Principles of polymer chemistry

Principles of polymer chemistry - Theory of Polymerization

• Classification of Monomers and Polyreactions

• Monomers with more than two reactive functions -> branched or crosslinked polymer.
• Polymerization of polyreactions: the former consists of polyreactions and the latter involves operations (dislocation).
• A polyreaction must be:
  • Microchemical - the product of a step must be the molecules increased in molecular weight with respect to the previous step.
  • Macrochemical - main products of a former act becoming the reactant for the latter.

Polyaddn.: A + B -> A-B -> A-B-A step

Polybphen.: A + B -> A-B + b

1. Bond Mechanism: Petrolytic. Breaking of the bond, no sternic part causes to the usage of sternicapedium. The reactivity will depend strongly on the degree of charge separation (fundamental role of catalyst and solvent).
2. Radical Mechanism: Homolytic breaking of the bond with the formation of prods with high energy. Impaired actions. High reactivity (no role of catalyst nor solvent).

• Classification of monomers

1. Monomers with electronically independent reactive functions (bipopic functions).
   • Examples: dipeptids, diamines, diacids. They generate indigenous reaction acts with the formation of stable intermediate. Kinmetic mechanism: step growth type.
   • Every reaction step have no energy increase, slow process. Each specie formed able to react with any other.

The base chemico mechanism is nonpolaronic. The base reaction can be condensation. Example: polycondensation -> polyesters, polyamides, polycarbonates.

Step-Growth Polymerizations

Bifunctional polycondensations give linear polymers. Two subcases: heteroclic functionial monomers A_mB + A_nB -> A_m+nB + b tala (not frequent) and homofunctional ones AA + BB that can give a(ABABA)_n, b(BAAB)_m or c(ABA)_t.

Stoichiometric ratio: ratio of the initial number of reactive terms of type A and B

r = N_0/B / N_0/A,    N₀ = N_0/B

Extent of reaction: fraction of reacted terminales at the generic time t

_1-p'

ti

Molecular weight distribution calculated with the same hypothesis the probability

of reactivity groups, with a degree of polymerization is equal to its mean

function of the terminal's (1-p), where π="....." probability of reaction and (1-p)

Probability of unreacting previous reactions

π

They are the most probable distribution or (...) rely distribution

X_i=1 ^→(N₀/n)

X_w=Σ_iNi<

Polydispersity = (Σ xi w_i) N M_n/ M_w = μ

Degree of polymerization graph 1

p=0.99 p=0.95 p=0.9

n

X >>

Cationic polymerization are based on the growth of carbocations (very reactive). Inert environment. Lewis (4°C > T > 100°C). Initiators: protic acids or Lewis acids. Polymerization monomers: quar substituted olefins, vinyl ethers (PO. CH-CH2) and epoxides. Initiation: Propagation: Termination: thermos dissociation (TD)

Anionic polymerization, carbanions or oxianions - chain-growth kinetics but in some cases even step-growth depending on monomeric solvent. Inert aprotic hydrocarbon environment or highly polar solvents (ethers, ammonia). Initiators nuclephs NaNH₂, alkylethylithium. Initiation: Propagation: Termination: NB, if the anionic polymerization is carried out in hydrocarbons (less reactivity) broad range and stereochemical control is obtained.

Some chain and step capabilities, but controlled experimental conditions (anhydrous, inert solvent, aprotic terminators) are needed. Living polymers. The polymer remains with an open chain terminations ready to resume the polymerization with further increase of m. More unimolecular adducts, even of a different type - block copolymers can be obtained. Polymerization living polymerizations are into control of MW and MW_D (1/100); Example SBS Heterogeneous structure Kelophasiac two anaphasous phass each charactenses by ditin Gg

Ring-opening polymerization - cyclic amines. Initiated by Lewis acids or water = anionic-pro thing occurs between chain and step-growth. More examples: polyresilosane in L series. Obtained by ionic or cationic polymerization

More enhanced by their rigid structure and chemical inhibition of conjulg. compounds. Structure can be obtained by chemical or specific organonietallic compounds based on transition metal salts. Extremely reactive monomers both in > with₂ and water and explaining the choice of coord. Catalyst, catalysts obtained by reaction of salts (NaCl) in illustration membranes with alkyle or aryle derivatives of P.

Mechanism of initiation and propagation with Zn.

Rubber Science and Technology

1) Introduction to Rubbers

A rubber, elastomer, is a natural or synthetic polymer which at room T can be stretched repeatedly to several times its original length and which, after removal of the stress, (load will immediately return to approximately its original length.

• High extensibility: any single chain assumes a random coil conformation when no external perturbation is applied. This conformation can be easily deformed.
• Requirement to behave as an elastomer:
  • 1) chain: and high MW
  • 2) amorphous
  • 3) flexichains: T_g well below room T (-20°C/-60°C) and low rigidity (low cohesive force).
  • 4) random coil conformation and high configurational disorder at undeformed
  • 5) elasticity must be decent chemically (vulcanisation or physically-phase segregation)

Elastomers with segregated phase may show a cruise crystalline order (thermoplastic one).

• Natural rubber (NR) is a polyisoprene, very high MW. It's an unsaturated rubber (can be C₂=C₂-CH₂-CH,,,Vulcanised)_sulfur Stereo union, very well controlled -> good mechanical properties at high elongation.
• General purpose synthetic rubbers. They contain only C and H. They can be both unsaturated or saturated/Two criteria:
  • 1) C=C them cannot be vulcanised by
  • 2) also saturated rubber: show generally better thermal

Specialty elastomers: Low Tg polymer, containing typically: S, O, Q, E, N. Better elastic for atoms (pen affinity) towards fuels; better thermal and chemical stability.

Entropic elasticity (except alone so clvds). The imposed deformation normally causes an increase in free energy of the material due to the increase of internal entropy (enthalpy related to the displacement of atoms from their equilibrium position (with internal The atoms are not mostly shifted thus the effect of the entropic part is negligible. The driving force to the elastic deformation is mainly due to the entropy part.

• Elasticity: the motion of rubbers is different since the undeformed (conformational transitions are of occurring, while the chains progressively pass from "couple o disordered, random state to a more oriented arrangement with entrop is gained. There' a significant decrease in conformational motion based on random elements with drones of fixed monomer a (good amount of free state). The driving force to the elastic recovery is entropic since the unstretched tends to return to its win entropy state by increasing its entropy level. The rubber has to store energydissipated upon elongation without dissipation in viscous forces (normandly this is achieved
• by the vulcanization process.

In absence of crosslinking the stretched unlinked cule return spontaneously this then behaves stick (random coil or by viscous thread follow

On the other hand, the elastomer after vulcanization maintained a secure entic state under load without dissipation, allowing for a fast and complete

• deformation recovery.

2) Classical Thermodynamics of Rubber Elasticity

• Pearson's reason: dE-dW = TdS + SdL - PdV because of the system an elastic body it will be subjected to a reversible deformation dL by the action of an external force dE = dQ - dV
• This work done on an isolated body gives a free energy content. The elastic compressive force of the body can be reached an increase in free energy at constant pressure (equal T) and divided into independent enthalpic and entropic contributions.