Ring opening, polymer synthesis

As disciissed in Chapter 1, under a scheme proposed by Carothers, polymers are classified as addition or condensation polymers depending on the type of polymerization reaction involved in their synthesis. This classification scheme, however, does not permit a complete difierentiation between the two classes of polymers. A more complete but still oversimplified scheme that is still based on the dilTerent polymerization processes places polymers into three classes condensation, addition, and ring-opening polymers. This scheme reflects the stractures of the starting monomers. Probably the most general classification scheme is based on the polymerization mechanism involved in polymer synthesis. Under this scheme, polymerization processes are classified as step-reaction (condensation) or chain-reaction (addition) polymerization. In this chapter, we will discuss the different types of polymers based on the different polymerization mechanisms. 

Polymer made by ring-opening thermal synthesis polymerization (MW uncontrolled, chain-ends unknown)... 

Additionally, it is known that cyclic SiaClg can be polymerized in an ROP via radical species to yield an insoluble but moisture sensitive polymer with an estimated degree of polymerization of about 35 and a molecular weight of about 3,500gmor [45]. Due to the insolubility and the high sensitivity to moisture, there have been no further reports on a ring opening type synthesis of perchlorinated polysilanes [42,45]. Nevertheless, a functionalization by reaction with isopropanol has been accomplished which led to an increased solubility and enabled a SEC-analysis [45]. 

Synthesis. The synthesis of poly(dichlotophosphazene) [25034-79-17, (N=PCl2) (4), the patent polymer to over 300 macromolecules of types (1) and (2), is carried out via controlled, ring-opening polymerization of the corresponding cycHc trimer, (N=PCl2)3 [940-71 -6]. 

A drawback to the Durham method for the synthesis of polyacetylene is the necessity of elimination of a relatively large molecule during conversion. This can be overcome by the inclusion of strained rings into the precursor polymer stmcture. This technique was developed in the investigation of the ring-opening metathesis polymerization (ROMP) of benzvalene as shown in equation 3 (31). 

As early as 1895, the synthesis of polydichlorophosphazene was attempted by H.N. Stokes by thermal ring-opening polymerization of hexachloro-triphosphazene [(NPCl2)3]. The product obtained by H.N. Stokes was a high-molecular weight cross-linked rubbery material called inorganic rubber which is insoluble in all solvents and hydrolytically decomposes into phosphates, ammonia, and hydrochloric acid in the presence of moisture. Because of its insolubility and hydrolytic instability, the polymer found no technological application and remained as a laboratory curiosity. 

In 1964, H.R. Allcock filed a patent claiming the preparation of a soluble polydichlorophosphazene. The polymer obtained can be soluble in organic solvents such as benzene and toluene. This thermal ring-opening polymerization of hexachloro-triphosphazene in the melt at 250°C is the most fully developed and commonly used method for polydichlorophosphazene synthesis to date. 

Lu H, Wang J, Lin Y, Oieng J (2009) One-pot synthesis of brush-like polymers via integrated ring-opening metathesis polymerization and polymerization of amino acid N-carboxyanhy-drides. J Am Oiem Soc 131 13582-13583... 

In polyester synthesis via ring-opening polymerizations, metal catalysts are often used. For medical applications of polyesters, however, there has been concern about harmful effects of the metallic residues. Enzymatic synthesis of a metal-free polyester was demonstrated by the polymerization of l,4-dioxan-2-one using Candida antarctica lipase (lipase CA). Under appropriate reaction conditions, the high molecular weight polymer (molecular weight = 4.1 x 10" ) was obtained. 

Terminal-functionalized polymers such as macromonomers and telechelics are very important as prepolymer for construction of functional materials. Single-step functionalization of polymer terminal was achieved via lipase catalysis. Alcohols could initiate the ring-opening polymerizahon of lactones by lipase catalyst. The lipase CA-catalyzed polymerizahon of DDL in the presence of 2-hydroxyethyl methacrylate gave the methacryl-type polyester macromonomer, in which 2-hydroxyethyl methacrylate acted as initiator to introduce the methacryloyl group quanhtatively at the polymer terminal ( inihator method ).This methodology was expanded to the synthesis of oo-alkenyl- and alkynyl-type macromonomers by using 5-hexen-l-ol and 5-hexyn-l-ol as initiator, respechvely. 

Controlling the exact architectnre of polymers has always attracted attention in macromolecular chemistry. Snccessfnl synthesis of alternating copolymers nsing ring opening metathesis polymerisation is of great interest also from a mechanistic perspective. NHC ligands were fonnd to be ideal to tune the selectivity of the metathesis initiators. 

In addition to solvent uses, esters of lactic acid can be used to recover pure lactic acid via hydrolysis, which in-tum is used to make optically active dilactide and subsequently polylactic acid used for drag delivery system.5 This method of recovery for certain lactic acid applications is critical in synthesis of medicinal grade polymer because only optically active polymers with low Tg are useful for drug delivery systems. Lactic acid esters themselves can also be directly converted into polymers, (Figure 1), although the commercial route proceeds via ring-opening polymerization of dilactide. 

The first report of ROMP activity by a well-characterized Mo or W species was polymerization of norbornene initiated by W(CH-t-Bu)(NAr)(0-f-Bu)2 [122]. In the studies that followed, functionality tolerance, the synthesis of block copolymers, and ring-opening of other monomers were explored [30, 123]. Two important issues in ROMP concern the cis or trans nature of the double bond formed in the polymer and the polymer s tacticity. Tacticity is a consequence of the presence of two asymmetric carbons with opposite configuration in each monomer unit. The four ROMP polymers (using polynorbornene as an example) that have a regular structure are shown in Scheme 3. 

Since one of the substrates is a cyclic alkene there is now the possibility of ring-opening metathesis polymerisation (ROMP) occurring which would result in the formation of polymeric products 34 (n >1). Since polymer synthesis is outside the scope of this review, only alkene cross-metathesis reactions resulting in the formation of monomeric cross-coupled products (for example 30) will be discussed here. 

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