabiHty and reduces the gas—Hquid permeabiHty. 

Polyoxypropylene urethane polymer, mercaptan-terminated, 22 40-41 Polyparabanic acid (PPA) tape material, 17 836... 

Poly sulfide compounds that are compatible with epoxy resins are liquid elastomers at room temperature. The most significant commercial resin of this type is LP-3 from Toray. The predominant product is a mercaptan-terminated liquid polymer (LP) that contains approximately 37% bound sulfur (see Fig. 7.2). It is the high concentration of sulfur linkages that provides these products with their unique chemical properties. A sulfur odor is noticeable during processing, making ventilation important. 

The mercaptan terminated liquid polymers are converted to high molecular weight disulfide elastomers using a variety of oxidizing agents. Cure rate can be controlled by the use of additives to the base compound or accelerator. 

In the mid-1970s, polysulfide polymer chemistry was advanced when Products Research and Chemical Corporation introduced a polyoxypropylene urethane backbone with mercaptan terminal groups [7]. Molecular weight regulation and minimization of side reactions are important features. The backbone is significantly different from that of conventional polysulfide polymers, yet the curing chemistry is the same and the eured product is a polysulfide rubber. A typical structure of this type of resin is shown below. 

Mercaptan-terminated liquid polysulfide polymers are polymerized to rubbery solids by oxidizing agents (e.g., lead dioxide, aetivated manganese dioxide, ealdum peroxide, cumene hydroperoxide, alkaline diehromates, and />-quinonedioxime). The euring proeess involves the oxidation of the terminal mereaptan groups in the polysulfide polymers to form the corresponding disulfide. 

Chem. Descrip. Polysulfide polymer, mercaptan-terminated CAS 9080-49-3... 

Villa [32—34] has modified Uquid polysulfide polymers by reacting them with either vinyl cyclohexane diepoxide or abietic acid. The modified products were used as an effective additive. While the reaction between liquid polysulfide polymer and abietic acid is not fully known. Villa has theorized that the reaction proceeds with inversion of a mercaptan terminal to hydroxyl terminal followed by esterification [32]. The following reaction takes place ... 

Hooker Chemical introduced pilot plant batches of still another commercial venture. These polymers had a polyethylene backbone and mercaptan terminals, but the polymer had a very high sulfur content of 55%. The venture was dropped when again the polymers had poorer properties than the Thiokol polysulfide poly-... 

Nummy prepared intermediates by reacting the divinyl ether of diethylene glycol with H2S to yield the bis(2-mercaptoethyl) ether of diethylene glycol, then oxidized this monomer to give a range of mercaptan-terminated liquid poly ether polymers. 

The mercaptan-terminated LP is readily polymerized to an elastomeric solid by the use of active oxidizing agents. The most widely used curing agent is lead dioxide which cures the LP as shown below. LP polymers are available with average molecular weights ranging from 1,000 to 80,000. 

The above explanation of autoacceleration phenomena is supported by the manifold increase in the initial polymerization rate for methyl methacrylate which may be brought about by the addition of poly-(methyl methacrylate) or other polymers to the monomer.It finds further support in the suppression, or virtual elimination, of autoacceleration which has been observed when the molecular weight of the polymer is reduced by incorporating a chain transfer agent (see Sec. 2f), such as butyl mercaptan, with the monomer.Not only are the much shorter radical chains intrinsically more mobile, but the lower molecular weight of the polymer formed results in a viscosity at a given conversion which is lower by as much as several orders of magnitude. Both factors facilitate diffusion of the active centers and, hence, tend to eliminate the autoacceleration. Final and conclusive proof of the correctness of this explanation comes from measurements of the absolute values of individual rate constants (see p. 160), which show that the termination constant does indeed decrease a hundredfold or more in the autoacceleration phase of the polymerization, whereas kp remains constant within experimental error. 

The free radical polymerization of HPMA in the presence of mercaptans involves two different initiation mechanisms (Scheme 2) [26]. One is the initiation by RS radicals from chain transfer agent the other appears to be the direct initiation by the primary isobutyronitrile (IBN) radicals formed by the decomposition of AIBN [27]. The RS are formed by either the free radical transfer reaction of alkyl mercaptans with the IBN radicals or the chain transfer reaction of an active polymer chain with the mercaptans. The initiation by the RS radicals produces the ST polymers with a functional group at one end of the polymer chain. The initiation by IBN radicals leads to nonfunctional polymer chains with an IBN end group. The presence of the polymers with IBN end groups effects the purity and the functionality of ST polymers. As expected, the production of nonfunctionalized polymer chains is affected by reaction conditions. The polymerization is mainly terminated by chain transfer reaction with the mercaptans, but other termination mechanisms, such as disproportionation and recombination, take place depending on the reaction conditions [26]. 

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