Exam 2

How does bulk polymerization influence polymer mechanical properties?

slower rate of polymerization decreases the molecular weight of polymers

What is an exothermic reaction?

reaction that generates heat

What type of polymerization is exothermic?

addition/chain polymerization

How do exothermic reactions affect reactor design?

Need an efficient means for heat dissipation in the reactor

What is autoacceleration?

heat generation causes an increase in the rate of reaction, and faster reactions in turn produce more heat

What is an endothermic reaction?

reaction requires an input of heat

What type of polymerization is endothermic?

step/condensation polymerization

How do endothermic reactions affect reactor design?

reactor must supply heat,

Removal or recycling of solvents and catalysts must be considered in reactor design,

reactor must provide efficient removal of volatile by-products

What is the basis for the classification of different polymerization processes?

physical states, reaction medium, mutual solubility

Polymerization classified by physical state

gas phase, suspension, and solution polymerization

Polymerization classified by reaction medium

bulk, solution, suspension, slurry, emulsion, and gas polymerization

Polymerization classified by mutual solubility

homogeneous and heterogeneous polymerization

What is homogeneous polymerization?

all reactants are mutually soluble and compatible with the resulting polymer

What is heterogeneous polymerization?

catalyst, monomer, and polymer product are mutually insoluble

What are examples of homogenerous polymerization?

bulk or solution systems

What are examples of heterogeneous polymerization?

bulk precipitation, bulk, solution, suspension, precipitation, emulsion, gas phase, and interfacial polymerization

What is homopolymerization?

polymer made from a single monomer

What is copolymerization?

polymer made from two or three different monomers

What does it mean for polymerization to be a continuous process?

monomers and catalyst are continuously fed into polymerization reactors, and polymers are continuously removed from the reactors

What is bulk polymerization?

polymerization of a pure liquid or gaseous monomer, mixed in a reactor and heated and cooled as desired, only the monomer and initiator are present, continuous

Bulk polymerization is suited for which type of reaction?

step growth (condensation) because less heat is released

What are the merits of bulk polymerization?

-yields highly transparent polymers

-can be used to produce the purest form of polymers

-greatest yield of polymer per unit volume (not diluted)

What are the demerits of bulk polymerization?

-very rapid and strongly exothermic (temperature control is difficult)

-overheating causes branching and formation of gels

-broad molecular weight distribution

-removal of unreacted monomer traces is difficult

high increase in viscosity makes processing difficult

What are the applications of bulk polymerization?

polystyrene, poly(methyl methacrylate), and low density polyethylene, casting of polymer products

What is solution polymerization?

monomer added to an inert solvent, and the monomer, initiator, and resulting polymer are all soluble in the solvent, and it reacts without the need for heating or cooling, performed as either batch or continuous

How does solution polymerization affect the properties of the polymer?

reduces both the reaction rate and the molecular weight of the compound

What is the Norrish-Trommsdorff effect?

aka autoacceleration effect, dramatic increase of viscosity during the reaction which reduces the rate of heat transfer

How does solution polymerization have a constant polymerization temperature?

the inert solvent has a boiling point that corresponds to the polymerization temperature; therefore, some solvent evaporates and thus helps to remove the heat of polymerization

Bulk polymerization is suited for which type of reaction?

ionic and coordination polymerization

What are the benefits of solution polymerization?

-both the heat generation and the viscosity of the polymerization mixture are lower because of dilution, which allows for more efficient cooling

-easier temperature control because of the added heat capacity of the solvent

-lower viscosity if solvent makes processing easier

-constant polymerization temperature

-better process control

-polymer solution is not pure but may be directly usable

-simply removal of catalyst residues from polymer

What are the demerits of solution polymerization?

-Polymer solutions have high viscosity, and the separation of the dissolved polymer from the solvent is difficult (if it is needed in pure form)

-lower yield per reactor volume

-reduction of reaction rate and average chain length

-not suited for the production of dry or pure polymers

-environmental pollution due to solvent release

-low molecular weight polymers

What are the applications of solution polymerization?

production of co-polymers, linear low-density polyethylene (LLDPE) and high-density polyethylene (HDPE), polybutadiene and butyl rubber

What is suspension polymerization?

aka bead or pearl polymerization, carried out in a liquid medium (either the inert solvent or liquified monomer), suspends catalysts and polymer particles, polymer remains insoluble in the liquid

How is the polymer separated from the suspending liquid in suspension polymerization?

centrifugation and by flash evaporation

What is the initiator in suspension polymerization?

peroxides i.e. benzoyl peroxide

How is the tendency for agglomeration of the particles in suspension polymerization reduced?

proper agitation and use of the suspension agent

In what ways is the suspension agent employed?

-surface-acting agents (surfactants) such as fatty acids, which reduce the surface tension between water and the monomer droplets and reduce the surface viscosity of the droplet

-water-soluble polymers like gelatin, which raise the viscosity of the suspending aqueous medium and act as protective coatings

What are the benefits of suspension polymerization?

-easy thermal control of the reactor

-constant viscosity of the reaction mass

-relatively pure polymers

-allows cooling of exothermic reaction and closer control of the chain-building process

-easy separation and removal of polymer product

-yields small, uniform, polymer sheets

-possible to control the particle size of the finished polymer

-use of water as the heat exchange medium is economical

-low viscosity due to the suspension

What are the demerits of suspension polymerization?

-continuous agitation required to prevent coalescence or agglomeration into a conglomerate mass

-continuous operation of the system is difficult

-cannot be used for polymers whose glass transition temperature is less than the polymerization temperature

-possibility of polymer contamination

What are the applications of suspension polymerization?

polymer resins, polyvinyl chloride, polystyrene and poly(methyl methacrylate) (PMMA)

What is slurry polymerization?

the monomer is soluble but the polymer is not soluble in the reaction medium and precipitates on the catalyst forming a slurry, a continuous process

In what form is the resultant polymer of slurry polymerization?

fine flocs (pellets) suspended in the solvent

What are the benefits of slurry polymerization?

-low temperature

-easy agitation of system

What are the demerits of slurry polymerization?

-separation or product is difficult and expensive

-Ziegler-Natta catalyst system is special and needs careful preparation

-molecular weight distribution depends on type of catalyst

What are the applications of slurry polymerization?

polyolefins (only C and H) i.e. polyethylene, polypropylene, polystyrene

What is emulsion polymerization?

carried out in a water (dispersing) medium containing an emulsifier (soap) and a water-soluble initiator

In what form is the resultant polymer of emulsion polymerization?

polymer particles of 50-1000 nm dispersed in a continuous medium (water)

What is the product of emulsion polymerization called and what is it composed of?

"latex" particles, virtually all the monomer is consumed and no solvents are involved

What are micelles, how are they formed, and why are they significant to emulsion polymerization?

small spheres of soap film that the monomer diffuses into during emulsion polymerization, formed by coalescence of some of the emulsifier particles, polymerization occurs within the micelles

Emulsion polymerization is suited for which type of reaction?

addition polymerization

What are the benefits of emulsion polymerization?

-rapid, possible to obtain high rates of polymerization

-high average chain length, very high molecular weights, high degree of polymerization

-easy temperature control (since viscosity changes very little)

-no agitation necessary

-low viscosity of reaction mass and increased heat capacity due to water

What are the demerits of emulsion polymerization?

-difficult to obtain pure polymers (surfacant and coagulant residues hard to remove)

-blocks the emulsifiers used in the process

-poor capacity and low electrical resistance

-difficult and expensive to obtain solid polymer

-water leads to lower yield per reactor volume

What are the applications of emulsion polymerization?

-very soft and tacky polymers

-vinyl monomers

-free radical polymerization

-ABS, styrene butadiene rubber, PVC

-graft polymers

What is gas polymerization?

monomer is in the gaseous state and a heterogeneous coordination catalyst (Ziegler-Natta) is used, continuous

Gas polymerization is suited for which type of reaction?

coordination polymerization

What are the benefits of gas phase polymerization?

-lower temp for polymerization

-lower pressure

-no diluent required and no residual catalyst remains

What are the demerits of gas phase polymerization?

-limited to the synthesis of polymers with a high to medium degree of crystallinity

What are the applications of gas phase polymerization?

-polyolefins

-polymers stable in the gaseous phase i.e. HDPE, PP, LLDPE

-LDPE, poly(ethylene vinyl acetate) (PEVA)

How are the costs of monomers of polymeric materials determined?

-cost of the raw material and energy

-number of stages or steps in their manufacture

-scale of the manufacture

What factors in the design of macromolecules affect the physical behavior of the bulk material?

-average length of chains

-strength of the forces among polymer chains

-regularity with which the chains can pack

-stiffness of the individual chains

What are steric hinderances and how does this affect the properties of polymers?

-refers to the interference caused by bulky groups or atoms within a molecule

-increases the molecular weight and melting temperature of the polymer

What is a vinyl polymer?

polymer with one hydrogen atom on the ethylene mer displaced by a side group

What is a vinylidene polymer?

polymer with two or more hydrogen atoms on the ethylene mer displaced by a side group

What are hydrocarbon rubbers?

polymers with a double bond in the C backbone

What is polybutadiene composed of?

double C in the backbone, H is the side group

What is polyisoprene composed of?

double C in the backbone, CH3 is the side group

What is polychloroprene composed of?

double C in the backbone, Cl is the side group

What is the weight average molecular weight?

chains are different sizes and will contribute differently to the molecular weight

What is the number average molecular weight?

number of chains determines the molecular weight, always smaller than the weight average

How does molecular weight affect the properties of polymers?

increasing mol. wt. increases mechanical strength and melting temperature (longer the chains, the greater the number of strong bonds between chains)

What is the degree of polymerization?

The number of repeat units (monomers) in a polymer molecule

How does the degree of polymerization affect the properties of polymers?

as DOP increases...

-increases the molecular weight, tensile strength, creep resistance, impact toughness, wear resistance, and melting temperature

How does the polydispersity index affect the properties of polymers?

-broad molecular weight distribution (high index) = good environmental crack resistance, good impact resistance, and good processability

-narrow molecular weight distribution (low index) = crystallizes at faster, more uniform rate, retains its shape, is less likely to warp, and conforms to its intended geometry

Which molecular configuration is the most stable?

head-to-tail

What are isotactic, syndiotactic, and atactic configurations?

isotactic - all R groups on the same side of the chain

syndiotactic - R groups are on alternating sides of the chain

atactic - R groups are randomly positioned

How does tacticity affect the properties of polymers?

-atactic results in poor packing, low density, low strength and stiffness, and poor heat and chemical resistance (amorphous)

-These will increase in syndiotactic and isotactic polymers, which have a higher propensity to crystallize

What is a cis isomer?

H atoms and side groups are on the same side as the double bond

What is a trans polymer?

H atoms and side groups are on opposite sides of the double bond

How does geometric isomerism affect the properties of polymers?

-cis reduces the glass transition temperature

-cis results in a highly coiled structure (amorphous rubbery polymer)

-trans results in crystalline, hard rigid polymer

-a mixture of cis and trans impedes crystallization

How does the structural shape affect the properties of polymers?

-density of branched polymers is lower than that of linear-chain

-high degrees of cross-linking lead to higher strength, rigidity, dimensional stability, and resistance to heat and chemicals

What is vulcanization and how does it affect the properties of polymers?

-The process of introducing cross-links between long-chain molecules, typically sulfur cross-links, only occurs in double-bonded carbon atoms

-higher elastic modulus

What are spherulites and how do they affect the properties of polymers?

Semi-crystalline polymers are composed of spherulites and as their size decreases the yield strength increases

What is the relationship between density and crystallinity?

properties that depend on crystallinity tend to increase with increasing density

How is the X-ray diffraction method used as a measurement of crystallinity?

-crystalline phase produces identifiable characteristic diffraction patterns like sharp peaks

-amorphous phase produces an indistinct or diffused pattern like halos

How does an increase in crystallinity affect the properties of polymers?

-increase in hardness

-less ductility

-increase in density

-less rubbery polymer

-increase in chemical resistance

-increase in heat resistance

-increase in mechanical strength

-decrease in transparency of polymers

In general, what determines a polymer's ability to crystallize?

the ease with which the molecules can move and be efficiently packed together to create long-range order

How does chain configuration affect polymer crystallinity?

-linear polymers support crystallization

-Branching impairs crystallization

How does the solidification rate affect polymer crystallinity?

slow solificiation allows time for random/entangled chains to become ordered and is required for crystallization

How does molecular chemistry affect polymer crystallinity?

Crystallization is favorable in chemically simple polymers with no bulky molecules or atoms

How does tacticity affect polymer crystallinity?

The regularity of the geometry of side groups enhances crystallization (isotactic and syndiotactic)

How do secondary bonds between chains affect polymer crystallinity?

The presence of polar groups along the polymer chain increases the attractive forces and enhances crystallization

How does copolymerization affect polymer crystallinity?

-reduces the structural symmetry of a polymer and decreases the crystallization tendency

-graft and random non-crystalline

What is polyethylene?

single-bonded C and all H's, CH2CH2

What is polypropylene?

single bonded C and one H replaced by a CH3 side group, CH2CHCH3

What is PTFE?

single-bonded C and all H replaced by F, CF2CF2

How do the size and complexity of side groups affect polymer crystallinity?

crystallization becomes harder as side groups become more complex (bulkier)

How does branching affect polymer crystallinity?

side branches interfere with crystallization because they prevent dense packing of chains

How does the addition of plasticizers affect polymer crystallinity?

The addition of plasticizers decreases the degree of crystallinity

How does chain flexibility affect polymer crystallinity?

chain flexibility reduces the tendency for crystallization as it gives rise to difficulty in chain packing

How does cross-linking affect polymer crystallinity?

Excessive rigidity due to extensive cross-linking can also cause an inability to crystallize