Macromolecular Synthesis CHEM 7010 Macromolecular Synthesis 2nd Introduction to Polymer Synthesis: General Aspects of Polymer Structures on Polymer Properties 2011

Multidimensional Order of Structure-Property Relationships in Polymers Microscopic *Crystallinity *Phases *Orientation Molecular Microscopic Synthesis and Processing Molecular *Chemical Composition *MW/MWD *Branching and/or Cross-linking Macroscopic *Strength *Modulus *Impact Resistance Permeability Clarity Macroscopic

IMPACT OF MOLECULAR WEIGHT ON MATERIAL PROPERTIES Increasing Degree of Polymerization, DP

Vinyl Monomers, CH2=CH-X X Polymer Abbreviation H Polyethylene PE CH3 Polypropylene PP Cl Poly(vinyl chloride) PVC Phenyl Polystyrene PSt CN Polyacrylonitrile PAN COOCH3 Poly(methyl acrylate) PMA O-COCH3 Poly(vinyl acetate) PVAc

Vinylidene Monomers, CH2=C(X)Y X Y Polymer Abbreviation CH3 CH3 Polyisobutylene PIB Cl Cl Poly(vinylidene chloride) PVDC F F Poly(vinylidene fluoride) PVDF Phenyl CH3 Poly(-methyl styrene) CH3 COOCH3 Poly(methyl methacrylate) PMMA CN COOR Poly(alkyl -cyanoacrylate

Structural Complexity of Polymers Homopolymers Head to Tail vs. Head to Head Adducts e.g. a-olefins 1,2- vs 1,4 Adducts; e.g. butadiene Tacticity of Enchainments Branching

Tacticity Isotactic All asymmetric carbons have same configuration Methylene hydrogens are meso Polymer forms helix to minimize substituent interaction Syndiotactic Asymmetric carbons have alternate configuration Methylene hydrogens are racemic Polymer stays in planar zig-zag conformation Heterotactic (Atactic) Asymmetric carbons have statistical variation of configuration

Structural Complexity of Polymers Copolymers Identity and Number of Comonomers Ratio and Distribution of Comonomers Statistical Alternating Gradient Blocks Grafts

Structural Complexity of Polymers Molecular Weight Molecular Weight Distribution, MWD Polydispersity Index, PDI Mn, Mw, Mz, Mv Averages Crosslinking Density Length of Crosslinks

Structural Complexity of Polymers Time Dependent Changes Chemical Reactions Hydrolysis Dehydrohalogenation Photodegradation Oxidation

Structural Complexity of Polymers Thermal Degradation Processing Aging Crystallization Changes in Polymorphism Weathering-- Combination of Above Plasticizer Loss -- Imbrittlement

Microscopic Properties (Intermolecular Interactions) Morphology Chain entanglement –amorphous Chain ordering--liquid crystalline Crystallinity Phase separations (microdomains)

Types of Intermolecular Forces Type of Force Relative Strength Low Molecular Analog Polymer Dispersion or Weak Methane Polyethylene Van der Waals Hexane Polypropylene Dipole-Dipole Medium CH3Cl PVC CH3CO2CH3 PMMA Hydrogen bonding Strong H2O Cellulose CH3CONH2 Proteins Electrostatic Very Strong CH3CO2-Na+ Ionomers

GLASS TRANSITION, Tg Definition: The onset of seqmental motion of seqments with 40-50 carbons atoms Physical Change Expansion of volume Free volume required to allow segmental motion Tg is an approximation Depends upon measurement technique Depends upon molecular weight Polystyrene MW = 4000 Tg = 40C = 300,000 = 100

GLASS TRANSITION, Tg Properties Affected Specific Volume / Density Specific Heat, Cp Refractive Index Modulus Dielectric Constant Permeability

FACTORS INFLUENCING Tg Tg is proportional to Rotational Freedom For symmetrical polymers Tg, / Tm in K  1/2 unsymmetical polymers  2/3 1. Chain flexibility Silicone  Ether  Hydrocarbon  Cyclic HC  Aromatics

FACTORS INFLUENCING Tg 2. Steric Bulk of Substituents Tg = -120C 5C -24C -50C Long side chains may act as plasticizers (C  6) Tg = -55C 88C

FACTORS INFLUENCING Tg 3. Molecular Symmetry Asymmetry increases chain stiffness. 4. Polar Interactions increase Tg Hydrogen bonding 5. Molecular Weight up to Critical Limit 6. Crosslinking Reduces Segment Mobility

FACTORS INFLUENCING Tm 1. Chain flexibility Silicone  Ether  Hydrocarbon  Cyclic HC  Aromatics 2. Substituents Producing Lateral Dipoles Hydrogen bonding 3. Molecular Symmetry Symmetry allows close packing

FACTORS INFLUENCING Tm 4. No Bulky Substituents to Disrupt Lattice if placement is Random 5. Structural Regularity monomer placement head to tail 1,2- vs 1,4- 1,2- vs 1,3- vs 1,4- aromatic substitution geometric isomers of enchainments cis or trans -C=C-; cyclic ring tacticity

FACTORS REQUIRED TO PROMOTE CRYSTALLIZATION Thermodynamic 1. Symmetrical chains which allow regular close packing in crystallite 2. Functional groups which encourage strong intermolecular attraction to stabilize ordered alignment.

FACTORS REQUIRED TO PROMOTE CRYSTALLIZATION Kinetic 1. Sufficient mobility to allow chain disentanglement and ultimate alignment Optimum range for mobility Tm -10  Tg + 30 at Tm segmental motion too high at Tg viscosity too high 2. Concentration of nuclei concentration of nucleating agents thermal history of sample

Macroscopic Properties (Physical Behavior) Tensile and/or Compressive Strength Elasticity Toughness Thermal Stability Flammability and Flame Resistance Degradability Solvent Resistance Permeability Ductility (Melt Flow)

Step Polymerization (Polycondensation) After many repetitions:                                                                                          Polyethyleneterephthalate (PET)

Step Polymerization Two Routes: A-A + B-B; or A-B 1 on 39 1 on 40 Either way, need: high conversion stoichiometric amount

Step vs. Chain Polymerization Step Polymerization Any two molecular species can react. Monomer disappears early. Polymer MW rises throughout. Growth of chains is usually slow (minutes to days). Long reaction times increase MW, but yield hardly changes. All molecular species are present throughout. Usually (but not always) polymer repeat unit has fewer atoms than had the monomer.

Step vs. Chain, cont. Chain Polymerization Growth occurs only by addition of monomer to active chain end. Monomer is present throughout, but its concentration decreases. High polymer forms immediately. MW and yield depend on mechanism details. Chain growth is usually very rapid (seconds to microseconds). Only monomer and polymer are present during reaction. Usually (but not always) polymer repeat unit has the same atoms as had the monomer.

Step Polymerization Concepts Comparison of Step and Chain Polymerization 1. Step: any 2 molecules in the system can react with each other Chain: chain growth occurs on end of growing polymer 2. Step: loss of monomer at early stage (dimers, tetramers, etc.) Chain: monomer concentration decreases steadily 3. Step: broad molecular weight distribution in late stages Chain: narrower distribution; just polymer and monomer

Step Polymerization Basis for Kinetics (2-1a): 3 on 40 4 on 40 5 on 40

Step Polymerization Basis for Kinetics: 1on 41 2 on 41

Step Polymerization Basis for Kinetics (2-1b): NOTE: Assume equal reactivity of functional groups table 2.1on 42 Thus, assumption looks ok

Step Polymerization Kinetics (2.2) Example: diacid + diol Mechanism (acid catalyzed): 2 on 44 1 on 45 2 on 45

Chain Polymerization Concepts General Mechanism Initiation Propagation

Chain Polymerization Concepts: Nature of Chain Polymerization; 3-1 3-1a. Comparison of Chain and Step Polymerizations Step: a. Monomer gone fast, get dimer, trimer, etc. b. MW increases with time c. No high mw until end Chain: a. Rapid high mw b. Snapshot: monomer, high poly, growing chains c. MW does not change with time

Chain Polymerization Concepts: Nature of Chain Polymerization; 3-1 3-1b-1. General Considerations of Polymerizability Thermodynamics (Chain t-fer)

Chain Polymerization Concepts: Nature of Chain Polymerization; 3-1 3-1b-2. Effects of Substituents on C=C monomers for Ionic Polymerization A. Initiation opportunities: 2 types of bond cleavage/resonance B. Radical, Anionic, Cationic: Depends on: i. Inductance ii. Resonance

Chain Polymerization Concepts: Nature of Chain Polymerization; 3-1 3-1b-2. Effects of Substituents on C=C monomers for Ionic Polymerization C. Donating Groups Good for cationic Inductance Resonance e.g. Polym of vinyl ether Example of resonance stabilization of cation by Delocalization of of positive charge. If oxygen not there, no stabilization

Chain Polymerization Concepts: Nature of Chain Polymerization; 3-1 3-1b-2. Effects of Substituents on C=C monomers for Ionic Polymerization C. Donating Groups ii. Resonance e.g. Polym of Styrene

Chain Polymerization Concepts: Nature of Chain Polymerization; 3-1 3-1b-2. Effects of Substituents on C=C monomers for Ionic Polymerization D. Withdrawing Groups i. inductance Good for anionic ii. resonance

Chain Polymerization Concepts: Nature of Chain Polymerization; 3-1 3-1b-2. Effects of Substituents on C=C monomers for radical polymerization A. Radicals not affected by charge, but can have resonance stabilization B. Also stabilized by primary<secondary<tertiary Draw your own pic

Chain Polymerization Concepts: Structural Arrangement of Monomer Units; 3-2 3-1a. Possible Modes of Propagation A. Two possible points of attachment i. On carbon 1: ii. On carbon 2:

Chain Polymerization Concepts: Structural Arrangement of Monomer Units; 3-2 3-1a. Possible Modes of Propagation B. If attachment is regular, get head-to-tail (H-T) C. If attachment is irregular, get head-to-head (H-H) [and T-T]

Chain Polymerization Concepts: Structural Arrangement of Monomer Units; 3-2 3-1a. Possible Modes of Propagation Usually only 1-2% H-H. How do we know? A. Chemical Methods B. NMR Methods (we’ll check this out in the Spectroscopy section)