Polymer backbone, degradable

With higher energy excitation, M-CO bond dissociation occurs (e.g., eq. 19). This type of reactivity does not necessarily lead to polymer backbone degradation. 

Chain scission produces hydrocarbons with terminal free radicals (structure II), which may be stabilized in several ways. If the free radical abstracts a hydrogen atom from a neighboring molecnle, it becomes a saturated end and creates another free radical in the neighboring molecule (structure III), which may stabilize in a number of ways. The most likely of these is beta scission, which accounts for most of the polymer backbone degradation by producing an unsaturated end and a new terminal free radical. 

A solution to the problem of ROMP polymer backbone degradability was reported recently [60]. The key was to identify 8-oxa-2-aza-bicyclo[3.2.1]oct-6-en-3-ones [61] as monomers with the important attributes (Figure 8.2b). First, these molecules are strained cycloolefins, rendering them candidates for polymerization. Second, their polymerization leads to the presence of an oxazinone hnkage incorporated along the polymer backbone (Figure 8.2c). This heterocyclic functional group is stable at neutral pH values but can be hydrolyzed in either... 

An attractive approach was developed by Vicent et al. containing the bioactive agent as an integral part of the polymer backbone. Polyacetals were synthesised incorporating a drug with bishydroxyl functionality into the polymer backbone. Degradation of the polymer backbone in the acidic environment of the lysosome or the extracellular fluid of some tumours woifld... 

Degradation of polyolefins such as polyethylene, polypropylene, polybutylene, and polybutadiene promoted by metals and other oxidants occurs via an oxidation and a photo-oxidative mechanism, the two being difficult to separate in environmental degradation. The general mechanism common to all these reactions is that shown in equation 9. The reactant radical may be produced by any suitable mechanism from the interaction of air or oxygen with polyolefins (42) to form peroxides, which are subsequentiy decomposed by ultraviolet radiation. These reaction intermediates abstract more hydrogen atoms from the polymer backbone, which is ultimately converted into a polymer with ketone functionahties and degraded by the Norrish mechanisms (eq. 

Acrylate polymers also have fully saturated polymer backbones free of any heteroatoms in the main chain. This makes the polymers highly resistant to oxidation, photo-degradation and chemical attack. The acrylate groups are esters, which could be hydrolyzed under severe conditions. However, the hydrophobic nature of most acrylic polymers minimizes the risk for hydrolysis and, even if this reaction happened to some extent, the polymer backbone would still be intact. Other desirable acrylate properties include the following ... 

Other polymers used in the PSA industry include synthetic polyisoprenes and polybutadienes, styrene-butadiene rubbers, butadiene-acrylonitrile rubbers, polychloroprenes, and some polyisobutylenes. With the exception of pure polyisobutylenes, these polymer backbones retain some unsaturation, which makes them susceptible to oxidation and UV degradation. The rubbers require compounding with tackifiers and, if desired, plasticizers or oils to make them tacky. To improve performance and to make them more processible, diene-based polymers are typically compounded with additional stabilizers, chemical crosslinkers, and solvents for coating. Emulsion polymerized styrene butadiene rubbers (SBRs) are a common basis for PSA formulation [121]. The tackified SBR PSAs show improved cohesive strength as the Mooney viscosity and percent bound styrene in the rubber increases. The peel performance typically is best with 24—40% bound styrene in the rubber. To increase adhesion to polar surfaces, carboxylated SBRs have been used for PSA formulation. Blends of SBR and natural rubber are commonly used to improve long-term stability of the adhesives. 

On heating PVC, as the glass transition temperature is approached, the tensions within the concentrations of like-poles may be released by atoms being pushed apart to an extent that some break from the polymer backbone, resulting in the initiation of dehydrochlorination of the polymer. Unstabilized PVC is known to start degrading at approximately its glass transition temperature [135]. 

Although, the heat resistance of NBR is directly related to the increase in acrylonitrile content (ACN) of the elastomer, the presence of double bond in the polymer backbone makes it susceptible to heat, ozone, and light. Therefore, several strategies have been adopted to modify the nitrile rubber by physical and chemical methods in order to improve its properties and degradation behavior. The physical modification involves the mechanical blending of NBR with other polymers or chemical ingredients to achieve the desired set of properties. The chemical modifications, on the other hand, include chemical reactions, which impart structural changes in the polymer chain. 

The effect of oxidative irradiation on mechanical properties on the foams of E-plastomers has been investigated. In this study, stress relaxation and dynamic rheological experiments are used to probe the effects of oxidative irradiation on the stmcture and final properties of these polymeric foams. Experiments conducted on irradiated E-plastomer (octene comonomer) foams of two different densities reveal significantly different behavior. Gamma irradiation of the lighter foam causes stmctural degradation due to chain scission reactions. This is manifested in faster stress-relaxation rates and lower values of elastic modulus and gel fraction in the irradiated samples. The incorporation of O2 into the polymer backbone, verified by IR analysis, conftrms the hypothesis of... 

Considering the high hydrophobicity of the palmitoyl side chain and the rigidity of the polymer backbone, we assumed that poly(N-palmitoylhydroxyproline ester) would degrade somewhat more slowly than poly (lactic acid) or polycaprolactone. In order to confirm this hypothesis, a series of long-term stability and degradation studies have been performed over the last 2 years at MIT (22). 

The thermal properties of tyrosine-derived poly(iminocarbonates) were also investigated. Based on analysis by DSC and thermogravi-metric analysis, all poly(iminocarbonates) decompose between 140 and 220 C. The thermal decomposition is due to the inherent instability of the iminocarbonate bond above 150°C and is not related to the presence of tyrosine derivatives in the polymer backbone. The molecular structure of the monomer has no significant influence on the degradation temperature as indicated by the fact that poly(BPA.-iminocarbonate) also decomposed at about 170 C, while the structurally analogous poly(BPA-carbonate) is thermally stable up to 350 C. 

The fluorine content of II gives it excellent resistance to fuels, oils, most hydraulic fluids and chemicals. Since there are no C-C and C-H bonds along the polymer backbone, II displays excellent resistance to degradation by atmospheric oxygen and ozone. Tn addition, the Inherently flexible nature of the P-N backbone allows this elastomer to be used at temperatures down to -65°C, and gives the polymer excellent flex fatigue resistance over a broad temperature range (-65 to 175°C). 

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